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Title: A Manual of Photographic Chemistry, Including the Practice of the Collodion Process

Author: T. Frederick Hardwich

Release date: November 12, 2020 [eBook #63710]
                Most recently updated: October 18, 2024

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK A MANUAL OF PHOTOGRAPHIC CHEMISTRY, INCLUDING THE PRACTICE OF THE COLLODION PROCESS ***

Transcriber Note

Text emphasis denoted as _Italics_ and =Bold=. Whole and fractional parts
of numbers as 123-4/5 and some fractions as 1/50th.


                                 A MANUAL

                                    OF

                         PHOTOGRAPHIC CHEMISTRY.


                                 A MANUAL

                                    OF

                         PHOTOGRAPHIC CHEMISTRY,

                              INCLUDING THE

                    PRACTICE OF THE COLLODION PROCESS.


                                    By

                          T. FREDERICK HARDWICH,

            LECTURER ON PHOTOGRAPHY IN KING'S COLLEGE, LONDON;
            LATE DEMONSTRATOR OF CHEMISTRY IN KING'S COLLEGE.

                             Fourth Edition.

                              [Illustration]

                                 LONDON:

                  JOHN CHURCHILL, NEW BURLINGTON STREET.

                                MDCCCLVII.


[_The Author reserves to himself the right of translating this Edition._]


PRINTED BY JOHN EDWARD TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS.




PREFACE TO THE THIRD EDITION.

[Illustration]


It is a source of much, gratification to the Author to find himself called
upon to prepare a Third Edition of his Manual in less than fourteen months
from the date of its first publication. No greater proof could have been
afforded of the rapid advance which the Photographic Art is now making in
this country.

On once more entering upon the task of revision, the Writer has been led
to reflect in what way the utility of the Work may be promoted; and from
numerous inquiries he believes that this result will best be attained
by carefully omitting everything which does not possess _practical_ as
well as scientific interest. The majority of Photographers look to the
Art to furnish them with amusement as well as instruction, and they are
deterred from entering upon a study which seems to involve a great amount
of technical detail: these remarks however are not intended to discourage
a habit of perseverance and careful observation, but simply to distinguish
between the essential and the non-essential in the theory of the subject.

The present Edition differs in many important particulars from those which
have preceded it. It has undergone a fresh arrangement throughout. In some
parts it is condensed, in others enlarged. The Chapters on Photographic
Printing are entirely re-written, and include the whole of the Author's
investigations, as published in the Society's Journal. The minute
directions given in this part of the Work will show how much success
in Photography is thought to depend upon a careful attention to minor
particulars.

Another point which has been kept in view, is to recommend, as far as
possible, the employment of chemical agents which are used in medicine
and vended by all druggists throughout the united kingdom. It is often
an advantage to the Amateur to be able to purchase his materials near at
hand; and, if the common impurities of the commercial articles are pointed
out, and directions given for their removal, the 'London Pharmacopœia'
will be found to include almost all the chemicals necessary for the
practice of the Art.

Great additions have been made to the Index of the present Edition, which
is now so complete that a reference to it will at once point out the most
important facts relating to each subject, and the different parts of the
Work at which they are described.

In conclusion, a hope is expressed that this 'Manual of Photographic
Chemistry' may be found to be a complete and trustworthy guide on every
point connected with the theory and practice of the Collodion process.

_London, June 2nd, 1856._




PREFACE TO THE FOURTH EDITION.

[Illustration]


The Author has endeavoured to keep pace with the improvements which are
daily being introduced in the science and art of Photography. In the
present Edition alterations have been made in the style and general
arrangement of the work, and additional matter has been inserted.

Since the publication of the Third Edition, a series of experiments have
been made on the manufacture of Collodion, the results of which have
thrown further light upon the conditions affecting the sensitiveness of
the excited film, and have enabled the writer to introduce an organic
substance, "Glycyrrhizine," which will be found of service in making
Photographic copies of Engravings and similar works of Art.

Dr. Norris, of Birmingham, has within the last few months communicated a
paper on _dry Collodion_, which places the theory of that subject upon a
better footing than before. The Oxymel preservative process is now also
thoroughly understood, and may be considered certain.

In addition to the above, the "Albuminized Collodion" of M. Taupenot,
which experience proves to be one of the best dry processes at present
known, is included in this Edition.

_King's College, London, April 6th, 1857._


ERRATA.

  Page  24, line  5, _for_ conditions _read_ condition.
  Page 115, line 32, _for_ Iodide _read_ Iodine.
  Page 194, line 15, _for_ p. 88 _read_ p. 188.

[_Transcriber Note--Corrections have been applied_]




CONTENTS.



PART I.

THE SCIENCE OF PHOTOGRAPHY.

Page

Introduction 1


CHAPTER I.

Historical Sketch of Photography 6


CHAPTER II.

THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY.

  Section I.--_The Chemistry of the Salts of Silver._--The preparation
     and properties of the Nitrate of Silver--of the Chloride, Bromide,
     and Iodide of Silver.--The Chemistry of the Oxides of Silver 12

  Section II.--_The Photographic Properties of the Salts of
     Silver._--The action of Light upon Nitrate of Silver--upon
     Chloride, Bromide, and Iodide of Silver.--The blackening of
     Chloride of Silver explained.--Simple experiments with sensitive
     Photographic Paper 18


CHAPTER III.

ON THE DEVELOPMENT OF AN INVISIBLE IMAGE.

  Simple experiments illustrating the process 25

  Section I.--_Chemistry of the Substances, employed as
     Developers._--Development shown to be a process of reduction.--The
     chemistry of the principal reducing agents, Gallic Acid,
     Pyrogallic Acid, and the Protosalts of Iron 26

  Section II.--_The Reduction of Salts of Silver by Developing
     Agents._--Reduction of Oxide of Silver--of Nitrate and Acetate
     of Silver.--Varied appearance of Metallic Silver when finely
     divided.--The reduction of the Iodide and other Salts of Silver
     containing no Oxygen 30

  Section III.--_Hypothesis on the Formation and Development of
     the Latent Image._--Explanation of the terms under- and
     over-exposure.--Diagram of Molecular change.--Moser's experiments
     on development.--Curious perversions of development 34


CHAPTER IV.

ON "FIXING" the PHOTOGRAPHIC IMAGE.

  Chemistry of the various substances which may be employed as
     Fixing Agents.--Ammonia, Alkaline Chlorides, Bromides, and
     Iodides.--Hyposulphite of Soda.--Cyanide of Potassium 41


CHAPTER V.

ON THE NATURE AND PROPERTIES OF LIGHT.

  Section I.--_The compound Nature of Light._--Its decomposition into
     elementary coloured rays.--Division of these rays into Luminous,
     Heat-producing, and Chemical Rays 46

  Section II.--_The Refraction of Light._--Phenomena of simple
     refraction by parallel and inclined surfaces.--Refraction from
     curved surfaces.--The various forms of Lenses.--The Foci of
     Lenses.--Formation of a Luminous Image by a Lens 49

  Section III.--_The Photographic Camera._--Its simplest form.--The
     field of the Camera.--Chromatic aberration.--Spherical
     aberration.--The use of Stops.--The double, or Portrait
     combination of Lenses.--Variation between the Visual and Chemical
     Foci in Lenses 54

  Section IV.--_The Photographic Action of Coloured Light._--Diagram
     of Chemical Spectrum.--Illustrative experiments.--Superior
     sensibility of Bromide of Silver to coloured light.--Mode in which
     dark-coloured objects are Photographed 60

  Section V.--_On Binocular Vision and the Stereoscope._--Phenomena
     of Binocular Vision.--Theory of the Stereoscope.--Wheatstone's
     reflecting Stereoscope.--Brewster's Stereoscope.--Rules for taking
     Stereoscopic pictures 66


CHAPTER VI.

THE PHOTOGRAPHIC PROPERTIES OF IODIDE OF SILVER UPON COLLODION.

  Section I.--_Collodion._--Pyroxyline--its varieties--its chemical
     composition.--Means of obtaining Nitro-Sulphuric Acid of the
     proper strength.--Solvents for Pyroxyline.--Variation of physical
     properties in different samples of Collodion.--The changes which
     Iodized Collodion undergoes by keeping 75

  Section II.--_The Chemistry of the Nitrate Bath._--Its solvent action
     on Iodide of Silver.--Acidity and Alkalinity of the Nitrate
     Bath.--Formation of Acetate of Silver in the Bath.--The substances
     which decompose the Bath.--Changes in the Nitrate Bath by use 86

  Section III.--_The Conditions which influence the Formation and
     Development of the Latent Image._--Causes which increase or
     diminish the sensitiveness of the film to Light.--Conditions which
     hasten or retard development 91

  Section IV.--_On certain irregularities in the Developing
     Process._--Effect of particular states of the Bath, and of the
     Collodion, in producing clouding of the Image, and of acids in
     obviating it 103


CHAPTER VII.

ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS.

  Definition of the terms Positive and Negative.--The same Photograph
     often capable of representing both varieties 106

  Section I.--_On Collodion Positives._--The Collodion and Nitrate
     Bath best suited for Positives.--Peculiarities of Pyrogallic
     Acid, and the Protosalts of Iron employed to develope Collodion
     Positives.--The colour of Positives affected by the length of
     exposure to light.--A Process for whitening Glass Positives by
     means of Bichloride of Mercury 108

  Section II.--_On Collodion Negatives._--The Collodion and Nitrate
     Bath best adapted for Negatives.--Use of Glycyrrhizine to increase
     the intensity.--Developing solutions for Negatives.--Conversion of
     finished Positives into Negatives 113


CHAPTER VIII.

ON THE THEORY OF POSITIVE PRINTING.

  Section I.--_The Preparation of the Sensitive Paper._--Its darkening
     by light.--The conditions which affect its sensitiveness and the
     vigour of the Image.--The colour of the print influenced by the
     preparation of the paper 121

  Section II.--_The Processes for Fixing and Toning the
     Proof._--Conditions of a proper fixing.--The Salts of Gold used
     as toning agents.--The properties of the single fixing and toning
     Bath, with the conditions which affect its action 128

  Section III.--_The Author's Photographic Researches._--The chemical
     composition of the Photographic Image.--The various agencies
     destructive to Photographs.--The action of damp air upon Positive
     Prints.--The change in properties of the fixing Bath by constant
     use 140

  Section IV.--_The Fading of Photographic Prints._--The causes which
     produce it.--The comparative permanency of Prints.--The mode of
     testing permanency 160


CHAPTER IX.

ON THE DAGUERREOTYPE AND TALBOTYPE PROCESSES.

  Section I.--_The Daguerreotype._--The nature of the sensitive
     film.--Development of the Latent Image.--The strengthening of the
     Image by means of Hyposulphite of Gold 171

  Section II.--_The Processes of Talbot, etc._--The Calotype.--The
     Waxed Paper Process of Le Grey.--The Albumen Negative
     Process.--Taupenot's Collodio-Albumen Process 176




PART II.

PRACTICAL DETAILS OF THE COLLODION PROCESS.


CHAPTER I.

PREPARATION OF COLLODION.

  Mode of preparing soluble Pyroxyline by the mixed Acids--by the Nitre
     process.--Purification of the Ether and Alcohol.--Preparation of
     the iodizing compounds in a state of purity 185


CHAPTER II.

FORMULÆ FOR SOLUTIONS REQUIRED FOR COLLODION PHOTOGRAPHS.

  Section I.--_Formulæ for direct Positive Solutions._--The
     Collodion.--The Nitrate Bath.--Developing fluids.--Fixing
     liquids.--Whitening solution 201

  Section II.--_Formula for Negative Solutions._--The Collodion.--The
     Nitrate Bath.--Developing fluid.--Fixing liquid 208


CHAPTER III.

MANIPULATIONS OF THE COLLODION PROCESS.

  Section I.--_Manipulations with moist
     Collodion._--Cleaning the Plates.--Coating with
     Collodion.--Exciting.--Exposing.--Developing. --Fixing 213

  Section II.--_Simple directions for the use of Photographic
     Lenses._--Portrait Lenses.--View Lenses.--Mode of finding the
     chemical Focus 227

  Section III.--_On copying Engravings, Etchings, Diagrams, etc._--Mode
     of intensifying the Collodion 231

  Section IV.--_Rules for taking Stereoscopic Photographs._--Mr.
     Latimer Clark's arrangement for working with a single Camera 232

  Section V.--_The Photographic delineation of Microscopic
     Objects.-_-Arrangement of the apparatus.--Mode of finding the
     chemical Focus.--Use of artificial light 235


CHAPTER IV.

THE DETAILS OF PHOTOGRAPHIC PRINTING.

  Section I.--_Positive Printing by the ordinary process._--Selection
     of the paper.--Preparation of Albuminized paper--of plain
     paper--of Ammonio-Nitrate Paper.--Preparation of the fixing and
     toning Bath.--Manipulatory details of Photographic Printing.--Mode
     of washing and mounting the Proofs 240

  Section II.--_Positive Printing by Development._--A process on
     Chloride and Citrate of Silver.--On Iodide of Silver.--On Bromide
     of Silver 259

  Section III.--_The Sel d'Or Process for toning
     Positives._--Preparation of the toning Bath.--Manipulatory details
     267

  Section IV.--_On Printing enlarged Positives, Transparencies, etc.,
     upon Collodion_ 272


CHAPTER V.

CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS.

  Section I.--_Imperfections in Negative and Positive Collodion
     Photographs._--Fogging.--Transparent and opaque spots.--Markings
     of all kinds--under- and over-exposure, etc. 276

  Section II.--_Imperfections in Paper Positives_ 285


CHAPTER VI.

LANDSCAPE PHOTOGRAPHY BY THE COLLODION PRESERVATIVE AND COLLODIO-ALBUMEN
PROCESSES.

  The Honey Keeping Process.--The Oxymel Process.--Photography on dry
     Collodion.--Taupenot's Collodio-Albumen Process 288




PART III.

OUTLINES OF GENERAL CHEMISTRY.


CHAPTER I.

THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS.

  The more important Elementary Bodies, with their Symbols and
     Atomic Weights.--The Compounds formed by their union.--The
     class of Salts.--Illustrations of the nature of Chemical
     Affinity.--Chemical Nomenclature.--Symbolic Notation.--The Laws of
     Combination.--The Atomic Theory.--The Chemistry of Organic Bodies
     305


CHAPTER II.

Vocabulary of Photographic Chemicals 327


APPENDIX.

  Quantitative testing of Nitrate Baths.--Recovery of Silver from
     waste solutions.--Reduction of Chloride of Silver.--Mode of
     taking the Specific Gravity of liquids.--On Filtration.--The use
     of Test-papers.--The removal of Silver stains from the hands,
     etc.--Dr. Ure's tables of the strength of Sulphuric and Nitric
     Acid of different densities.--Table of Weights and Measures 371




A MANUAL

OF

PHOTOGRAPHIC CHEMISTRY.

[Illustration]




INTRODUCTION.


In attempting to impart knowledge on any subject, it is not sufficient
that the writer should himself be acquainted with that which he professes
to teach. Even supposing such to be the case, yet much of the success
of his effort must depend upon the manner in which the information is
conveyed; for as, on the one hand, a system of extreme brevity always
fails of its object, so, on the other, a mere compilation of facts
imperfectly explained tends only to confuse the reader.

A middle course between these extremes is perhaps the best to adopt; that
is, to make selection of certain fundamental points, and to explain them
with some minuteness, leaving others of less importance to be dealt with
in a more summary manner, or to be altogether omitted.

But independently of observations of this kind, which apply to educational
instruction in general, it may be remarked, that there are sometimes
difficulties of a more formidable description to be overcome. For
instance, in treating of any science, such as that of Photography, which
may be said to be comparatively new and unexplored, there is great danger
of erroneously attributing effects to their wrong causes! Perhaps none but
he who has himself worked in the laboratory can estimate this point in its
proper light. In an experiment where the quantities of material acted upon
are infinitesimally small, and the chemical changes involved of a most
refined and subtle description, it is soon discovered that the slightest
variation in the usual conditions will suffice to alter the result.

Nevertheless Photography is truly _a science_, governed by fixed laws; and
hence, as our knowledge increases, we may fairly hope that uncertainty
will cease, and the same precision at length be attained as that with
which chemical operations are usually performed.

The intention of the author in writing this work, is to impart a thorough
knowledge of what may be termed the "First Principles of Photography,"
that the amateur may arm himself with a theoretical acquaintance with the
subject before proceeding to the practice of it. To assist this object,
care will be taken to avoid needless complexity in the formulæ, and all
ingredients will be omitted which are not proved to be of service.

The impurities of chemicals will be pointed out as far as possible, and
special directions given for their removal.

Amongst the variety of Photographic processes devised, those only will
be selected which are correct on theoretical grounds, and are found in
practice to succeed.

As the work is addressed to one supposed to be unacquainted both with
Chemistry and Photography, pains will be taken to avoid the employment of
all technical terms of which an explanation has not previously been given.


A SKETCH OF THE MAIN DIVISIONS TO BE ADOPTED, WITH THE PRINCIPAL
SUBJECT-MATTER OF EACH.

The title given to the Work is "A Manual of Photographic Chemistry," and
it is proposed to include in it a familiar explanation of the nature of
the various chemical agents employed in the Art of Photography, with the
rationale of the manner in which they are thought to act.

The division adopted is threefold:--

Part I. enters minutely into the _theory_ of Photographic
processes; Part II. treats of the _practice_ of Photography upon
Collodion; Part III. embraces a simple statement of the main laws
of Chemistry, with the principal properties of the various substances,
elementary or compound, which are employed by Photographers.

Part I., or "the Science of Photography," includes a full
description of the chemical action of Light upon the Salts of Silver, with
its application to artistic purposes; all mention of manipulatory details,
and of quantities of ingredients, being, as a rule, omitted.

In this division of the Work will be found nine Chapters, the contents of
which are as follows:--

Chapter I. is a sketch of the history of Photography, intended to convey a
general notion of the origin and progress of the Art, without dwelling on
minute particulars.

Chapter II. describes the Chemistry of the Salts of Silver employed by
Photographers; their preparation and properties; the phenomena of the
action of Light upon them, with experiments illustrating it.

Chapter III. leads us on to the formation of _an invisible image_ upon a
sensitive surface, with the development or bringing out to view of the
same by means of chemical re-agents. This point, being of elementary
importance, is described carefully;--the reduction of metallic oxides, the
properties of the bodies employed to reduce, and the hypotheses which have
been entertained on the nature of the Light's action, are all minutely
explained.

Chapter IV. treats of the fixing of Photographic impressions, in order to
render them indestructible by diffused light.

Chapter V. contains a sketch of the _Optics_ of Photography--the
decomposition of white Light into its elementary rays, the Photographic
properties of the different colours, the refraction of Light, and
construction of Lenses. In the last Section of the same Chapter will be
found a short sketch of the history and use of the Stereoscope.

Chapter VI. embraces a more minute description of the sensitive
Photographic processes upon Collodion. In it is explained the chemistry
of Pyroxyline, with its solution in Alcoholized Ether, or _Collodion_;
also the Photographic properties of Iodide of Silver upon Collodion, with
the causes which affect its sensitiveness to Light, and the action of the
developing solutions in bringing out the image.

Chapter VII. continues the same subject, describing the classification of
Collodion Photographs as Positives and Negatives, with the distinctive
peculiarities of each.

Chapter VIII. contains the theory of the production of Positive
Photographs upon paper. In this Chapter will be found an explanation of
the somewhat complex chemical changes involved in printing Positives, with
the precautions which are required to ensure the permanency of the proofs.

Chapter IX. is supplementary to the others, and a brief notice of it
will suffice. It explains the theory of the Photographic processes of
Daguerre and Talbot; especially noticing those points in which they may be
contrasted with Photography upon Collodion, but omitting all description
of manipulatory details, which if included would extend the Work beyond
its proposed limits.

The title of the second principal division of the Work, viz. "The practice
of Photography upon Collodion," explains itself. Attention however may be
invited to the fifth Chapter, in which a classification is given of the
principal imperfections in Photographs, with short directions for their
removal; and to Chapter VI., which describes the preservation of the
sensitiveness of Collodion plates and the mode of operating upon films of
Albumenized Collodion.

In Part III. will be found, in addition to a statement of the laws
of chemical combination, etc., a list of Photographic chemicals,
alphabetically arranged, including their preparation and properties as far
as required for their employment in the Art.

The reader will at once gather from this sketch of the contents of the
volume before him, that whilst the general theory of every Photographic
process is described, with the preparation and properties of the chemicals
employed, minute directions in the minor points of manipulation are
restricted to Photography upon Collodion, that branch of the Art being the
one to which the time and attention of the author have been especially
directed. Collodion is allowed by all to be the best vehicle for the
sensitive Silver Salts which is at present known, and successful results
can be obtained with a very small expenditure of time and trouble, if the
solutions employed in the process are prepared in a state of purity.




CHAPTER I.

HISTORICAL SKETCH OF PHOTOGRAPHY.


The Art of Photography, which has now attained such perfection, and has
become so popular amongst all classes, is one of comparatively recent
introduction.

The word Photography means literally "writing by means of Light;" and it
includes all processes by which any kind of picture can be obtained by the
chemical agency of Light, without reference to the nature of the sensitive
surface upon which it acts.

The philosophers of antiquity, although chemical changes due to the
influence of Light were continually passing before their eyes, do not
appear to have directed their attention to them. Some of the _Alchemists_
indeed noticed the fact that a substance which they termed "Horn Silver,"
which was probably a Chloride of Silver which had undergone fusion, became
_blackened_ by exposure to Light; but their ideas on such subjects being
of the most erroneous nature, nothing resulted from the discovery.

The first philosophical examination of the decomposing action of Light
upon compounds containing Silver was made by the illustrious Scheele, no
longer than three-quarters of a century ago, viz. in 1777. It was also
remarked by him that some of the coloured rays of Light were peculiarly
active in promoting the change.

_Earliest application of these facts to purposes of Art._--The first
attempts to render the blackening of Silver Salts by Light available for
artistic purposes were made by Wedgwood and Davy about A.D. 1802.
A sheet of white paper or of white leather was saturated with a solution
of Nitrate of Silver, and the _shadow_ of the figure intended to be copied
projected upon it. Under these circumstances the part on which the shadow
fell remained white, whilst the surrounding exposed parts gradually
darkened under the influence of the sun's rays.

Unfortunately these and similar experiments, which appeared at the
outset to promise well, were checked by the experimentalists being
unable to discover any means of fixing the pictures, so as to render
them indestructible by diffused Light. The unchanged Silver Salt being
permitted to remain in the white portions of the paper, naturally caused
the proofs to blacken in every part, unless carefully preserved in the
dark.

_Introduction of the Camera Obscura, and other Improvements in
Photography._--The "Camera Obscura," or darkened chamber, by means of
which a luminous image of an object may be formed, was invented by
Baptista Porta, of Padua; but the preparations employed by Wedgwood were
not sufficiently sensitive to be easily affected by the subdued light of
that instrument.

In the year 1814, however, twelve years subsequent to the publication of
Wedgwood's paper, M. Niépce, of Chalons, having directed his attention to
the subject, succeeded in perfecting a process in which the Camera could
be employed, although the sensibility was still so low that an exposure of
some hours was required to produce the effect.

In the process of M. Niépce, which was termed "Heliography," or
"sun-drawing," the use of the Silver Salts was discarded, and a resinous
substance, known as "Bitumen of Judæa," substituted. This resin was
smeared on the surface of a metal plate, and exposed to the luminous
image. The light in acting upon it so changed its properties, that it
became _insoluble_ in certain essential oils. Hence, on subsequent
treatment with the oleaginous solvent, the shadows dissolved away, and
the _lights_ were represented by the unaltered resin remaining on the
plate.

_The Discoveries of M. Daguerre._--MM. Niépce and Daguerre appear at one
time to have been associated as partners, for the purpose of mutually
prosecuting their researches; but it was not until after the death of the
former, viz. in 1839, that the process named the Daguerreotype was given
to the world. Daguerre was dissatisfied with the slowness of action of the
Bitumen sensitive surface, and directed his attention mainly to the use of
the Salts of Silver, which are thus again brought before our notice.

Even the earlier specimens of the Daguerreotype, although far inferior
to those subsequently produced, possessed a beauty which had not been
attained by any Photographs prior to that time.

The sensitive plates of Daguerre were prepared by exposing a silvered
tablet to the action of the vapour of _Iodine_, so as to form a layer of
Iodide of Silver upon the surface. By a short exposure in the Camera an
effect was produced, not visible to the eye, but appearing when the plate
was subjected to the vapour of Mercury. This feature, viz. the production
of a _latent_ image upon Iodide of Silver, with its subsequent development
by a chemical reagent, is one of the first importance. Its discovery at
once reduced the time of taking a picture from hours to minutes, and
promoted the utility of the Art.

Daguerre also succeeded in fixing his proofs, by removal of the unaltered
Iodide of Silver from the shadows. The processes employed however were
imperfect, and the matter was not set at rest until the publication of a
paper by Sir John Herschel, on the property possessed by "Hyposulphites"
of dissolving the Salts of Silver insoluble in water.

_On a means of Multiplying Photographic Impressions, and other Discoveries
of Mr. Fox Talbot._--The first communication made to the Royal Society
by Mr. Fox Talbot, in January, 1839, included only the preparation of a
sensitive paper for copying objects by application. It was directed that
the paper should be dipped first in solution of Chloride of Sodium, and
then in Nitrate of Silver. In this way a white substance termed Chloride
of Silver is formed, more sensitive to light than the Nitrate of Silver
originally employed by Wedgwood and Davy. The object is laid in contact
with the prepared paper, and, being exposed to light, a copy is obtained,
which is Negative,--_id est_, with the light and shade reversed. A second
sheet of paper is then prepared, and the first, or Negative impression,
laid upon it, so as to allow the sun's light to pass through the
transparent parts. Under these circumstances, when the Negative is raised,
a natural representation of the object is found below; the tints having
been again reversed by the second operation.

This production of a Negative Photograph, from which any number of
Positive copies may be obtained, is a cardinal point in Mr. Talbot's
invention, and one of great importance.

The patent issued for the process named Talbotype or Calotype dates from
February, 1841. A sheet of paper is first coated with Iodide of Silver by
soaking it alternately in Iodide of Potassium and Nitrate of Silver; it
is then washed with solution of Gallic Acid containing Nitrate of Silver
(sometimes termed Gallo-Nitrate of Silver), by which the sensibility to
light is greatly augmented. An exposure in the Camera of some seconds or
minutes, according to the brightness of the light, impresses an invisible
image, which is brought out by treating the plate with a fresh portion of
the mixture of Gallic Acid and Nitrate of Silver employed in exciting.

On the use of Glass Plates to retain Sensitive Films.--The principal
defects in the Calotype process are attributable to the coarse and
irregular structure of the fibre of paper, even when manufactured with the
greatest care, and expressly for Photographic purposes. In consequence of
this, the same amount of exquisite definition and sharpness of outline as
that resulting from the use of metal plates, cannot be obtained.

We are indebted to Sir John Herschel for the first employment of glass
plates to receive sensitive Photographic films.

The Iodide of Silver may be retained upon the glass by means of a layer of
Albumen or white of egg, as proposed by M. Niépce de Saint-Victor, nephew
to the original discoverer of the same name.

A more important improvement still is the employment of "Collodion" for a
similar purpose.

Collodion is an ethereal solution of a substance almost identical with
Gun-Cotton. On evaporation it leaves a transparent layer, resembling
gold-beater's skin, which adheres to the glass with some tenacity. M. Le
Grey of Paris originally suggested that this substance might possibly be
rendered available in Photography, but our own countryman, Mr. Archer, was
the first to carry out the idea practically. In a communication to 'The
Chemist' in the autumn of 1851, this gentleman gave a description of the
Collodion process much as it now stands; at the same time proposing the
substitution of _Pyro_-gallic acid for the Gallic acid previously employed
in developing the image.

At that period no idea could have been entertained of the stimulus which
this discovery would render to the progress of the Art; but experience has
now abundantly demonstrated, that, as far as all qualities most desirable
in a Photographic process are concerned, none at present known can excel,
or perhaps equal, the Collodion process.




CHAPTER II.

THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY.


By the term Salt of Silver we understand that the compound in question
contains Silver, but not in its elementary form; the metal is in fact in
a state of chemical union with other elements which disguise its physical
properties, so that the Salt possesses none of the external characters of
the Silver from which it was produced.

Silver is not the only metal which forms Salts; there are Salts of Lead,
Copper, Iron, etc. Sugar of Lead is a familiar instance of a Salt of Lead.
It is a white crystalline body, easily soluble in water, the solution
possessing an intensely sweet taste; chemical tests prove that it contains
Lead, although no suspicion of such a fact could be entertained from a
consideration of its general properties.

Common Salt, or Chloride of Sodium, which is the type of the salts
generally, is constituted in a similar manner; that is to say, it contains
a metallic substance, the characters of which are masked, and lie hid in
the compound.

The contents of this Chapter may be arranged in three Sections: the first
describing the Chemistry of the Salts of Silver; the second, the action of
Light upon them; the third, the preparation of a sensitive surface, with
experiments illustrating the formation of the Photographic image.


SECTION I.

Chemistry of the Salts of Silver.

The principal Salts of Silver employed in the Photographic processes are
four in number, viz. Nitrate of Silver, Chloride of Silver, Iodide of
Silver, and Bromide of Silver. In addition to these, it will be necessary
to describe the Oxides of Silver.


THE PREPARATION AND PROPERTIES OF THE NITRATE OF SILVER.

Nitrate of Silver is prepared by dissolving metallic Silver in Nitric
Acid. Nitric Acid is a powerfully acid and corrosive substance, containing
two elementary bodies united in definite proportions. These are Nitrogen
and Oxygen; the latter being present in greatest quantity.

Nitric Acid is a powerful solvent for the metallic bodies generally. To
illustrate its action in that particular, as contrasted with other acids,
place pieces of silver-foil in two test-tubes, the one containing dilute
Sulphuric, the other dilute Nitric Acid; on the application of heat a
violent action soon commences in the latter, but the former is unaffected.
In order to understand this, it must be borne in mind that when a metallic
substance dissolves in an acid, the nature of the solution is different
from that of an aqueous solution of salt or sugar. If salt water be boiled
down until the whole of the water has evaporated, the salt is recovered
with properties the same as at first; but if a similar experiment be made
with a solution of Silver in Nitric Acid, the result is different: in that
case metallic Silver is not obtained on evaporation, but Silver combined
with Oxygen and Nitric Acid, both of which are strongly retained, being in
fact in a state of chemical combination with the metal.

If we closely examine the effects produced by treating Silver with Nitric
Acid, we find them to be of the following nature:--first, a certain
amount of Oxygen is imparted to the metal, so as to form an _Oxide_, which
Oxide dissolves in another portion of the Nitric Acid, producing _Nitrate_
of the Oxide, or, as it is shortly termed, Nitrate of Silver.[1]

[1] The preparation of Nitrate of Silver from the standard coin of the
realm is described in Part III., Art. "Silver."

It is the instability of Nitric Acid therefore--its proneness to part with
Oxygen--which renders it superior to the Sulphuric and to most acids in
dissolving Silver and various other substances, both organic and inorganic.

_Properties of Nitrate of Silver._--In preparing Nitrate of Silver, when
the metal has dissolved, the solution is boiled down and set aside to
crystallize. The salt however as so obtained is still acid to test-paper,
and requires either re-crystallization, or careful heating to about 300°
Fahrenheit. It is this retention of small quantities of Nitric Acid, and
sometimes probably of Nitrous Acid, which renders much of the commercial
Nitrate of Silver useless for Photography, until rendered neutral by
fusion and a second crystallization.

Pure Nitrate of Silver occurs in the form of white crystalline plates,
which are very heavy and dissolve readily in an equal weight of cold
water. The solubility is much lessened by the presence of free Nitric
Acid, and in the _concentrated_ Nitric Acid the crystals are almost
insoluble. Boiling Alcohol takes up about one-fourth part of its weight
of the crystallized Nitrate, but deposits nearly the whole on cooling.
Nitrate of Silver has an intensely bitter and nauseous taste; acting as a
caustic, and corroding the skin by a prolonged application. Its aqueous
solution does not redden blue litmus-paper.

Heated in a crucible the salt melts, and when poured into a mould and
solidified, forms the white _lunar caustic_ of commerce. At a still higher
temperature it is decomposed, and bubbles of Oxygen Gas are evolved: the
melted mass cooled and dissolved in water leaving behind a black powder,
and yielding a solution, which is faintly alkaline to test-paper, from
the presence of minute quantities of Nitrite or basic Nitrite of Silver.[2]

[2] Nitrite of Silver differs from the Nitrate in containing less Oxygen,
and is formed from it by the abstraction of two atoms of that element; it
is described in the vocabulary, Part III.


THE CHEMISTRY OF THE CHLORIDES OF SILVER.

_Preparation of Protochloride of Silver._--The ordinary white Chloride of
Silver may be prepared in two ways,--by the direct action of Chlorine upon
metallic Silver, and by double decomposition between two salts.

If a plate of polished silver be exposed to a current of Chlorine Gas,[3]
it becomes after a short time coated on the surface with a superficial
film of white powder. This powder is Chloride of Silver, containing the
two elements Chlorine and Silver united in single equivalents.

[3] For the properties of the element "Chlorine," see the third division
of the Work.

_Preparation of Chloride of Silver by double decomposition._--In order
to illustrate this, take a solution in water of Chloride of Sodium or
"common salt," and mix it with a solution containing Nitrate of Silver;
immediately a dense, curdy, white precipitate falls, which is the
substance in question.

In this reaction the elements change places; the Chlorine leaves the
Sodium with which it was previously combined, and crosses over to the
Silver; the Oxygen and Nitric Acid are released from the Silver, and unite
with the Sodium; thus

           Chloride of Sodium _plus_ Nitrate of Silver
  _equals_ Chloride of Silver _plus_ Nitrate of Soda.

This interchange of elements is termed by chemists double decomposition;
further illustrations of it, with the conditions necessary to the proper
establishment of the process, are given in the first Chapter of Part III.

The essential requirements in two salts intended for the preparation of
Chloride of Silver, are simply that the first should contain Chlorine,
the second Silver, and that both should be soluble in water; hence the
Chloride of Potassium or Ammonium may be substituted for the Chloride of
Sodium, and the Sulphate or Acetate for the Nitrate of Silver.

In preparing Chloride of Silver by double decomposition, the white clotty
masses which first form must be washed repeatedly with water, in order to
free them from soluble Nitrate of Soda, the other product of the change.
When this is done, the salt is in a pure state, and may be dried, etc., in
the usual way.

_Properties of Chloride of Silver._--Chloride of Silver differs in
appearance from the Nitrate of Silver. It is not usually crystalline,
but forms a soft white powder resembling common chalk or whiting. It is
tasteless and insoluble in water; unaffected by boiling with the strongest
Nitric Acid, but sparingly dissolved by concentrated Hydrochloric Acid.

Ammonia dissolves Chloride of Silver freely, as do solutions of
Hyposulphite of Soda and Cyanide of Potassium. Concentrated solutions
of alkaline Chlorides, Iodides, and Bromides are likewise solvents of
Chloride of Silver, but to a limited extent, as will be more fully shown
in Chapter IV., when treating of the modes of fixing the Photographic
proofs.

Dry Chloride of Silver carefully heated to redness fuses, and concretes on
cooling into a tough and semi-transparent substance, which has been termed
_horn silver_ or _luna cornea_.

Placed in contact with metallic Zinc or Iron acidified with dilute
Sulphuric Acid, Chloride of Silver is reduced to the metallic state, the
Chlorine passing to the other metal under the decomposing influence of the
galvanic current which is established.

_Preparation and Properties of the Subchloride of Silver._--If a plate
of polished Silver be dipped in solution of Perchloride of Iron, or of
Bichloride of Mercury, a _black stain_ is produced, the Iron or Mercury
Salt losing a portion of Chlorine, which passes to the Silver and converts
it superficially into Subchloride of Silver. This compound differs from
the white Chloride of Silver in containing less Chlorine; the composition
of the latter being represented by the formula AgCl, that of the former
may perhaps be written as Ag{2}Cl(?).

Subchloride of Silver is interesting to the Photographer as corresponding
in properties and composition with the ordinary Chloride of Silver
blackened by light. It is a pulverulent substance of a bluish-black colour
not easily affected by Nitric Acid but decomposed by fixing agents such as
Ammonia, Hyposulphite of Soda, or Cyanide of Potassium, into Chloride of
Silver which dissolves, and insoluble metallic Silver.


THE CHEMISTRY OF IODIDE OF SILVER.

The properties of _Iodine_ are described in the third division of the
Work: they are analogous to those of Chlorine and Bromine, the Silver
Salts formed by these elements bearing also a strong resemblance to each
other.

_Preparation and Properties of Iodide of Silver._--Iodide of Silver may be
formed in an analogous manner to the Chloride, viz. by the direct action
of the vapour of Iodine upon metallic Silver, or by double decomposition,
between solutions of Iodide of Potassium and Nitrate of Silver.

When prepared by the latter mode it forms an impalpable powder, the colour
of which varies slightly with the manner of precipitation. If the Iodide
of Potassium be in excess, the Iodide of Silver falls to the bottom of the
vessel nearly white; but with an excess of Nitrate of Silver it is of a
straw-yellow tint. This point may be noticed, because the yellow salt is
the one adapted for Photographic use, the other being insensible to the
influence of light.

Iodide of Silver is tasteless and inodorous; insoluble in water and in
dilute Nitric Acid. It is scarcely dissolved by Ammonia, which serves
to distinguish it from the Chloride of Silver, freely soluble in that
liquid. Hyposulphite of Soda and Cyanide of Potassium both dissolve Iodide
of Silver; it is also soluble in solutions of the alkaline Bromides and
Iodides, as will be further explained in Chapter IV.

Iodide of Silver is reduced by Metallic Zinc in the same manner as the
Chloride of Silver, forming soluble Iodide of Zinc and leaving a black
powder.


THE PREPARATION AND PROPERTIES OF BROMIDE OF SILVER.

This substance so closely resembles the corresponding salts containing
Chlorine and Iodine, that a short notice of it will suffice.

Bromide of Silver is prepared by exposing a silvered plate to the vapour
of Bromine, or by adding solution of Bromide of Potassium to Nitrate of
Silver. It is an insoluble substance, slightly yellow in colour, and
distinguished from Iodide of Silver by dissolving in strong Ammonia and in
Chloride of Ammonium. It is freely soluble in Hyposulphite of Soda and in
Cyanide of Potassium.

The properties of the element Bromine are described in Part III.


CHEMISTRY OF THE OXIDES OF SILVER.

_The Protoxide of Silver_ (Ag O).--If a little Potash or Ammonia be added
to solution of Nitrate of Silver, an olive-brown substance is formed,
which, on standing, collects at the bottom of the vessel. This is Oxide of
Silver, displaced from its previous state of combination with Nitric Acid
by the stronger oxide. Potash. Oxide of Silver is soluble to a very minute
extent in pure water, the solution possessing an alkaline reaction to
Litmus; it is easily dissolved by Nitric or Acetic Acid, forming a neutral
Nitrate or Acetate; also soluble in Ammonia (Ammonio-Nitrate of Silver),
and in Nitrate of Ammonia, Hyposulphite of Soda, and Cyanide of Potassium.
Long exposure to light converts it into a black substance, which is
probably a Suboxide.

_The Suboxide of Silver_ (Ag{2}O?)--This substance was obtained by Faraday
on exposing a solution of the Ammonio-Nitrate of Silver to the action of
the air. It bears a relation to the ordinary brown Protoxide of Silver
similar to that which the Subchloride bears to Protochloride of Silver.

Suboxide of Silver is a black or grey powder, which assumes the metallic
lustre on rubbing, and when treated with dilute Acids is resolved into
Protoxide of Silver which dissolves, and metallic Silver.


SECTION II.

_On the Photographic Properties of the Salts of Silver._

In addition to the Salts of Silver described in the first Section of this
Chapter there are many others well known to chemists, as the Acetate of
Silver, the Sulphate, the Citrate of Silver, etc. Some occur in crystals
which are soluble in water, whilst others are pulverulent and insoluble.

The Salts of Silver formed by colourless Acids are white when first
prepared, and remain so if kept in a dark place; but they possess the
remarkable peculiarity of being darkened in colour by exposure to Light.

Action of Light upon the Nitrate of Silver.--The Nitrate of Silver is one
of the most permanent of the Silver salts. It may be preserved unchanged
in the crystalline form, or in solution in distilled water, for an
indefinite length of time, even when constantly exposed to the diffused
light of day. This is partly explained by the nature of the acid with
which Oxide of Silver is associated in the Salt; Nitric Acid, possessing
strong oxidizing properties, being opposed to the darkening influence of
Light upon the Silver compounds.

Nitrate of Silver may, however, be rendered susceptible to the influence
of Light, by adding to its solution _organic matter_, vegetable or animal.
The phenomena produced in this case are well illustrated by dipping a
pledget of cotton-wool, or a sheet of white paper, in solution of Nitrate
of Silver, and exposing it to the direct rays of the sun; it slowly
darkens, until it becomes nearly black. The stains upon the skin produced
by handling Nitrate of Silver are caused in the same way, and are seen
most evidently when the part has been exposed to light.

The varieties of organic matter which especially facilitate the blackening
of Nitrate of Silver are such as tend _to absorb Oxygen_; hence pure
vegetable fibre, free from Chlorides, such, for instance, as the Swedish
filtering-paper, is not rendered very sensitive by being simply brushed
with solution of the Nitrate, but a little grape sugar added soon
determines the decomposition.

_Decomposition of Chloride, Bromide, and Iodide of Silver by Light._--Pure
moist Chloride of Silver[4] changes slowly from white to violet on
exposure to light. Bromide of Silver becomes of a grey colour, but is
less affected than the Chloride. Iodide of Silver (if free from excess
of Nitrate of Silver) does not alter in appearance by exposure even to
the sun's rays, but retains its yellow tint unchanged. Of these three
compounds therefore _Chloride_ of Silver is the most readily acted on
by light, and papers prepared with this salt will become far darker on
exposure than others coated with Bromide or Iodide of Silver.

[4] The Chloride here spoken of is the compound prepared by adding a
soluble Chloride to a solution of Nitrate of Silver: the product of the
direct action of Chlorine upon metallic Silver is sometimes insensitive to
light.

There are certain conditions which accelerate the action of light upon the
Chloride of Silver. These are, first, _an excess of Nitrate of Silver_,
and second, _the presence of organic matter_. Pure Chloride of Silver
would be useless as a Photographic agent, but a Chloride with excess of
Nitrate is very sensitive. Even Iodide of Silver, ordinarily unaffected,
is blackened by light when moistened with a solution of the Nitrate of
Silver.[5]

[5] The reader will understand that the Acetate, Sulphate, or any other
soluble Salt of Silver, might be substituted for the Nitrate in this
experiment.

Organic matter combined with Chloride and Nitrate of Silver gives a still
higher degree of sensibility, and in this way the Photographic papers are
prepared.

_The blackening of Chloride of Silver by Light explained._--This may
be studied by suspending pure Chloride of Silver in distilled water,
and exposing it to the sun's rays for several days. When the process of
darkening has proceeded to some extent, the supernatant liquid is found to
contain _free Chlorine_, or, in place of it. _Hydrochloric Acid_ (H Cl),
the result of a subsequent action of the Chlorine upon the water.

The luminous rays appear to loosen the affinity of the elements Chlorine
and Silver for each other; hence a portion of Chlorine is separated, and
the white Protochloride is converted into the violet _Sub_chloride of
Silver. If an atom of Nitrate of Silver be present, the liberated Chlorine
unites with it, displacing Nitric Acid, and forming again Chloride of
Silver, which is decomposed in its turn. The excess of Nitrate of Silver
thus exerts an accelerating influence upon the darkening of Chloride of
Silver, by rendering the chain of chemical affinities more complete, and
preventing an accumulation of Chlorine in the liquid, which would be a
check to the continuance of the action.

_Action of Light upon organic Salts of Silver._--On adding diluted
Albumen, or white of egg, to solution of Nitrate of Silver, a flocculent
deposit forms which is a compound of the animal matter with Protoxide of
Silver, and is known as "Albuminate of Silver." This substance is at first
quite white, but on exposure to light it turns to a brick-red colour. The
change which takes place is one of _deoxidation_, the Protoxide of Silver
losing a portion of its Oxygen, and a Suboxide of Silver, the product
of the reduction, remaining in union with the oxidized Albumen. The red
compound may therefore be loosely designated as an Albuminate of Suboxide
of Silver.

_Gelatine_ does not precipitate Nitrate of Silver in the same manner as
Albumen: but if a sheet of transparent Gelatine be allowed to imbibe a
solution of the Nitrate, it becomes of a clear ruby-red tint on exposure
to light, and a true chemical compound of Gelatine, or a product of its
oxidation, with a low Oxide of Silver, is produced.

Caseine, the animal principle of milk, is coagulated by Nitrate of Silver,
and the red substance formed on exposing the curds to light may be viewed
as analogous in composition to the corresponding compounds with Albumen
and Gelatine.

Many other organic salts of Silver are darkened by light. The white
Citrate of Protoxide of Silver changes to a red substance, reacting with
chemical tests in the same manner as Wöhler's Citrate of Suboxide of
Silver, which he obtained by reducing the ordinary Citrate in Hydrogen
Gas. Glycyrrhizin, the Sugar of Liquorice, also forms a white compound
with Oxide of Silver which becomes brown or red in the sun's rays.[6]

[6] For further particulars on the action of light upon the Salts of
Silver associated with organic matter, see the Author's paper on the
composition of the photographic image, in the eighth Chapter.


SIMPLE EXPERIMENTS ILLUSTRATING THE ACTION OF LIGHT UPON A SENSITIVE LAYER
OF CHLORIDE OF SILVER ON PAPER.

In the performance of the most simple experiments on the decomposition
of Silver Salts by Light, the student may employ ordinary test-tubes,
in which small quantities of the two liquids required for the double
decomposition may be mixed together.

When however concentrated solutions are used in this way, the insoluble
Silver Salt falls in dense and clotted masses, which, exposed to the
sun's rays, quickly blacken on the exterior, but the inside is protected,
and remains white. It is of importance therefore in Photography that the
sensitive material should exist in the form of _a surface_, in order that
the various particles of which it is composed may each one individually be
brought into relation with the disturbing force.

Full directions for the preparation of sensitive Photographic paper are
given in the second division of this work. The following is the theory of
the process:--A sheet of paper is treated with solution of Chloride of
Sodium or Ammonium, and subsequently with Nitrate of Silver; hence results
a formation of Chloride of Silver in a fine state of division, with an
excess of Nitrate of Silver, the Silver bath having been purposely made
stronger in proportion than the salting solution.

_Illustrative Experiment No. I._--Place a square of sensitive paper
(prepared according to the directions given in the Second Part of the
work) in the direct rays of the sun, and observe the gradual process of
darkening which takes place; the surface passes through a variety of
changes in colour until it becomes of a deep chocolate-brown. If the Light
is tolerably intense, the brown shades are probably reached in from three
to five minutes; but the sensibility of the paper, and also the nature of
the tints, will vary much with the character of the organic matter present.

_Experiment No. II._--Lay a device cut from black paper upon a sheet
of sensitive paper, and compress the two together by means of a sheet
of glass. After a proper length of exposure the figure will be exactly
copied, the tint however being reversed: the black paper protecting the
sensitive Chloride beneath, produces a _white_ figure upon a dark ground.

_Experiment No. III._--Repeat the last experiment, substituting a piece
of lace or gauze-wire for the paper device. This is intended to show the
minuteness with which objects can be copied, since the smallest filament
will be distinctly represented.

_Experiment No. IV._--Take an engraving in which the contrast of light
and shade is tolerably well marked, and having laid it closely in contact
with the sensitive paper, expose as before. This experiment shows that the
surface darkens in degrees proportionate to the intensity of the light, so
that the _half_ shadows of the engraving are accurately maintained, and a
pleasing gradation of tone produced.

In the darkening of Photographic papers, the action of the light is quite
superficial, and although the black colour may be intense, yet the amount
of reduced Silver which forms it is so small that it cannot conveniently
be estimated by chemical reagents. This is well shown by the results of
an analysis performed by the Author, in which the total weight of Silver
obtained from a blackened sheet measuring nearly 24 by 18 inches amounted
to less than _half a grain_. It becomes therefore of great importance in
preparing sensitive paper to attend to the condition of the surface layer
of particles, the action rarely extending to those beneath. The use of
Albumen, Gelatine, etc., which will be explained in the eighth Chapter,
has reference to this amongst other advantages, and secures a better and
more sharply defined print.




CHAPTER III.

ON THE DEVELOPMENT OF AN INVISIBLE IMAGE BY MEANS OF A REDUCING AGENT.


It has been shown in the previous Chapter that the majority of the Salts
of Silver, both organic and inorganic, are darkened in colour on exposure
to light, and, by the loss of Oxygen, Chlorine, etc., become reduced to
the condition of _Sub_salts.

Many of the same compounds are also susceptible of a change under the
influence of light, which is even more remarkable. This change takes
place after a comparatively short exposure, and as it does not affect the
appearance of the sensitive layer, for some time it escaped notice: but
it was afterwards discovered that an impression, before invisible, might
be brought out by treating the plate with certain chemical agents which
are without effect on the original unchanged salt, but quickly blacken it
after exposure.

It is a remarkable fact that the Silver compounds most readily affected by
light alone, are not the most sensitive to the reception of the invisible
image. Thus, of Photographic papers prepared with Chloride, Bromide, or
Iodide of Silver, the former assume the deepest shade of colour under the
influence of the sun's rays, but if all be exposed _momentarily_, and
then removed, the greatest amount of effect will be developed upon the
Iodide paper. Iodide of Silver therefore is the salt commonly used when
sensibility is an object, but it should be noted that images nearly or
quite latent can be impressed upon many other of the compounds of Silver,
including those belonging to the animal and vegetable kingdoms.

_Experiments illustrating the Formation of an Invisible Image._--Take a
sheet of sensitive paper, prepared with Iodide of Silver by the method
given in the fourth Chapter of Part II., and having divided it into two
parts, expose one of them to the luminous rays for a few seconds. No
visible decomposition takes place, but on removing the pieces to a room
dimly illuminated, and brushing with a solution of _Gallic Acid_, a
manifest difference will be observed; the one being unaffected, whilst the
other darkens gradually until it becomes black.

_Experiment II._--A prepared sheet is shielded in certain parts by an
opaque substance, and then after the requisite exposure, which is easily
ascertained by a few trials, treated with the Gallic Acid as before; in
this case the protected part remains white, whilst the other darkens to a
greater or less extent.

In the same way, copies of leaves, engravings, etc. may be made, very
correct in the shading and much resembling those produced by the prolonged
action of light alone upon the Chloride of Silver.

The object of employing a substance like Gallic Acid to _develope_ or
bring out to view an invisible image, in preference to forming the picture
by the direct action of light, unassisted by a developer, is the _economy
of time_ thereby effected. This is well shown in the results of some
experiments conducted by M. Claudet in the Daguerreotype process: he found
that with a sensitive layer of Bromo-Iodide of Silver, an intensity of
light three thousand times greater was required if the use of a developer
was omitted, and the exposure continued until the picture became visible
upon the plate.

To increase the sensitiveness of Photographic preparations is a point
of great consequence; and indeed, when the Camera is used, from the low
intensity of the luminous image formed in that instrument, no other plan
than the one above described would be practicable. Hence the advancement,
and indeed the very origin, of the Photographic Art, may be dated from the
first discovery of a process for bringing out to view an invisible image
by means of a reducing agent.

The present Chapter is divided into three Sections:--first, the chemical
properties of the substances usually employed as developers;--second,
their mode of action in reducing the Salts of Silver;--third, hypotheses
on the action of light in impressing a latent image.


SECTION I.

_Chemistry of the various Substances employed as Developers._

Development is essentially a process of _reduction_, or, in other words,
of _deoxidation_. If we take a certain metal, we can, by means of Nitric
Acid, impart Oxygen to it, so that it becomes first an Oxide, and
afterwards, by solution of the Oxide in the excess of acid, _a salt_. When
this salt is formed, by a series of chemical operations the reverse of
the former it may be deprived of all its Oxygen, and the metallic element
again isolated.

The degree of facility with which oxidation as well as reduction is
performed, depends upon the affinity for Oxygen which the particular
metal under treatment possesses. In this respect there is considerable
difference, as may be shown by a reference to the two well-known metals,
Iron and Gold. How speedily does the first become tarnished and covered
with rust, whilst the other remains bright even in the fire! It is indeed
possible, by a careful process, to form Oxide of Gold; but it retains its
Oxygen so loosely that the mere application of heat is sufficient to
drive it off, and leave the metal in a pure state.

Silver, Gold, and Platinum all belong to the class of _noble_ metals,
having the least affinity for Oxygen: hence their Oxides are unstable,
and any body tending strongly to absorb Oxygen will reduce them to the
metallic state.

Observe, therefore, that the substances employed by the Photographer
to assist the action of the light, and to develope the picture, act by
removing Oxygen. The sensitive Salt of Silver is thus _reduced_, more or
less completely, in the parts touched by light, and an opaque deposit
results which forms the image.[7]

[7] These remarks do not apply to the vapour of Mercury employed as a
developing agent in the Daguerreotype. The chemistry of that process will
be explained in a separate Chapter.

The most important of the developers are as follows:-- Gallic Acid,
Pyrogallic Acid, and the _Proto_salts of Iron.


CHEMISTRY OF GALLIC AND PYROGALLIC ACIDS.

a. _Of Gallic Acid._--Gallic Acid is obtained from _Gall Nuts_, which
are peculiar excrescences formed upon the branches and shoots of the
_Quercus infectoria_ by the puncture of a species of insect. The best
kind is imported from Turkey, and sold in commerce as Aleppo Galls. Gall
Nuts do not contain Gallic Acid ready formed, but an analogous chemical
principle termed _Tannic Acid_, well known for its astringent properties
and employment in the process of tanning raw hides.

Gallic Acid is produced by the _decomposition and oxidation_ of Tannic
Acid when powdered galls are exposed for a long time in a moist state to
the action of the air. By boiling the mass with water and filtering whilst
hot, the acid is extracted, and crystallizes on cooling, on account of its
sparing solubility in cold water.

Gallic Acid occurs in the form of long silky needles, soluble in 100 parts
of cold and 3 of boiling water; they are also readily soluble in Alcohol,
but sparingly in Ether. The aqueous solution becomes mouldy on keeping,
to obviate which, the addition of Acetic Acid or a drop or two of Oil of
Cloves is recommended.

Gallic Acid is a feeble acid, scarcely reddening litmus; it forms salts
with the alkaline and earthy bases, such as Potash, Lime, etc., but not
with the oxides of the noble metals. When added to Oxide of Silver the
metallic element is separated and the Oxygen absorbed.

b. _Pyrogallic Acid._--The term _pyro_ prefixed to Gallic Acid implies
that the new substance is obtained by the _action of heat_ upon that body.
At a temperature of about 410° Fahr., Gallic Acid is decomposed, and a
white sublimate forms, which condenses in lamellar crystals; this is
Pyrogallic Acid.

Pyrogallic Acid is very soluble in cold water, and in Alcohol and Ether;
the solution decomposes and becomes brown by exposure to the air. It gives
an indigo blue colour with Protosulphate of Iron, which changes to dark
green if any Persulphate be present.

Although termed an acid, this substance is strictly _neutral_; it does not
redden litmus-paper, and forms no salts. The addition of Potash or Soda
decomposes Pyrogallic Acid, at the same time increasing the attraction
for Oxygen; hence this mixture may conveniently be employed for absorbing
the Oxygen contained in atmospheric air. The compounds of Silver and Gold
are reduced by Pyrogallic Acid even more rapidly than by Gallic Acid, the
reducing agent absorbing the Oxygen, and becoming converted into Carbonic
Acid and a brown matter insoluble in water.

Commercial Pyrogallic Acid is often contaminated with empyreumatic oil,
and also with a black insoluble substance known as _Metagallic Acid_,
which is formed when the heat is raised above the proper temperature in
the process of manufacture.


CHEMISTRY OF THE PROTOSALTS OF IRON.

The combinations of Iron with Oxygen are somewhat numerous. There are two
distinct Oxides which form Salts, viz. the Protoxide of Iron, containing
an atom of Oxygen to one of metal; and the Peroxide, with an atom and a
half of Oxygen to one of metal. As _half atoms_ however are not allowed in
chemical language, it is usual to say that the Peroxide of Iron contains
three equivalents of Oxygen to two of metallic Iron.

Expressed in symbols, the composition is as follows:--

  Protoxide of Iron, Fe O.
  Peroxide of Iron, Fe{2}O{3}.

The Proto- and Persalts of Iron do not resemble each other in their
physical and chemical properties. The former are usually of an apple-green
colour, and the aqueous solutions almost colourless, if not highly
concentrated. The latter, on the other hand, are dark, and give a yellow
or even blood-red solution.

The Protosalts of Iron are alone useful in Photography; but the following
experiment will serve to illustrate the properties of both classes of
salts:--Take a crystal of Protosulphate of Iron, and, having reduced
it to powder, pour a little Nitric Acid upon it in a test-tube. On the
application of heat, abundance of fumes will be given off, and a red
solution obtained. The Nitric Acid in this reaction imparts Oxygen, and
converts the _Proto_sulphate entirely into a _Per_sulphate of Iron. It is
this feature, viz. the tendency to absorb Oxygen, and to pass into the
state of Persalts, which makes the Protosalts of Iron useful as developers.

There are two Protosalts of Iron commonly employed by Photographers: the
Protosulphate and the Protonitrate of Iron.

a. _Protosulphate of Iron._--This salt, often termed _Copperas_ or _Green
Vitriol_, is an abundant substance, and used for a variety of purposes in
the arts. Commercial Sulphate of Iron however, being prepared on a large
scale, requires re-crystallization to render it sufficiently pure for
Photographic purposes.

Pure Sulphate of Iron occurs in the form of large transparent, prismatic
crystals, of a delicate green colour: by exposure to the air they
gradually absorb Oxygen and become rusty on the surface. Solution of
Sulphate of Iron, colourless at first, afterwards changes to a red tint,
and deposits a brown powder; this powder is a _basic_ Persulphate of Iron,
that is, a Persulphate containing an excess of the oxide or _base_. By the
addition of Sulphuric or Acetic Acid to the solution, the formation of a
deposit is prevented, the brown powder being soluble in acid liquids.

The Crystals of Sulphate of Iron include a large quantity of water of
crystallization, a part of which they lose by exposure to dry air. By a
higher temperature, the salt may be rendered perfectly _anhydrous_, in
which state it forms a white powder.

b. _Protonitrate of Iron._--This salt is prepared by double decomposition
between Nitrate of Baryta or of Lead and Protosulphate of Iron. It is an
unstable substance and crystallizes with great difficulty; its aqueous
solution is pale green at first, but very prone to decomposition, even
more so than the corresponding Sulphate of Iron.


SECTION II.

_The Reduction of Salts of Silver by Developing Agents._

The general theory of the reduction of metallic oxides having been
explained, it may be desirable to enter more minutely into the exact
nature of the process as applied to the compounds of Silver.

First, the Reduction of the Oxide of Silver will be taken, as the most
simple illustration; then that of Salts of Silver formed by Oxygen-acids;
and lastly, of the Chloride, Iodide, and Bromide of Silver containing no
Oxygen.

_Reduction of Oxide of Silver._--To illustrate this conveniently, the
Oxide of Silver should be in a state of solution; water dissolves Oxide
of Silver very sparingly, but it is freely soluble in Ammonia, forming
the liquid known as Ammonio-Nitrate of Silver. If, therefore, a little of
the Ammonio-Nitrate of Silver be placed in a test-tube, and solution of
Sulphate of Iron be added to it, immediately it becomes discoloured, and a
deposit settles to the bottom.

This deposit is metallic Silver, produced by the reducing agent
appropriating to itself the Oxygen previously combined with the metal. As
metallic Silver does not dissolve in Ammonia, the liquid becomes turbid,
and the metal subsides in the form of a bulky precipitate.

_Reduction of the Oxyacid Salts of Silver._--The term _Oxyacid_ includes
those salts which contain the Oxide of Silver intimately combined with
Oxygen-acids; as _e. g._ the Nitrate of Silver, the Sulphate, the Acetate
of Silver, etc.

These salts, soluble in water, are reduced by developing agents in the
same manner as Oxide of Silver, but more slowly. The presence of an
acid united with the base is a hindrance to the process and tends to
keep the oxide in solution, especially when that acid is powerful in
its affinities. To illustrate the effect of the acid constituent of the
salt in retarding reduction, take two test-tubes, the one containing
Ammonio-Nitrate, and the other ordinary Nitrate of Silver--a single drop
of solution of Sulphate of Iron added to each will indicate an evident
difference in the rapidity of deposition.

The precipitate of metallic Silver obtained by the action of reducing
agents upon the Nitrate, varies much in colour and in general appearance.
If Gallic or Pyrogallic Acid be employed, it is a black powder;[8] whilst
the salts of Iron, and especially the same with free Nitric Acid added,
produce a sparkling precipitate, resembling what is termed _frosted
silver_. Grape Sugar and many of the essential oils, such as the Oil of
Cloves, etc., separate the metal from Ammonio-Nitrate of Silver in the
form of a brilliant mirror film, and are often employed in silvering glass.

[8] Silver precipitated by Gallic or Pyrogallic Acid does not appear to be
free from organic matter, and probably contains also a small proportion of
Oxygen.

In remarking upon these peculiarities in the molecular condition of
precipitated Silver, it should be observed that the appearance of a metal
whilst in mass is no indication of its colour when in the state of fine
powder. Platinum and Iron, both bright metals, and susceptible of a high
polish, are dull and intensely black when in a fine state of division;
Gold is of a purple or yellowish brown; Mercury a dirty grey.

_Reduction of the Hydracid Salts of Silver._--By the term _Hydracid_ is
meant Salts of Silver which contain no Oxygen or Oxygen-acids, but simply
elements like Chlorine or Iodine combined with Silver. These elements are
characterized by forming acids with Hydrogen, which acids are hence called
_Hydr_acids. Hydrochloric Acid (HCl) is an example; so also is Hydriodic
Acid (HI).

The reduction of the Hydracid Salts requires to be discussed separately,
because it is evidently different from that already described; the
reducing agent tending only to absorb _Oxygen_, which is not present in
these salts. The explanation is as follows: When a Chloride of a noble
metal is reduced by a developer, _an atom of water_, composed of Oxygen
and Hydrogen, takes a part in the reaction. The Oxygen of the water passes
to the developer, the Hydrogen to the Chlorine.

To illustrate this, take a solution of Chloride of Gold, and add to it a
little Sulphate of Iron. A yellow deposit of metallic Gold soon forms,
and the supernatant liquid is found, by testing, to be acid from free
Hydrochloric Acid. The following simple diagram, in which however the
_number_ of the atoms concerned is omitted, may assist the comprehension
of the change.

[Illustration:

  Compound Atom of   Compound Atom     Atom of
  Chloride of Gold.    of Water.    Sulphate of Iron.
]

The symbol Au represents Gold, Cl Chlorine, H Hydrogen, and O Oxygen.
Observe that the molecules H and O separate from each other and pass in
opposite directions: the latter unites with the Sulphate of Iron; the
former meets Cl, and produces Hydrochloric Acid (HCl), whilst the atom of
Gold is left alone.

Hence there is no theoretical difficulty in supposing a reduction of
Iodide of Silver by a developer, if we associate with the Iodide an atom
of water to furnish the Oxygen. Unless the sensitive plate however has
been exposed to the light, the reduction does not readily take place;
nor can it be produced under any circumstances, with or without light,
when the whole of the free Nitrate of Silver has been washed away from
the plate. Pure Iodide of Silver is therefore unaffected by a developer,
and the compound which blackens on the application of Sulphate of Iron or
Pyrogallic acid is an Iodide with excess of Nitrate of Silver.

[Illustration:

  Compound Atom of   Compound Atom of       Atom of
  Iodide of Silver.  Nitrate of Silver.  Sulphate of Iron.
]

The mode in which a Salt of Silver, such as the Nitrate, soluble in
water, may act in facilitating the reduction of Iodide of Silver, is shown
in the preceding diagram, which corresponds closely with the last.

Notice that the compound atom of Nitrate of Silver contains a molecule of
Oxygen for the developer, one of Silver (Ag) for the separated Iodine, and
an atom of Nitric Acid (NO{5}), which is liberated, and takes no further
part in the change.

The chain of chemical affinities is more complete in this diagram than
in the last, where an atom of water only was present, the affinity of
Iodine for Silver being greater than that of Iodine for Hydrogen. Hence
it is possible that an excess of Nitrate of Silver may, by furnishing an
elementary basis for which Iodine has an attraction, assist in drawing off
that element, so to speak, from the original particle of Iodide of Silver
touched by light.[9]

[9] The reader must not suppose from the remarks which have been made in
this Section that images obtained by development consist invariably of
pure metallic Silver. It can be shown that such is not the case,--that the
process of reduction is in many cases suspended when a part only of the
Oxygen has been removed; and hence results a _subsalt_ similar to that
produced by the direct action of light upon organic compounds of Silver,
and differing in properties from metallic Silver. For further particulars
see the Author's Photographic researches in the eighth Chapter.


SECTION III.

_The formation and development of the Latent Image._

It was shown in the second Chapter that the continued action of white
light upon certain of the Salts of Silver resulted in the separation
of elements like Chlorine and Oxygen and the partial reduction of the
compound. We have also seen that bodies possessing affinity for Oxygen,
such as Sulphate of Iron and Pyrogallic Acid, tend to produce a similar
effect; acting in some cases with great energy and precipitating metallic
Silver in a pure state.

In forming an extemporaneous theory on the production of the latent
image in the Camera, it would therefore be natural to suppose that the
process consisted in setting up a reducing action upon the sensitive
surface by means of light, afterwards to be continued by the application
of the developing solution. This idea is to a certain extent correct, but
it requires some explanation. The effects produced by the light and the
developer are not so precisely similar that the one agency can always
be substituted for the other: an insufficient exposure in the Camera
cannot be remedied by prolonging the development of the image. In the
Photographic processes on paper it is indeed found that a certain latitude
may be allowed; but, as a rule, it should be stated that a definite time
is occupied in the formation of the invisible image, which may not be
shortened or extended beyond its proper limits with impunity. There is a
maximum point beyond which no advance is made; hence if the plate be not
then removed from the Camera, those portions of the image formed by the
brightest lights are speedily overtaken by the "half tones," so that, on
developing, an image appears without that contrast between lights and
shadows which is essential to the artistic effect. On the other hand, in
a case of insufficient exposure, the feeble rays of light not having been
allowed time to impress the plate, the half shadows cannot be brought out
on subsequent treatment with the developing agent.

A careful study of the phenomena involved in this part of the process
cannot fail to show that the ray of Light determines a _molecular_ change
of some kind in the particles of Iodide of Silver forming the sensitive
surface. This change is not of a nature to alter the composition or the
chemical properties of the salt. The Iodine does not leave the surface,
or there would be a difference in the appearance of the film, or in its
solubility in Hyposulphite of Soda.

The following diagrams may perhaps be useful in mechanically illustrating
what is meant by a molecular change.

Fig. 1 represents a compound molecule of Iodide of Silver, the component
atoms of which are closely associated.

Fig. 2. The same after the action of a disturbing force. The simple
molecules have not altogether separated, but they are prepared to do so,
touching only at a single point.

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]

Now the effect produced on this combination by a developer is understood,
if we suppose that in the first case the affinity of the Iodine for Silver
is too great to allow of its separation; but in the second, this affinity
having been loosened, the structure gives way, and metallic Silver is the
result.

This hypothesis has the merit of simplicity, and is not opposed to known
facts; it may therefore for the present be received. The point however on
which a doubt must rest is--whether the molecular disturbance produced
by light upon Iodide of Silver leads to a reduction of that Salt by the
developer. No image can be produced on the application of Pyrogallic Acid
_unless the particles of Iodide are in contact with Nitrate of Silver;_
and hence it may be the Nitrate and not the Iodide which is reduced--that
is, the impressed molecule of Iodide may determine the decomposition of a
contiguous particle of Nitrate, itself remaining unchanged. This view is
supported to some extent by Moser's experiments, shortly to be quoted; and
also by the fact that the delicate image first formed can be _intensified_
by treating it with a mixture of the developing solution and Nitrate of
Silver, even after the Iodide has been removed by a fixing agent. The
following experiment will serve to illustrate this.--

Take a sensitive Collodion plate, and having impressed an invisible
image upon it by a proper exposure in the Camera, remove it to the dark
room, and pour over it the solution of Pyrogallic Acid. When the picture
has fully appeared, stop the action by washing the plate with water,
and remove the unaltered Iodide of Silver by Cyanide of Potassium. An
examination of the image at this stage will show that it is perfect in
the details, but pale and translucent. The plate is then to be taken back
again to the dark room and treated with fresh Pyrogallic Acid, _to which
Nitrate of Silver has_ been added; immediately the picture becomes much
blacker, and continues to darken, even to complete opacity, if the supply
of Nitrate be kept up.

Now in this experiment it is evident that the additional deposit upon the
image is produced from the Nitrate of Silver, the whole of the Iodide
having been previously removed. Observe also, _that it forms only upon
the image, and not upon the transparent parts of the plate_. Even if the
Iodide, untouched by light, be allowed to remain, the same rule holds
good;--the Pyrogallic Acid and Nitrate of Silver react upon each other and
produce a metallic deposit; this deposit however has no affinity for the
unaltered Iodide upon the part of the plate corresponding to the shadows
of the picture, but attaches itself in preference to the Iodide already
blackened by light.

This second stage of the development, by which a feeble image may be
strengthened and rendered more opaque, is sometimes termed "development
by precipitation," and should be correctly understood by the practical
operator.

_Researches of M. Moser._--The papers of M. Ludwig Moser 'On the
Formation and Development of Invisible Images,' published in 1842,
explain so clearly many remarkable phenomena of occasional occurrence in
the Collodion and paper processes, that no apology need be offered for
referring to them somewhat at length.

His first proposition may be stated thus:--"If a polished surface has
been touched in particular parts by anybody, it acquires the property of
precipitating certain vapours on these spots differently to what it does
on the other untouched parts." To illustrate this, take a thin plate of
metal, having characters _excised_; warm it gently, and lay it upon the
surface of a clean mirror glass for a few minutes: then remove, allow to
cool, and _breathe_ upon the glass, when the outlines of the device will
be distinctly seen. A plate of polished Silver may be substituted for
the glass, and in place of developing the image by the breath, it may be
brought out by Mercurial vapour.

The second proposition of M. Moser is as follows:--"_Light_ acts on
bodies, and its influence may be tested by vapours that adhere to the
substance."--A plate of mirror glass is exposed in the Camera to a bright
and intense light; it is then removed and breathed upon, when an image
before invisible will be developed, the breath settling most strongly upon
the parts where the light has acted. A plate of polished Silver may be
used as before instead of glass, the vapour of Mercury or of water being
employed to develope the image. An _iodized Silver plate_ is still more
sensitive to the influence of the light, and receives a very sharp and
perfect impression under the action of the Mercury.

It seems therefore from these experiments and others not quoted, that
the surfaces of various bodies are capable of being modified by contact
with each other, or by contact with a ray of light, in such a way as to
impart an affinity for a vapour; and further, that many of the Salts of
Silver are in the list of substances admitting of such modification. But
it is also evident that the same condition of surface which causes a
vapour to settle in a peculiar manner also affects the behaviour of the
Silver Salt when treated with a reducing agent. Thus, if a clean glass
plate be touched in certain spots by the warm finger, the impression soon
disappears, but is again seen on breathing upon the glass; and if this
same plate be coated with a very delicate layer of Iodized Collodion and
passed through the Nitrate bath, the solution of Pyrogallic acid will
commonly produce a well-defined outline of the figure even before the
plate has been exposed to the light. This experiment, although it does
not invariably succeed, is nevertheless an instructive one, and shows the
necessity of cleaning the plates used in Photography with care. If there
be any irregularity in the manner in which the breath settles upon the
glass when it is breathed on, a condition of surface exists at that point
which will probably so modify the layer of Iodide of Silver, that the
action of the developing fluid will be in some way interfered with.

One more remarkable fact observed by M. Moser may be quoted. He finds that
the action of light upon the Daguerreotype plate is of an _alternating_
kind: it first gives an affinity for Mercury, and then removes it. "If
light acts on Iodide of Silver," he says, "it imparts to it the power of
condensing mercurial vapours; but if it acts beyond a certain time, it
then diminishes this power and at length takes it away altogether." This
is precisely in accordance with phenomena observed also in the Collodion
process, where the deposit of metallic Silver is sometime less marked than
usual if the plate has been exposed in the Camera beyond the proper period
of time.

A curious perversion of the developing process is occasionally met with,
in which on the application of the Pyrogallic Acid, the deposit of Silver
takes place upon the _shadows_ of the picture, and not upon the lights;
hence on viewing the image by transmitted light, the usual appearance is
reversed. This may perhaps be explained by an alternating action of the
light as above suggested.

A phenomenon at first sight even more remarkable has occurred, in
which, on developing the plate, _two_ images start out instead of
one. The secondary image in such a case is probably the remains of a
previous impression which, although apparently removed by washing, had
nevertheless modified the surface of the glass so as to affect the layer
of Iodide of Silver; and if the glass were _breathed_ upon before again
coating it with Collodion, there is every reason to suppose that the
outlines of the accidental image would be seen.[10]

[10] Since writing the above, the Author has perused with pleasure a paper
by Mr. Grove on the production of latent images by electricity, with a
mode of fixing them. In the experiments described, a plate of glass,
electrized in certain portions only, was breathed upon, or exposed to the
fumes of Hydrofluoric Acid. In either case the vapour settled exclusively
upon the non-electrical part of the glass, thus developing a latent image.
When the plate was first submitted to electrization, and then coated
with Iodide of Silver upon Collodion, and exposed to light,--solution of
Pyrogallic Acid produced a reduction of Silver only upon the parts of the
glass corresponding to those on which the breath settled in the previous
experiment; thus indicating that the electricity neutralized the effect of
light upon the sensitive Iodide of Silver.




CHAPTER IV.

ON FIXING THE PHOTOGRAPHIC IMAGE.


A sensitive layer of Chloride or Iodide of Silver on which an
image has been formed, either with or without the aid of a developing
agent, must pass through further treatment in order to render it
indestructible by diffused light.

It is true that the image itself is sufficiently permanent, and cannot be
said, in correct language, to need _fixing_; but the unchanged Silver Salt
which surrounds it, being still sensitive to light, tends to be decomposed
in its turn, and so the picture is lost. It is therefore necessary to
remove this salt by applying some chemical agent capable of dissolving
it. The list of solvents of Chloride and Iodide of Silver has been given
in Chapter II., but some are better adapted for fixing than others. In
order that any body may be employed with success as a fixing agent, it is
required not only that it should dissolve unchanged Chloride or Iodide of
Silver, but that it should produce no injurious effect upon the same salts
reduced by light.

This _solvent action upon the image_, as well as upon the parts which
surround it, is most liable to happen when the agency of light alone,
without a developer, has been employed. In that case the darkened surface,
not being reduced perfectly to the metallic state, remains soluble to a
certain extent in the fixing liquid.


CHEMISTRY OF THE VARIOUS FIXING AGENTS.

The following will be mentioned:--Ammonia--Alkaline Chlorides--Alkaline
Iodides--Alkaline Hyposulphite--Alkaline Cyanides.


AMMONIA.

The properties of the alkaline liquid "Ammonia" are given in Part III.
Ammonia dissolves Chloride of Silver readily, but not Iodide of Silver:
hence its use is necessarily confined to the paper proofs upon Chloride
of Silver. Even these however cannot advantageously be fixed in Ammonia
unless a deposit of Gold has been previously produced upon the surface
by a process of "toning," presently to be explained: a peculiar and
unpleasant red tint is always caused by Ammonia acting upon the darkened
material of a sun picture as it comes from the printing-frame: but this is
obviated by the employment of the Gold.


ALKALINE CHLORIDES, IODIDES, AND BROMIDES.

The Chlorides of Potassium, Ammonium, and Sodium possess the property of
dissolving a small portion of Chloride of Silver. In the act of solution
a double salt is formed; that is, a compound of Chloride of Sodium with
Chloride of Silver, which may be crystallized out by allowing the liquid
to evaporate spontaneously.

The earlier Photographers employed a saturated solution of common Salt for
fixing paper prints; but the fixing action of the Alkaline Chlorides is
slow and imperfect, and their use may now be said to be obsolete.

The Iodide and Bromide of Potassium have both been used as fixing agents.
They dissolve Iodide of Silver, forming with it a double salt in the
manner before described.

It is important to remark in the solution of the insoluble Silver Salts
by Alkaline Chlorides, Iodides, etc., that the amount dissolved is not
in proportion to the _quantity_ of the solvent, but to the degree of
concentration of its aqueous solution. This is not usual with solvents
which act by entering into chemical combination with the substance
dissolved. Commonly a given weight of the one salt dissolves a given
weight of the other, independent of the amount of water present. The
peculiarity in the case before us depends upon the fact that the double
salt formed is _decomposed_ by a large quantity of water. Hence it is a
_saturated_ solution of Chloride of Sodium which possesses the greatest
power of fixing paper prints; and with the Bromide or Iodide of Potassium
the same rule holds good--the stronger the solution the more Iodide of
Silver will be taken up. The addition of water produces milkiness and a
deposit of the silver Salt previously dissolved.


ALKALINE HYPOSULPHITES.

Hyposulphurous Acid is one of the Oxides of Sulphur. It is, as its name
implies, of an acid nature, and takes its place upon the list immediately
below Sulphurous Acid ("υρο," under).

The Hyposulphite of Soda commonly employed by Photographers is a neutral
combination of Hyposulphurous Acid and the alkali Soda. It is selected as
being more economical in preparation than any other Hyposulphite adapted
for fixing.

Hyposulphite of Soda occurs in the form of large translucent groups of
crystals, which include five atoms of water. These crystals are soluble
in water almost to any extent, the solution being attended with the
production of cold; they have a nauseous and bitter taste.

In the solution of Silver compounds by Hyposulphite of Soda a _double
decomposition_ always takes place; thus:--

    Hyposulphite of Soda   + Chloride of Silver
  = Hyposulphite of Silver + Chloride of Sodium.

The Hyposulphite of Silver with an excess of Hyposulphite of Soda forms
a soluble double salt, which may be crystallized out by evaporating the
solution. It possesses an intensely sweet taste, and contains one atom of
Hyposulphite of Silver, chemically combined with two of Hyposulphite of
Soda. In addition to this there is a second double Salt, differing from
the first in being _very sparingly_ soluble in water. It is formed by
acting upon Chloride of Silver with a solution of Hyposulphite of Soda
already saturated, or nearly so, with Silver Salts; and contains single
atoms of each constituent.

The fact that the Silver contained in an ordinary fixing Bath is present
in the state of _Hyposulphite_ must be borne in mind, because this salt is
liable to undergo peculiar chemical changes, as will be better shown in
Chapter VIII.

Iodide of Silver is dissolved by Hyposulphite of Soda more slowly than
Chloride of Silver, and the amount eventually taken up is less. This is
explained as follows:-- During the solution of Iodide of Silver, _Iodide
of Sodium_ is formed, and this alkaline Iodide has a prejudicial effect
upon the continuance of the process. _Chloride_ of Sodium has not the same
action, neither has Bromide of Sodium, consequently the corresponding
Silver Salts dissolve to a greater extent than the Iodide.


ALKALINE CYANIDES.

The chemistry of Cyanogen is sketched in Part III.

The Cyanide of _Potassium_ is the salt most frequently employed in fixing.
It occurs in commerce in the form of fused lumps of considerable size. In
this state it is usually contaminated with a large percentage of Carbonate
of Potash, amounting in some cases to more than half its weight. By
boiling in proof Spirit the Cyanide may be extracted and crystallized, but
this operation is scarcely required as far as its use in Photography is
concerned.

Cyanide of Potassium absorbs moisture on exposure to the air. It is very
soluble in water, but the solution decomposes on keeping; changing in
colour and evolving the odour of _Prussic Acid_, which is a Cyanide of
Hydrogen. Cyanide of Potassium is highly poisonous, and must be used with
caution.

Solution of Cyanide of Potassium is a most energetic agent in dissolving
the insoluble Silver Salts: far more so, in proportion to the quantity
used, than the Hyposulphite of Soda. The Salts are in all cases converted
into Cyanides, and exist in the solution in the form of soluble double
Salts, which, unlike the double Iodides, are not affected by dilution with
water. Cyanide of Potassium is unadapted for fixing positive proofs upon
Chloride of Silver; and even when a developer has been used, unless the
solution is tolerably dilute, it is apt to attack the image and dissolve
it.




CHAPTER V.

ON THE NATURE AND PROPERTIES OF LIGHT.


The present Chapter is devoted to a discussion of the more remarkable
properties of Light; the object being to select certain prominent points,
and to state them as clearly as possible, referring, for information of a
more complete kind, to acknowledged works on the subject of Optics.

The Chapter will be divided into five Sections:--first, the compound
nature of Light; second, the laws of refraction of Light; third, the
construction of Lenses and of the Camera; fourth, the Photographic action
of coloured Light; fifth, on Binocular Vision and the Stereoscope.


SECTION I.

_The Compound Nature of Light._

The ideas entertained on the subject of Light, before the time of Sir
Isaac Newton, were vague and unsatisfactory. It was shown by that eminent
philosopher, that a ray of sunlight was not _homogeneous_, as had been
supposed, but consisted of several rays of vivid colours, united and
intermingled.

This fact may be demonstrated by throwing a pencil of Sunlight upon one
angle of a _prism_, and receiving the oblong image, so formed, upon a
white screen.

The space illuminated and coloured by a pencil of rays analyzed in this
way is called "the Solar Spectrum." The action of a prism in decomposing
white light will be more fully explained in the next Section. At present
we notice only that seven principal colours may be distinguished in the
Solar Spectrum, viz. red, orange, yellow, green, blue, indigo, and violet.
Sir David Brewster has made observations which lead him to suppose that
the _primary_ colours are in reality but three in number, viz. red,
yellow, and blue, and that the others are _compound_, being produced by
two or more of these overlapping each other; thus the red and yellow
spaces intermingled constitute _orange_; the yellow and blue spaces,
_green_.

[Illustration]

The composition of white light from the seven prismatic colours may be
roughly proved by painting them on the face of a wheel, and causing it to
rotate rapidly; this blends them together, and a sort of greyish-white
is the result. The white is imperfect, because the colours employed
cannot possibly be obtained of the proper tints or laid on in the exact
proportions.

The decomposition of light is effected in other ways besides that already
given:---

First, by _reflection_ form the surfaces of coloured bodies. All
substances throw off rays of light, which impinge upon the retina of the
eye and produce the phenomena of vision. Colour is caused by a _portion
only_, and not the whole, of the elementary rays, being projected in this
way. Surfaces termed _white_ reflect all the rays; coloured surfaces
absorb some and reflect others: thus _red_ substances reflect only red
rays, _yellow_ substances, yellow rays, etc, the ray which is reflected in
all cases deciding the colour of the substance.

Secondly, light may be decomposed by _transmission_ through media which
are transparent to certain rays, but opaque to others.

Ordinary transparent glass allows all the rays constituting white light
to pass; but by the addition of certain metallic oxides to it whilst in a
state of fusion, its properties are modified, and it becomes _coloured_.
Glass stained by Oxide of Cobalt is permeable only to blue rays. Oxide of
Silver imparts a pure yellow tint; Oxide of Gold or Suboxide of Copper a
ruby red, etc.


DIVISION OF THE ELEMENTARY RAYS OF WHITE LIGHT INTO LUMINOUS,
HEAT-PRODUCING, AND CHEMICAL. RAYS.

The agency of Light produces a variety of distinct effects upon the bodies
which surround us. These may be classed together as the properties of
light. They are of three kinds--the phenomena of colour and vision, of
heat, and of chemical action.

By resolving white light into its constituent rays, we find that these
properties are associated each one with certain of the elementary colours.

The _yellow_ is decidedly the most luminous ray. On examining the Solar
Spectrum, it is seen that the brightest part is that occupied by the
yellow, and that the light diminishes rapidly on either side. So again,
rooms glazed with yellow glass always appear abundantly illuminated,
whilst the effect of red or blue glass is dark and sombre. The yellow
colour therefore constitutes that portion of white light by which
surrounding objects are rendered visible; it is essentially the _visual_
ray.

The _heating properties_ of the sunlight reside principally in the red
ray, as is shown by the expansion of a mercurial thermometer placed in
that part of the spectrum.

The chemical action of light corresponds more to the indigo and violet
rays, and is wanting, as regards its influence upon Iodide of Silver, both
in the red and yellow. Strictly speaking however it cannot be localized in
either of the coloured spaces, as will be more fully shown in the Fourth
Section of this Chapter, to which the reader is referred.



SECTION II.

_The Refraction of Light._

A ray of light, in its passage through any transparent medium, travels in
a straight line as long as the density of the medium continues unchanged.
But if the density varies, becoming either greater or less, then the ray
is _refracted_, or bent out of the course which it originally pursued. The
degree to which the refraction or bending takes place depends upon the
nature of the new medium, and in particular upon its _density_ as compared
with that of the medium which the ray had previously traversed. Hence
Water refracts light more powerfully than Air, and Glass more so than
Water.

The following diagram illustrates the refraction of a ray of light.

[Illustration]

The dotted line is drawn perpendicularly to the surface, and it is
seen that the ray of light on entering is bent towards this line. On
emerging, on the other hand, it is bent to an equal extent _away from
the perpendicular_, so that it proceeds in a course parallel to, but not
coincident with, its original direction. If we suppose the new medium,
in place of being more dense than the old, to be _less dense_, then
the conditions are exactly reversed,--the ray is bent away from the
perpendicular on entering, and towards it on leaving.

It must be observed that the laws of refraction apply only to rays
of light which fall upon the medium _at an angle:_ if they enter
perpendicularly--in the direction of the dotted lines in the last
figure--they pass straight through without suffering refraction.

Notice also, that it is _at the surfaces of bodies_ that the deflecting
power acts. The ray is bent on entering, and bent again on leaving; but
whilst within the medium it continues in a straight line. Hence it is
evident that by variously modifying the surfaces of refractive media the
rays of light may be diverted almost at pleasure. This will be rendered
clear by a few simple diagrams.

In the figures given below, and in the following page, the dotted lines
represent perpendiculars to the surface at the point where the ray falls,
and it is seen that the usual law of bending _towards_ the perpendicular
on entering, and away from it on leaving the dense medium, is in each case
correctly observed.

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]

Fig. 2, termed a prism, bends the ray permanently to one side; fig. 3,
consisting of two prisms placed base to base, causes rays before parallel
to meet in a point; and conversely, fig. 4, having prisms placed edge to
edge, diverts them further asunder.

[Illustration: Fig. 3.]

[Illustration: Fig. 4.]

_The various forms of Lenses._--The phenomena of the refraction of light
are seen in the case of curved surfaces in the same manner as with those
which are plane.

Glasses ground of a curvilinear form are termed _Lenses_. The following
are examples.

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]

[Illustration: Fig. 3.]

Fig. 1 is a biconvex lens; fig. 2, a biconcave lens; and fig. 3, a
_meniscus_ lens.

As far as regards their refractive powers, such figures may be
represented, nearly, by others bound by straight lines, and thus it
becomes evident that a biconvex lens tends to condense rays of light to a
point, and a biconcave to scatter them. A meniscus combines both actions,
but the rays are eventually bent together, the convex curve of a meniscus
lens being always greater than the concave.

_The Foci of Lenses._--It has been shown that convex lenses tend to
condense rays of light and bring them together to a point. This point is
termed "the focus" of the Lens.

The following laws as regards the focus may be laid down:--

That rays of light which are pursuing a parallel course at the time they
enter the Lens are brought to a focus at a point nearer to the Lens
than diverging rays. The rays proceeding from very distant objects are
parallel; those from objects near at hand diverge. The sun's rays are
always parallel, and the divergence of the others becomes greater as the
distance from the Lens is less.

The focus of a Lens for parallel rays is termed the "principal focus,"
and is not subject to variation; this is the point referred to when the
_focal length_ of a Lens is spoken of. When the rays are not parallel, but
diverge from a point, that point is associated with the focus, and the two
are termed "conjugate foci."

[Illustration]

In the above diagram A is the principal focus, and B and C are conjugate
foci. Any object placed at B has its focus at G, and conversely when
placed at C it is in focus at B.

Therefore, although the principal focus of a Lens (as determined by the
degree of its convexity) is always the same, yet the focus for objects
near at hand varies, being longer as they are brought closer to the Lens.

_Formation of a Luminous Image by a Lens._--As the rays of light
proceeding from a point are brought to a focus by means of a Lens, so are
they when they proceed from an object, and in that case _an image of the
object_ is the result.

[Illustration]

The above figure illustrates this. The size of the image varies with the
distance of the arrow from the glass--being larger and formed at a point
further from the Lens as the object is brought nearer. The refracting
power of the Lens also influences the result--lenses of short focal
length, _i. e._ more convex, giving a smaller image.

In order that the course pursued by pencils of rays proceeding from an
object may be easily traced, the lines from the barb of the arrow in
the last figure are _dotted_. Observe that the object is necessarily
_inverted_, and also that those rays which traverse the central point of
the Lens, or the centre of the _axis_, as it is termed, are not bent away,
but pursue a course either coincident with, or parallel to, the original,
as in the case of refracting media with parallel surfaces.


SECTION III.

The Photographic Camera.

The Photographic Camera is in its essential nature an extremely simple
instrument. It consists merely of a _dark chamber_, having an aperture
in front in which a Lens is inserted. The accompanying figure shows the
simplest form of Camera.

[Illustration]

The body is represented as consisting of two portions which slide within
each other; but the same object of lengthening or shortening the focal
distance may be attained by making the Lens itself movable. A luminous
image of any object placed in front of the Camera is formed by means of
the Lens, and received upon a surface of ground glass at the back part
of the instrument. When the Camera is required for use, the object is
_focussed_ upon the ground glass, which is then removed, and a slide
containing the sensitive layer inserted in its place.

The luminous image, as formed upon the ground glass, is termed the
"Field" of the Camera; it is spoken of as being flat or curved, sharp
or indistinct, etc. These and other peculiarities which depend upon the
construction of the Lens will now be explained.

_Chromatic Aberration of Lenses._--The outside of a biconvex lens is
strictly comparable with the sharp edge of a _prism_, and therefore
necessarily produces decomposition in the white light which passes through
it.

The action of a prism in separating white light into its constituent rays
may be simply explained;--all the coloured rays are refrangible, but not
to the same extent. The indigo and violet are more so than the yellow and
red, and consequently they are separated from them, and occupy a higher
position in the Spectrum. (See the diagram at p. 47.)

A little reflection will show that in consequence of this unequal
refrangibility of the coloured rays, white light must invariably be
decomposed on entering any dense medium. This is indeed the case; but if
the surfaces of the medium _are parallel to each other_ the effect is
not seen, because the rays recombine on their emergence, being bent to
the same extent in the opposite direction. Hence light is transmitted
colourless through an ordinary pane of glass, but yields the tints of the
Spectrum in its passage through a prism or a lens, where the two surfaces
are inclined to each other at an acute angle.

Chromatic aberration is corrected by combining two lenses cut from
varieties of glass which differ in their power of separating the coloured
rays. These are the dense flint-glass containing Oxide of Lead, and the
light crown-glass. Of the two lenses, the one is _biconvex_, and the
other _biconcave_; so that when fitted together they produce a compound
Achromatic lens of a meniscus form, thus:--

[Illustration]

The first Lens in this figure is the flint- and the second the
crown-glass. Of the two the biconvex is the most powerful, so as to
overcome the other, and produce a total of refraction to the required
extent. Each of the Lenses produces a spectrum of a different length;
and the effect of passing the rays through both, is, by overlapping the
coloured spaces, to unite the complementary tints, and to form again white
light.

Spherical Aberration of Lenses.--The field of a Camera is not often
equally sharp and distinct at every part. If the centre be rendered clear
and well defined, the outside is misty; whilst, by slightly altering the
position of the ground glass, so as to define the outside portion sharply,
the centre is thrown out of focus. Opticians express this by saying that
there is a want of proper flatness of field; two causes may be mentioned
as concurring to produce it.

The first is "spherical aberration," by which is meant the property
possessed by Lenses which are segments of spheres, of refracting rays
of light unequally at different parts of their surfaces. The following
diagram shows this:--

[Illustration]

Observe that the dotted lines which fall upon the circumference of the
Lens are brought to a focus at a point nearer to the Lens than those
passing through the centre; in other words, the outside of the Lens
refracts light the most powerfully. This causes a degree of confusion and
indistinctness in the image, from various rays crossing, and interfering
with, each other.

Spherical aberration may be avoided by increasing the convexity of the
centre part of the Lens, so as to add to its refracting power at that
particular point. The surface is then no longer a segment of a sphere, but
of an ellipse, and refracts light more equally. The difficulty of grinding
Lenses to an elliptical form however is so great, that the spherical Lens
is still used, the aberration being corrected in other ways.

A second cause interfering with the distinctness of the outer portions
of the image in the Camera is the obliquity of some rays proceeding from
the object; in consequence of which the image has a curved form, with the
concavity inwards, as may be seen by referring to the figure given at page
53. The following diagram is meant to explain curvature of the image.

The centre line running at right angles to the general direction of the
Lens is the axis; an imaginary line, on which the Lens may be said to
rotate as a wheel turns on its axle. The lines A A represent rays of light
falling parallel to the axis; and the dotted lines, others which have
an oblique direction; B and C show the points at which the two foci are
formed. Observe that these points, although equidistant from the centre
of the Lens, do not fall in the same vertical plane, and therefore they
cannot both be received distinct upon the ground glass of the Camera,
which would occupy the position of the perpendicular double line in the
diagram. Hence it is that with most lenses, when the centre of the field
has been focussed, the glass must be shifted forwards a little to define
the outside sharply.

[Illustration]

_The Use of Stops in Lenses._--Curvature of the image and indistinctness
of outline from spherical aberration are both remedied to a great extent
by fixing in front of the Lens a diaphragm having a small central
aperture. The diagram gives a sectional view of a Lens with a "stop"
attached; the exact position it should occupy with reference to the Lens
is a point of importance, and influences the flatness of the field.

[Illustration]

By using a diaphragm the quantity of light admitted into the Camera
is diminished in proportion to the size of the aperture. The image is
therefore less brilliant, and a longer exposure of the sensitive plate is
required. In other respects however the result is improved; the spherical
aberration is lessened by cutting off the outside of the Lens, and a
portion of the oblique rays being intercepted, the focus of the remainder
is lengthened out, and the image is rendered flatter, and improved in
distinctness. Hence also, when a small stop is affixed to a Lens, a
variety of objects, situated at different distances, are all in focus at
once; whereas, with the full aperture of the Lens, objects near at hand
cannot be rendered distinct upon the ground glass at the same time with
distant objects, or _vice versâ_.

_The Double or Portrait Combination of Achromatic Lenses._--The brightness
of illumination of an image formed by a Lens is in proportion to the
diameter of the Lens, that is, to the size of the aperture by which the
Light is admitted. The _clearness or distinctness of outline_ however is
independent of this, being improved by using a stop, which lessens the
diameter.

The Portrait combination of Lenses is constructed to ensure rapidity of
action by admitting a large volume of light. The following diagram gives a
sectional view.

[Illustration]

In this combination the front Lens is an Achromatic plano-convex, with,
the convex side turned toward the object; and the second, which takes up
the rays and refracts them further, is a compound Biconvex Lens; there are
therefore in all four distinct glasses concerned in forming the image,
which may appear at first to be an unnecessarily complex arrangement. It
is found however that a good result cannot be secured by using a single
Lens, when a "stop" is inadmissible. By combining two glasses of different
curves, the aberrations of one correct those of the other to a great
extent, and the field is both flatter and more distinct than in the case
of an Achromatic Meniscus employed without a diaphragm.

The manufacture of Portrait Lenses is a point of great difficulty, the
glasses requiring to be ground with extreme care, in order to avoid
_distortion_ of the image: hence the most rapid Portrait Lenses, having
large aperture and short focus, are often useless unless purchased of a
good maker.

_The Variation between the Visual and Actinic Foci in Lenses._--The same
causes which produce chromatic aberration in a Lens, tend also to separate
the chemical from the visual focus.

The violet and indigo rays are more strongly bent in than the yellow, and
still more than the red; consequently the focus for each of those colours
is at a different point. The following diagram shows this.

[Illustration]

V represents the focus of the violet ray, Y of the yellow, and E of the
red.

Hence, as the chemical action corresponds more to the violet, the most
marked actinic effect would be produced at V. The luminous portion of the
spectrum however is _the yellow_, consequently the visual focus is at Y.

Photographers have long recognized this point; and therefore, with
ordinary Lenses, not corrected for colour, rules are laid down as to the
exact distance which the sensitive plate should be shifted away from the
visual focus in order to obtain the greatest amount of distinctness of
outline in the image impressed by chemical action.

These rules do not apply to the Achromatic Lenses recently described. The
coloured rays being in that case bent together again and reunited, the two
foci also nearly correspond. By a little further correction to a point
higher in the Spectrum, they are made to do so perfectly.


SECTION IV.

_On the Photographic Action of Coloured Light._

It has already been mentioned in the First Section of this Chapter that
certain of the elementary colours of white light, viz. the violet and
indigo, are peculiarly active in decomposing the Photographic Salts of
Silver; but there are some points of importance relating to the same
subject which require a further notice.

The term "actinism" (Gr. ἁκτἱς, a ray or flash) has been proposed as
convenient to designate the property possessed by light of producing
chemical change; the rays to which the effect is especially due being
known as actinic rays.

If the pure Solar Spectrum formed by prismatic analysis in the manner
represented at page 47 be allowed to impinge upon a prepared sensitive
surface of Iodide of Silver, the latent image being subsequently developed
by a reducing agent, the effect produced will be something similar to that
represented in the following diagram:--

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]

Fig. 1 shows the visible spectrum as it appears to the eye; the brightest
part being in the yellow space, and the light gradually shading off
until it ceases to be seen. Fig. 2 represents the chemical effect
produced by throwing the Spectrum upon Iodide of Silver. Observe that
the darkening characteristic of chemical action is most evident in the
upper spaces, where the light is feeble, and is altogether absent at the
point corresponding to the bright yellow spot of the visible spectrum.
The actinic and luminous spectra are therefore totally distinct from each
other, and the word "Photography," which signifies the process of taking
pictures by light, is in reality inaccurate.

To those who have not the opportunity of working with the Solar Spectrum,
the following experiments will be useful in illustrating the Photographic
value of coloured light.

_Experiment I._--Take a sheet of sensitive paper prepared with Chloride
of Silver, and lay upon it strips of blue, yellow, and red glass. On
exposure to the sun's rays for a few minutes, the part beneath the blue
glass darkens rapidly, whilst that covered by the red and yellow glass is
perfectly protected. This result is the more striking from the extreme
_transparency_ of the yellow glass, giving the idea that the Chloride
would certainly be blackened first at that point. On the other hand, the
blue glass appears very dark, and effectually conceals the tissue of the
paper from view.

_Experiment II._--Select a vase of flowers of different shades of scarlet,
blue, and yellow, and make a Photographic copy of them, by development,
upon Iodide of Silver. The blue tints will be found to act most violently
upon the sensitive compound, whilst the reds and yellows are scarcely
visible; were it not that it is difficult to procure in nature pure and
homogeneous tints, free from admixture with other colours, they would make
no impression whatever upon the plate.

In exemplifying further the importance of distinguishing between visual
and actinic rays of light, we may observe that if the two were in all
respects the same. Photography must cease to exist as an Art. It would
be impossible to make use of the more sensitive chemical preparations
from the difficulties which would attend the previous preparation and
subsequent development of the plates. These operations are now conducted
in what is termed a dark room; but it is dark only in a _Photographic_
sense, being illuminated by means of yellow light, which, whilst it
enables the operator easily to watch the progress of the work, produces
no injurious effect upon the sensitive surfaces. If the windows of the
room were glazed with _blue_ in place of yellow glass, then it would
be strictly a "dark room," but one altogether unfitted for the purpose
intended.

Another point connected with the same subject and worthy of note
is--the extent to which the sensibility of the Photographic compounds is
influenced by atmospheric conditions not visibly interfering with the
_brightness_ of the light. It is natural to suppose that those days on
which the sun's rays are the most powerful would be the best for rapid
impression, but such is not by any means the case. If the light is at all
of a yellow cast, however bright it may be, its actinic power will be
small.

It will also be often observed in working towards the evening, that a
sudden diminution of sensibility in the plates begins to be perceptible at
a time when but little difference can be detected in the brilliancy of the
light; the setting sun has sunk behind a golden cloud, and all chemical
action is soon at an end.

In the same manner is explained the difficulty of obtaining Photographs
in the glowing light of tropical climates; the superiority Of the early
months of spring over those of the midsummer; of the morning sun to that
of the afternoon, etc. April and May are usually considered the best
months for rapid impression in this country; but the light continues good
until the end of July. In August and September a longer exposure of the
plates will be required.


THE SUPERIOR SENSIBILITY OF BROMIDE OF SILVER TO COLOURED LIGHT.

In copying the Solar Spectrum alternately upon a surface of Iodide and
Bromide of Silver, we notice a difference in the Photographic properties
of these two salts. The latter is affected more extensively, to a point
lower in the spectrum, than the former. In the case of the Iodide of
Silver, the action ceases in the Blue space; but with the Bromide it
reaches to the Green. This is shown in the following diagrams, which are
drawn from the observations of Mr. Crookes ('Photographic Journal,' vol.
i. p. 100):--

[Illustration: Fig. 1. Fig. 2. Fig. 3.]

Fig. 1 represents the chemical spectrum on Bromide of Silver; fig. 2, the
same upon Iodide of Silver; and fig. 3, the visible spectrum.

It might perhaps be supposed that the superior sensibility of the Bromide
of Silver to green rays of light would render that salt useful to the
Photographer in copying landscape scenery; and indeed it is the opinion of
many that, in the _Calotype_ paper process, the dark colour of foliage is
better rendered by a mixture of Bromide and Iodide of Silver than by the
latter salt alone. This however cannot depend upon the greater sensibility
of the Bromide to coloured light, as may easily be proved.--

The diagrams given above are shaded to represent nearly, the relative
intensity of the chemical action exerted by the rays at different points
of the spectrum; and on referring to them it will be seen that the
maximum point of blackness is in the indigo and violet space, the action
being more feeble in the blue space lower down; there are also highly
refrangible rays extending upwards far beyond the visible colours, and
these invisible rays are actively concerned in the formation of the image.

It is evident therefore that the amount of effect produced by a pure
green, or even a light blue tint, upon a surface of Bromide of Silver is
very small as compared with that of an indigo or violet; and hence, as in
copying natural objects radiations of all kinds are present at the same
time, the green tints have not time to act before the image is impressed
by the more refrangible rays.

Sir John Herschel proposed to render coloured light more available in
Photography by separating the actinic rays of high refrangibility, and
working only with those which correspond to the blue and green spaces
in the spectrum. This may be done by placing in front of the Camera a
vertical glass trough containing a solution of Sulphate of Quinine.
Professor Stokes has shown that this liquid possesses curious properties.
In transmitting rays of light it _modifies_ them so that they emerge _of
lower refrangibility_, and incapable of producing the same actinic effect.
Sulphate of Quinine is, if we may use the term, _opaque_ to all actinic
rays higher than the blue-coloured space. The proposition of Sir John
Herschel above referred to was therefore to employ a bath of Sulphate of
Quinine, and having eliminated the actinic rays of high refrangibility, to
work upon Bromide of Silver with those corresponding to the lower-coloured
spaces. In this way he conceived that a more natural effect might be
obtained.

If Photographic compounds should be discovered of greater sensibility
than any we at present possess, the use of the Quinine bath will perhaps
be adopted; but at present we trust to the superior intensity of the
invisible rays for the formation of the image, and hence the employment of
Bromide of Silver is less strongly indicated.

These remarks apply to Photographs taken by sunlight. Mr. Crookes states
that in working with artificial light, such as gas or camphine, the case
is different. Actinic rays of high refrangibility are comparatively
wanting in gas-light, the great bulk of the Photographic rays beings found
to lie within the limits of the visible spectrum, and consequently acting
more energetically upon Bromide than on Iodide of Silver.

_Explanation of the mode in which Coloured Objects impress the Sensitive
Film._--The fact of which we have been speaking, viz. that the natural
colours are not always correctly represented in photography, is often
urged in depreciation of the art,--"when lights, are represented by
shadows," it is said, "how can a truthful picture be expected?" The
insensitiveness of Iodide of Silver to the colours occupying the lower
portion of the spectrum would indeed present an insuperable difficulty _if
the tints of Nature were pure and homogeneous:_ such however is not the
case. Even the most sombre colours are accompanied by scattered rays of
white light in quantity amply sufficient to affect the sensitive film.

This is especially seen when the coloured body _possesses a good
reflecting surface;_ and hence some varieties of foliage, as for instance
the Ivy, with its smooth and polished leaf, are more easily photographed
than others. So again with regard to drapery in the department of
portraiture--it is necessary to attend not only to the colour, but also to
the material of which it is composed. Silks and satins are favourable, as
reflecting much light, whilst velvets and coarse stuffs of all kinds, if
at all dark, produce very little effect upon the sensitive film.


SECTION V.

_On Binocular Vision and the Stereoscope._

An object is said to be "stereoscopic" (στρεοϛ solid, and σκοπεω, I see)
when it stands out in relief, and gives to the eye the impression of
solidity.

This subject was first explained by Professor Wheatstone in a memoir on
binocular vision, published in the 'Philosophical Transactions' for 1838;
in which he shows that solid bodies project different perspective figures
upon each retina, and that the illusion of solidity may be artificially
produced by means of the "Stereoscope."

The phenomena of binocular vision may be simply sketched as follows:--If a
cube, or a small box of an oblong form, be placed at a short distance in
front of the observer, and viewed attentively with the right and left eye
separately and in succession, it will be found that the figure perceived
in the two cases is different; that each eye sees more of one side of the
box, and less of the other; and that in neither instance is the effect
exactly the same as that given by the two eyes employed conjointly.

A silver pencil-case, or a pen-holder, may be used to illustrate the same
fact. It should be held at about six or eight inches distant from the root
of the nose, and quite at right angles to the face, so that the length of
the pencil is concealed by the point. Then, whilst it remains fixed in
this position, the left and right eye are to be alternately closed: in
each case a portion of the opposite side of the pencil will be rendered
visible.

[Illustration: Fig. 1. Fig. 2.]

The preceding diagrams exhibit the appearance of a bust as seen by each
eye successively.

Observe that the second figure, which represents the impression received
by the right eye, is more of a full face than fig. 1, which, being viewed
from a point removed a little to the left, partakes of the character of a
profile.

The human eyes are placed about 2-1/2 inches, or from that to 2-5/8
inches, asunder; hence it follows that, the points of sight being
separated, a _dissimilar_ image of a solid object is formed by each eye.
We do not however see two images, but a single one, which is stereoscopic.

In looking at a picture painted on a flat surface the case is different:
the eyes, as before, form two images, but these images are in every
respect similar; consequently the impression of solidity is wanting. A
single picture, therefore, cannot be made to appear stereoscopic. To
convey the illusion _two_ pictures must be employed, the one being a right
and the other a left perspective projection of the object. The pictures
must also be so arranged, that each is presented to its own eye, and that
the two appear to proceed from the same spot.

The reflecting stereoscope, employed to effect this, forms _luminous
images_ of the binocular pictures, and throws these images together, so
that, on looking into the instrument, only a single image is seen, in
a central position. It should, however, be understood, that no optical
arrangement of any kind is indispensably required, since it is quite
possible, with a little effort, to combine the two images by the unaided
organs of vision. The following diagram will make this obvious:--

[Illustration]

The circles A and B represent two wafers, which are stuck on paper at
a distance of about three inches from each other. They are then viewed
by _squinting_ strongly, or turning the eyes inwards towards the nose,
until the right eye looks at the left wafer, and the left eye at the right
wafer. Each wafer will then appear to become double, four images being
seen, the two central of which will gradually approach each other until
they coalesce. Stereoscopic pictures, properly arranged, may be examined
in the same manner; and it will be found that the resultant solid image is
formed midway, at a point where two lines, drawn across from the eyes to
the pictures, cut one another. The experiment here mentioned is sometimes
a painful one, and cannot easily be made if the eyes are not of equal
strength; but it will serve to show that the essential principle resides
in the binocular representation of the object, and not in the instrument
employed to view it.

In Mr. Wheatstone's reflecting Stereoscope _mirrors_ are used. The
principle of the instrument is as follows:-- objects placed in front of
a mirror have their reflected images apparently _behind_ the mirror. By
arranging two mirrors at a certain inclination to each other, the images
of the double picture may be made to approach until they coalesce, and the
eye perceives a single one only. The following diagram will explain this.

[Illustration]

The rays proceeding from the star on either side pass in the direction of
the arrows, being thrown off from the mirror (represented by the thick
black line) and entering the eyes at R and L. The reflected images appear
behind the mirror, uniting at the point A.

The reflecting Stereoscope is adapted principally for viewing large
pictures. It is a very perfect instrument, and admits of a variety of
adjustments, by which the apparent size and distance of the Stereoscopic
image may be varied almost at pleasure.

The "lenticular" Stereoscope of Sir David Brewster is a more portable form
of apparatus. A sectional view is given in the diagram.

[Illustration]

The brass tubes to which the eyes of the observer are applied contain
each a semi-lens, formed by dividing a common lens through the centre and
cutting each half into a circular form (fig. 1 in the following page). The
half-lens viewed in section (fig. 2) is therefore of a prismatic shape,
and when placed with its sharp edge as in the diagram above, alters the
direction of the rays of light proceeding from the picture, bending them
outwards or away from the centre, so that in accordance with well-known
optical laws they appear to come in the direction of the dotted lines in
the diagram (in the last page), and the two images coalesce at their point
of junction. In the instrument as it is often sold, one of the lenses is
made movable, and by turning it round with the finger and thumb it will be
seen that the positions of the images may be shifted at pleasure.

Fig. 1.

[Illustration]

Fig. 2.

_Rules for taking Binocular Photographs._--In viewing very distant objects
with the eyes, the images formed on the retinæ are not sufficiently
dissimilar to produce a very Stereoscopic effect; hence it is often
required, in taking binocular pictures, to separate the Cameras more
widely than the two eyes are separated, in order to give a sufficient
appearance of relief. Mr. Wheatstone's original directions were, to allow
about one foot of separation for each twenty-five feet of distance, but
considerable latitude may be permitted.

If the Cameras be not placed far enough apart, the dimensions of the
stereoscopic image from before backwards will be too small,--statues
looking like bas-reliefs, and the circular trunks of trees appearing oval,
with the long diameter transverse. On the other hand, when the separation
is too wide, the reverse obtains,--objects for instance which are square,
assuming an oblong shape pointing towards the observer.

To understand the cause of this, the following law in optics should be
studied:--"The distance of objects is estimated by the extent to which
the axes of the eyes must be converged to view them." If we have to turn
our eyes strongly inwards, we judge the object to be near; but if the eyes
remain nearly parallel, we suppose it to be distant.

[Illustration]

The above figures represent six-sided truncated pyramids, each with
its apex towards the observer, the centres of the two smaller interior
hexagons being more widely separated than those of the larger exterior
ones. By converging the eyes upon them so as to unite the central images
in the manner represented in page 68 a greater amount of convergence
will be required to bring together the two summits than the bases, and
hence the summits will appear the nearest to the eye; that is to say,
the resultant central figure will acquire the additional dimension of
_height_, and appear as a solid cone, standing perpendicularly upon its
base: further, the more widely the summits are separated in relation to
the bases, the taller will the cone be, although a greater effort will be
required to coalesce the figures.

Binocular Photographs taken with too much separation of the Cameras, are
distorted from a similar cause,--so strong a convergence being required to
unite them that certain parts of the picture appear to approach near to
the eye; and the depth of the solid image is increased.

This effect is most observable when the picture embraces a variety of
objects, situated in different planes. In the case of views which are
quite distant, no near objects being admitted, the Cameras may be placed
with especial reference to them, even as far as twelve feet apart, without
producing distortion.

It is sometimes observable, in looking at Stereoscopic pictures, that
they convey an erroneous impression of the real size and distance of the
object. For instance, in using the large reflecting Stereoscope, if,
when the adjustments have been made and the images properly united, the
two pictures be moved slowly forward, the eyes remaining fixed upon the
mirrors, the Stereoscopic image will gradually change its character,
the various objects it embraces appearing to become diminished in size,
and approaching near to the observer: whilst if the pictures be pushed
_backwards_, the image will enlarge and recede to a distance. So, again,
if an ordinary slide for the lenticular Stereoscope be divided in the
centre, and, looking into the instrument until the images coalesce, the
two halves be slowly separated from each other, the solid picture will
seem to become larger and to recede from the eye.

It is easy to understand the cause of this. When the pictures in the
reflecting Stereoscope are moved _forwards_, the convergence of the optic
axes is increased: the image therefore appears _nearer_, in accordance
with the last-mentioned law. But to convey the impression of nearness
is equivalent to an apparent diminution in size, for we judge of the
dimensions of a body very much in relation to its supposed distance. Of
two figures, for instance, appearing of the same height, one known to
be a hundred yards off might be considered colossal, whilst the other,
obviously near at hand, would be viewed as a statuette.

These facts, with others not mentioned, are of great interest and
importance, but their further consideration does not fall within the
bounds originally prescribed to us. The practical details of Stereoscopic
Photography have been arranged in a distinct Section, and will be found
included in the Second Part of the Work.[11]

[11] For a more full and detailed explanation of the Stereoscopic
phenomena, see an abstract of Professor Tyndall's lectures in the third
volume of the 'Photographic Journal.'




CHAPTER VI.

THE PHOTOGRAPHIC PROPERTIES OF IODIDE OF SILVER UPON COLLODION.


In the preceding part of this Work the physical and chemical properties of
Chloride and Iodide of Silver have been described, with the changes which
they experience by the action of Light. Nothing however has been said of
the surface used to support the Iodide of Silver, and to expose it in a
finely divided state to the influence of the actinic radiations. This
omission will now be supplied, and the use of Collodion will engage our
attention.

The sensibility of Iodide of Silver upon Collodion is greatly superior to
that of the same salt employed in conjunction with any other vehicle at
present known. Hence the Collodio-Iodide film will supersede the paper
and Albumen processes in all cases where objects liable to move are to be
copied. The causes of this superior sensitiveness, as far as ascertained,
may be referred to the state of _loose coagulation_ of a Collodion film
and other particulars presently to be noticed. It must however be allowed
that there are yet some points affecting the sensitiveness of Iodide of
Silver, both mechanical and chemical, of the exact nature of which we are
ignorant.

The present Chapter may be divided into four Sections:--the nature of
Collodion; the chemistry of the Nitrate Bath; the causes affecting the
formation and development of the Image upon Collodion; the various
irregularities in the development of the Image.


SECTION I.

_Collodion._

Collodion (so named from the Greek word κολλἁω, to _stick_) is a
glutinous, transparent fluid, procured, as generally said, by dissolving
Gun-Cotton in Ether. It was originally used for surgical purposes only,
being smeared over wounds and raw surfaces, to preserve them from
contact with the air by the tough film which it leaves on evaporation.
Photographers employ it to support a delicate film of Iodide of Silver
upon the surface of a smooth glass plate.

Two elements enter into the composition of Collodion: first, the
Gun-Cotton; second, the fluids used to dissolve it. Each of these will be
treated in succession.


CHEMISTRY OF PYROXYLINE.

Gun-Cotton or _Pyroxyline_ is Cotton or Paper which has been altered in
composition and properties by treatment with strong acids.

Both Cotton and Paper are, chemically, the same. They consist of fibres
which are found on analysis to have a constant composition, containing
three elementary bodies, Carbon, Hydrogen, and Oxygen, united together
in fixed proportions. To this combination the term _Lignine_ or
_Cellulose_[12] has been applied.

[12] Lignine and Cellulose are not precisely identical substances. The
latter is the material composing the cell-wall; the former, the contained
matter in the cell.

Cellulose is a definite chemical compound, in the same sense as Starch or
Sugar, and consequently, when treated with various reagents, it exhibits
properties peculiar to itself. It is insoluble in most liquids, such as
Water, Alcohol, Ether, etc., and also in dilute acids; but when acted
upon by Nitric Acid of a certain strength it liquefies and dissolves.

It has been already shown (p. 12) that when a body dissolves in Nitric
Acid the solution is not usually of the same nature as an aqueous
solution; and so in this case--the Nitric Acid imparts Oxygen first to the
Cotton, and afterwards dissolves it.

_Preparation of Pyroxyline._--If, instead of treating Cotton with Nitric
Acid, a mixture of Nitric and Sulphuric Acids in certain proportions be
used, the effect is peculiar. The fibres contract slightly, but undergo
no other visible alteration. Hence we are at first disposed to think the
mixed Acids ineffectual. This idea however is not correct, since on making
the experiment the properties of the cotton are found to be changed. Its
weight has increased by more than one-half; it has become soluble in
various liquids, such as Acetic Ether, Ether and Alcohol, etc., and, what
is more remarkable, it no longer burns in the air quietly, but explodes on
the application of flame with greater or less violence.

This change of properties clearly shows, that although the fibrous
structure of the material is unaffected, it is no longer the same
substance, and consequently chemists have assigned it a different name,
viz. Pyroxyline.

To produce the peculiar change by which Cotton is converted into
Pyroxyline, both Nitric and Sulphuric Acids are, as a rule, required; but
of the two the former is the most important. On analyzing Pyroxyline,
Nitric Acid, or a body analogous to it, is detected in considerable
quantity, but not Sulphuric Acid. The latter Acid, in fact, serves but
a temporary purpose, viz. to prevent the Nitric Acid from dissolving
the pyroxyline, which it would be liable to do if employed alone. The
Sulphuric Acid prevents the solution by removing water from the Nitric
Acid, and so producing a higher degree of concentration; Pyroxyline,
although soluble in a dilute, is not so in the strong Acid, and hence it
is preserved.

The property possessed by Oil of Vitriol of removing water from other
bodies, is one with which it is well to be acquainted. A simple experiment
will serve to illustrate it. Let a small vessel of any kind be filled to
about two-thirds with Oil of Vitriol, and set aside for a few days; at the
end of that time, and especially if the atmosphere be damp, it will have
absorbed sufficient moisture to cause it to flow over the edge.

Now even the strongest reagents employed in chemistry contain, almost
invariably, water in greater or lesser quantity. Pure Anhydrous Nitric
Acid is a white, solid substance; Hydrochloric Acid is a gas: and the
liquids sold under those names are merely solutions. The effect then of
mixing strong Oil of Vitriol with aqueous Nitric Acid is to remove water
in proportion to the amount used, and to produce a liquid containing
Nitric Acid in a high state of concentration, and Sulphuric Acid more or
less diluted. This liquid is the Nitro-Sulphuric Acid employed in the
preparation of Pyroxyline.

Various forms of Pyroxyline.--Very soon after the first announcement of
the discovery of Pyroxyline, most animated discussions arose amongst
chemists with regard to its solubility and general properties. Some
spoke of a "solution of Gun-Cotton in Ether;" whilst others denied its
solubility in that menstruum; a third class, by following the process
described, obtained a substance which was not explosive, and therefore
could scarcely be termed Gun-Cotton.

On further investigations some of these anomalies were cleared up, and
it was found that there were varieties of Pyroxyline, depending mainly
upon the degree of strength of the Nitro-Sulphuric Acid employed in the
preparation. Still the subject was obscure until the publication of
researches by Mr. E. A. Hadow. These investigations, conducted in the
Laboratory of King's College, London, were published in the Journal of the
Chemical Society. Constant reference will be made to them in the following
remarks.

We notice--first, the chemical constitution of Pyroxyline; secondly, its
varieties; and thirdly, the means adopted to procure a Nitro-Sulphuric
Acid of the proper strength.

a. _Constitution of Pyroxyline._--Pyroxyline has been sometimes spoken
of as a Salt of Nitric Acid, a Nitrate of Lignine. This view however is
erroneous, since it can be shown that the substance present is not Nitric
Acid, although analogous to it. It is the Peroxide of Nitrogen, which is
intermediate in composition between Nitrous Acid (NO{3}) and Nitric Acid
(NO{5}). Peroxide of Nitrogen (NO{4}) is a gaseous body of a dark red
colour; it possesses no acid properties, and is incapable of forming a
class of salts. In order to understand in what state this body is combined
with cotton fibre to form Pyroxyline, it will be necessary to digress for
a short time.

Law of Substitution.--By the careful study of the action of Chlorine,
and of Nitric Acid, upon various organic substances, a remarkable series
of compounds has been discovered, containing a portion of Chlorine or
of Peroxide of Nitrogen in the place of Hydrogen. The peculiarity of
these substances is, that they strongly resemble the originals in their
physical, and often in their chemical properties. It might have been
supposed that agents of such active chemical affinities as Chlorine and
Oxide of Nitrogen would, by their mere presence in a body, produce a
marked effect; yet it is not so in the case before us. The primitive type
or constitution of the substance modified remains the same, even the
crystalline form being often unaffected. It seems as if the body by which
the Hydrogen had been displaced had stepped in quietly and taken up its
position in the framework of the whole without disturbance. Many compounds
of this kind are known; they are termed by chemists "substitution
compounds." The law invariably observed is, that the substitution takes
place in equal atoms: a single atom of Chlorine, for instance, displaces
one of Hydrogen; two of Chlorine displace two of Hydrogen, and so on,
until, in some cases, the whole of the latter element is separated.

In illustration of these remarks, take the following instances:--Acetic
Acid contains Carbon, Hydrogen, and Oxygen; by the action of Chlorine
the Hydrogen may be removed in the form of Hydrochloric Acid, and an
equal number of atoms of Chlorine be substituted. In this way a new
compound is formed, termed _Chloracetic Acid_, resembling in many
important particulars the Acetic Acid itself. Notice particularly that
the peculiar properties characteristic of Chlorine are completely masked
in the substitution body, and no indication of its presence is obtained
by the usual tests! A soluble _Chloride_ gives with Nitrate of Silver a
white precipitate of Chloride of Silver, unaffected by Acids, but the
Chloracetic Acid does not; hence it is plain that the Chlorine exists in a
peculiar and Ultimate state of combination different from what is usual.

The substance we have been previously considering, viz. Pyroxyline,
affords another illustration of the Law of Substitution. Omitting, for
the sake of simplicity, the number of atoms concerned in the change,
the action of concentrated Nitric Acid upon ligneous fibre may be thus
explained:--

              { Carbon
  Cotton _or_ { Hydrogen  +  Nitric Acid
              { Hydrogen
              { Oxygen

equals

                  { Carbon
  Pyroxyline _or_ { Hydrogen          + Water
                  { Peroxide Nitrogen
                  { Oxygen

Or in symbols:--

  CH{11}O + NO{5} = C (H{n-1}NO{4}) O + HO

By a reference to the formula, it is seen that the fifth atom of Oxygen
contained in the Nitric Acid takes one of Hydrogen, and forms an atom of
Water; the NO{4} then steps in, to fill the gap which the atom of Hydrogen
has left. All this is done with so little disturbance that even the
fibrous structure of the cotton remains as before.

b. _Chemical Composition of the varieties of Pyroxyline._--Mr. Hadow has
succeeded in establishing _four_ different substitution compounds, which,
as no distinctive nomenclature has been at present proposed, may be termed
compounds A, B, C, and D.

_Compound A_ is the most explosive Gun-Cotton, and contains the largest
amount of Peroxide of Nitrogen. It dissolves _only in Acetic Ether_, and
is left on evaporation as a white powder. It is produced by the strongest
Nitro-Sulphuric Acid which can be made.

_Compounds B and C_, either separate or in a state of mixture, form the
soluble material employed by the Photographer. They both dissolve in
Acetic Ether, and also in a mixture of Ether and Alcohol. The latter,
viz. C, also dissolves in glacial Acetic Acid. They are produced by a
Nitro-Sulphuric Acid slightly weaker than that used for A, and contain a
smaller amount of Peroxide of Nitrogen.

_Compound D_ resembles what has been termed _Xyloidine_, that is, the
substance produced by acting with Nitric Acid upon Starch. It contains
less Peroxide of Nitrogen than the others, and dissolves in Ether
and Alcohol, and also in Acetic Acid. The ethereal solution leaves,
on evaporation, an opaque film, which is highly combustible, but not
explosive.

By bearing in mind the properties of these compounds, many of the
anomalies complained of in the manufacture of Gun-Cotton disappear. If the
Nitro-Sulphuric Acid employed is too strong, the product will be insoluble
in Ether; whilst if it is too weak, the fibres are gelatinized by the Acid
and partly dissolved.

c. _Means adopted to procure a Nitro-Sulphuric Acid of the requisite
strength for preparing Pyroxyline._--This is a point of more difficulty
than would at first appear. It is easy to determine an exact formula for
the mixture, but not so easy to hit upon the proper proportions of the
acids required to produce that formula; and a very slight departure from
them altogether modifies the result. The main difficulty lies in _the
uncertain strength of commercial Nitric Acid_. Oil of Vitriol is more to
be depended upon, and has a tolerably uniform Sp. Gr. of 1·836;[13] but
Nitric Acid is constantly liable to variation; hence it becomes necessary
to make a preliminary determination of its real strength, which is done
either by taking the specific gravity and referring to tables, or, better
still, by a direct analysis. As each atom of Sulphuric Acid removes only a
given quantity of water, it follows that the weaker the Nitric Acid, the
larger the amount of Sulphuric which will be required to bring it up to
the proper degree of concentration.

[13] The later experience of the writer induces him to believe, that the
specific gravity of Oil of Vitriol cannot always be taken as an indication
of its real strength; which is best ascertained by analysis.

To avoid the trouble necessarily attendant upon these preliminary
operations, many prefer to use, in place of Nitric Acid itself, one of
the salts formed by the combination of Nitric Acid with an alkaline
base. The composition of these salts, provided they are pure and nicely
crystallized, can be depended on.

Nitrate of Potash, or _Saltpetre_, contains a single atom of Nitric Acid
united with one of Potash. It is an _anhydrous_ salt, that is, it has
no water of crystallization. When strong Sulphuric Acid is poured upon
Nitrate of Potash in a state of fine powder, in virtue of its superior
chemical affinities it appropriates to itself the Alkali and liberates the
Nitric Acid. If care be taken to add a sufficient excess of the Sulphuric
Acid, a solution is obtained containing Sulphate of Potash dissolved in
Sulphuric Acid, and free Nitric Acid. The presence of the Sulphate of
Potash (or, more strictly speaking, of the _Bi_-Sulphate) does not in any
way interfere with the result, and the effect is the same as if the mixed
acids themselves had been used.

The reaction may be thus represented:--

    Nitrate of Potash _plus_ Sulphuric Acid in excess
  = Bisulphate Potash _plus_ Nitro-Sulphuric Acid.


CHEMISTRY OF THE SOLUTION OF PYROXYLINE IN ETHER AND ALCOHOL, OR
"COLLODION."

The substitution compounds B and C, already alluded to as forming the
Soluble Cotton of Photographers, are both abundantly soluble in Acetic
Ether. This liquid however is not adapted for the purpose required,
inasmuch as on evaporation it leaves the Pyroxyline in the form of a white
powder, and not as a transparent layer.

The rectified Ether of commerce has been found to answer better than any
other liquid as a solvent for Pyroxyline.

If the sp. gravity be about ·750, it contains invariably a small
proportion of _Alcohol_, which appears to be necessary; the solution not
taking place with absolutely pure Ether. The Pyroxyline, if properly
prepared, begins almost immediately to gelatinize by the action of the
Ether, and is soon completely dissolved. In this state it forms a slimy
solution, which, when poured out on a glass plate, dries up into a horny
transparent layer.

In preparing Collodion for Photographic purposes, we find that its
physical properties are liable to considerable variation. Sometimes
it appears very thin and fluid, flowing on the glass almost like
water, whilst at others it is thick and glutinous. The causes of these
differences will now engage our attention. They may be divided into two
classes: first, those relating to the Pyroxyline; second, to the solvents
employed.

a. _Variation of Properties in different Samples of soluble
Pyroxyline._--The substitution compounds A, B, C, and D differ, as already
shown, in the percentage amount of Peroxide of Nitrogen present, and the
former are more explosive and insoluble than the latter. But it often
happens in preparing Pyroxyline, that two portions of Nitro-Sulphuric
Acid taken from the same bottle yield products which vary in properties,
although they are necessarily the same in composition.

Taking _extremes_ in illustration, we notice two principal modifications
of soluble Pyroxyline.

The first, when treated with the mixture of Ether and Alcohol, sinks down
to a gummy or gelatinous mass, which gradually dissolves on agitation The
solution is very fluid in proportion to the number of grains used, and
when poured out spreads into a beautifully smooth and glassy surface,
which is quite structureless, even when highly magnified. The film adheres
tightly to the glass, and when the finger is drawn across it, separates in
short fragments, and broken pieces.

The second variety produces a Collodion which is thick and glutinous,
flowing over the glass in a slimy manner, and soon setting into numerous
small waves and cellular spaces. The film lies loose upon the glass, is
apt to contract on drying, and may be pushed off by the finger in the form
of a connected skin.

This subject is not thoroughly understood, but it is known that the
_temperature_ of the Nitro-Sulphuric Acid at the time of immersing the
Cotton influences the result. The soluble variety is produced by _hot_
acids; the second, or glutinous, by the same acids employed cold, or
only slightly warm. The best temperature appears to be from 130° to 155°
Fahrenheit; if it rises much beyond that point, the acids act upon and
dissolve the Cotton.

b. _The physical properties of Collodion affected by the proportions and
purity of the Solvents._--Pyroxyline of the varieties termed B and C
dissolves freely in a mixture of Ether and Alcohol; but the characters
of the resulting solution vary with the relative proportions of the two
solvents.

When the Ether is in large excess, the film is inclined to be strong and
tough, so that it can often be raised by one corner and lifted completely
off the plate without tearing. It is also very contractile, so that a
portion of the Collodion poured on the hand draws together and puckers the
skin as it dries. If spread upon a glass plate in the usual way, the same
property of contractility causes it to retract and separate from the sides
of the glass.

These properties, produced by Ether in large proportion, disappear
entirely on the addition of more Alcohol. The transparent layer is now
soft and easily torn, possessing but little coherency. It adheres to the
surface of the glass more firmly, and exhibits no tendency to contract and
separate from the sides.

From these remarks it will be gathered that an excess of Ether, and a
low temperature in preparing the Pyroxyline, both favour the production
of a contractile Collodion; whilst on the other hand an abundance of
Alcohol, and a hot Nitro-Sulphuric Acid, tend to produce a short and
non-contractile Collodion.

The physical properties of Collodion are affected by another cause, viz.
by the _strength_ and purity of the solvents, or, in other words, their
freedom from dilution with water. If a few drops of water be purposely
added to a sample of Collodion, the effect is seen to be to precipitate
the Pyroxyline in flakes to the bottom of the bottle. There are many
substances known in chemistry which are soluble in spirituous liquids, but
behave in the same manner as Pyroxyline in this respect.

The manner in which water gains entrance into the Photographic Collodion
is usually by the employment of Alcohol or Spirit of Wine which has not
been highly rectified. In that case the Collodion is thicker, and flows
less readily than if the Alcohol were stronger. Sometimes the texture of
the film left upon evaporation is injured; it is no longer homogeneous and
transparent, but semi-opaque, reticulated, or honeycombed, and so rotten
that a stream of water projected upon the plate washes it away.

These effects are to be attributed not to the Alcohol, but to the water
introduced with it; and the remedy will be to procure a stronger spirit,
or, if that cannot be done, to increase the amount of Ether. Collodion
prepared with a large proportion of Ether, and water, but a small quantity
of Alcohol, is often very fluid and structureless at first, adhering to
the glass with some tenacity and having a short texture; but it tends to
become rotten when used to coat many plates successively, the water on
account of its lesser volatility accumulating in injurious quantity in the
last portions.


THE COLORATION OF IODIZED COLLODION EXPLAINED.

Collodion iodized with the Iodides of Potassium, Ammonium, or Zinc,
soon assumes a yellow tint, which in the course of a few days or weeks,
according to the temperature of the atmosphere, deepens to a full brown.
This gradual coloration, due to a development of Iodine, is caused partly
by the Ether and partly by the Pyroxyline.

Ether may, with proper precautions, be preserved for a long time in a
pure state, but on exposure to the joint action of air and light it
undergoes a slow process of oxidation, attended with formation of Acetic
Acid and a peculiar principle resembling in properties ozone, or Oxygen
in an allotropic and active condition. Iodide of Potassium or Ammonium is
decomposed by Ether in this state. Acetate of the Alkali, and Hydriodic
Acid (HI), being first produced. The ozonized substance then removes
Hydrogen from the latter compound, and liberates Iodine, which dissolves
and tinges the liquid yellow.

A simple solution of an Alkaline Iodide in Alcohol and Ether does not,
however, become so quickly coloured as Iodized Collodion; and hence it is
evident that the presence of the Pyroxyline produces an effect. It may
be shown that Alkaline Iodides slowly decompose Pyroxyline, and that a
portion of Peroxide of Nitrogen is set free: this body, containing loosely
combined oxygen, tends powerfully to eliminate Iodine, as may be seen by
adding a few drops of the yellow commercial Nitrous acid to a solution of
Iodide of Potassium.

The _stability_ of the particular Iodide used in Iodizing Collodion,
influences mainly the rate of coloration, though elevation of temperature
and exposure to light are not without effect. Iodide of Ammonium is the
least stable, and Iodide of Cadmium the most so; Iodide of Potassium being
intermediate. Collodion iodized with _pure_ Iodide of Cadmium usually
remains nearly colourless to the last drop, if kept in a cool and dark
place.

As the presence of free Iodine in Collodion affects its photographic
properties, it may sometimes be necessary to remove it. This is done by
inserting a strip of Silver-foil; which decolorizes the liquid, by forming
Iodide of Silver, soluble in the excess of Alkaline Iodide (p. 42).
Metallic Cadmium, and metallic Zinc, have the same effect.

When Methylated Spirits are employed in the manufacture of Collodion,
the Iodine first liberated is afterwards either partially or entirely
reabsorbed, the liquid acquiring at the same time an acid reaction to
test-paper.


SECTION II.

_The Chemistry of the Nitrate Bath._

The solution of Nitrate of Silver in which the plate coated with iodized
Collodion is dipped, to form the layer of Iodide of Silver, is known
technically as _the Nitrate Bath_. The chemistry of Nitrate of Silver
has been explained at page 13, but there are some points relating to the
properties of its aqueous solution which require a further notice.

_Solubility of Iodide of Silver in the Nitrate Bath._--Aqueous solution of
Nitrate of Silver may be mentioned in the list of solvents of Iodide of
Silver. The proportion dissolved is in all cases small, but it increases
with the _strength_ of the solution. If no attention were paid to this
point, and the precaution of previously saturating the Nitrate Bath with
Iodide of Silver neglected, the film would be dissolved when left too long
in the liquid.

This solvent power of Nitrate of Silver on the Iodide is well shown by
taking the excited Collodion plate out of the Bath, and allowing it to dry
spontaneously. The layer of Nitrate on the surface, becoming concentrated
by evaporation, eats away the film, so as to produce a transparent,
spotted appearance.

In the solution of Iodide of Silver by Nitrate of Silver a _double
salt_ is formed, which corresponds in properties to the double Iodide
of Potassium and Silver in being _decomposed_ by the addition of water.
Consequently, in order to saturate a Bath with Iodide of Silver it is
only necessary to dissolve the total weight of Nitrate of Silver in a
small bulk of water, and to add to it a few grains of an Iodide; perfect
solution takes place, and on subsequent dilution with the full amount of
water, the excess of Iodide of Silver is precipitated in the form of a
milky deposit.

_Acid condition of Nitrate of Silver._--A solution of _pure Nitrate of
Silver_ is neutral to blue litmus-paper, but that prepared from the
commercial Nitrate has usually an acid reaction; the crystals having been
imperfectly drained from the acid mother-liquor in which they were formed.
Hence, in making a new Bath it is often advisable not only to saturate it
with Iodide of Silver, but to neutralize the free Nitric acid it contains.

There is also a peculiar condition of Nitrate of Silver crystallized from
a solution of the metal in Nitric Acid, which renders it quite unfit for
photographic purposes (see p. 101). It is thought to depend upon the
presence of an oxide of Nitrogen, possibly of Nitrous Acid, and the remedy
is to dry the crystals very strongly, or, better still, to fuse them at a
moderate heat: mere neutralization with Carbonate of Soda does not suffice.

In melting Nitrate of Silver great care should be taken not to raise the
heat so high as to decompose the salt, or a basic Nitrite will be formed,
which affects the properties of the solution (p. 13): fused Nitrate of
Silver ought, when cold, to be quite white, and to dissolve perfectly in
water without leaving any residue. The only objection to the employment
of Nitrate of Silver in this form is the facility with which it may be
adulterated with Nitrates of Potash and Soda, the presence of which would
lessen the available strength of the Bath.

The Nitrate Bath, although perfectly neutral when first prepared, may
become _acid_ by continued use, if Collodion containing much _free
Iodine_ be constantly employed. In that case a portion of Nitric Acid is
liberated, thus:--

    Nitrate of Silver + Iodine
  = Iodide of Silver  + Nitric Acid + Oxygen.

When Collodion is iodized entirely with alkaline Iodides, it liberates
Iodine by keeping; and hence the occasional use of Ammonia may be required
to remove acidity from the Bath. But since the introduction of the Iodide
of Cadmium, which preserves the Collodion nearly or quite colourless, the
necessity for neutralizing Nitric Acid in the Bath has ceased.

_Alkaline condition of the Bath._--By "alkalinity" of the Bath is meant
a condition in which the blue tint is rapidly restored to reddened
litmus-paper. This indicates that an Oxide of some kind is present in
solution, which, by combining with the acid in the reddened paper,
neutralizes it and removes the red colour.

If a small portion of caustic Potash or Ammonia be added to a strong
solution of Nitrate of Silver, it produces a brown precipitate, which is
Oxide of Silver.

    Ammonia + Nitrate of Silver
  = Oxide of Silver + Nitrate Ammonia.

The solution however, from which the precipitate has separated, is not
left in a neutral state, but possesses a faint alkaline reaction. Oxide
of Silver and Carbonate of Silver are also _abundantly_ soluble in water
containing Nitrate of Ammonia; which salt is continually accumulating in
the Bath when compounds of Ammonium are used for iodizing.

An alkaline Bath is perhaps of all conditions the one most fatal to
success in photography. It leads to that universal darkening of the film
on applying the developer to which the name of "fogging" has been given.
Hence care must be used in adding to the Bath substances which tend to
make it alkaline.

Collodion containing free Ammonia, often sold in the shops, gradually does
so. The use of Potash, Carbonate of Soda, Chalk, or Marble, to remove free
Nitric Acid from the Bath, has the same effect; and hence, when they are
employed, a trace of Acetic acid must afterwards be added.

The mode of testing a bath for alkalinity is as follows:-- a strip of
porous blue litmus-paper is taken and held to the mouth of a bottle of
glacial Acetic acid until it becomes reddened; it is then placed in the
liquid to be examined and left for ten minutes or a quarter of an hour. If
Oxide of Silver be present in solution, the original blue colour of the
paper will slowly but gradually be restored.

_Occasional formation of Acetate of Silver in the Nitrate Bath._--In
preparing a new Bath, if the crystals of Nitrate of Silver are acid, it is
usual to add an alkali in small quantity. This removes the Nitric Acid,
but leaves the solution faintly alkaline. Acetic Acid is then dropped in,
which, by combining with the Oxide of Silver, forms Acetate of Silver.

Acetate of Silver is not formed by the simple addition of Acetic Acid to
the Bath, because its production under such circumstances would imply the
liberation of Nitric Acid; but if an alkali be present to neutralize the
Nitric Acid, then the double decomposition takes place, thus--

    Acetate of Ammonia + Nitrate of Silver
  = Acetate of Silver  + Nitrate of Ammonia.

Acetate of Silver is a white flaky salt, sparingly soluble in water. It
dissolves in the Bath only in small proportion, but yet sufficiently to
affect the Photographic properties of the film (see p. 111 and 117). The
observance of the following simple rules will obviate its production in
injurious quantity:--_First_, when it is required to remove free Nitric
Acid from a bath _not containing Acetic Acid_ a solution of Potash or
Carbonate of Soda may be dropped in _freely_; but the liquid must be
filtered before adding any Acetic Acid, otherwise the brown deposit of
Oxide of Silver will be taken up by the Acetic Acid, and the Bath will
be charged with Acetate of Silver. _Secondly_, in dealing with a Bath
containing both Nitric and Acetic Acids, employ an alkali _much diluted_
(Liquor Ammoniæ with 10 parts of Water), and add a single drop at a
time, coating and trying a plate between each addition; the Nitric Acid
will neutralize itself before the Acetic, and with care there will be no
formation of Acetate of Silver in quantity.

_Substances which decompose the Nitrate Bath._--Most of the common
metals, having superior affinity for Oxygen, separate the Silver from a
solution of the Nitrate; hence the Bath must be kept in glass, porcelain,
or gutta-percha, and contact with Iron, Copper, Mercury, etc., must
be avoided, or the liquid will be discoloured, and a black deposit of
metallic Silver precipitated.

All developing agents, such as Gallic and Pyrogallic Acids, the Protosalts
of Iron, etc., blacken the Nitrate Bath, and render it useless by reducing
metallic Silver.

Chlorides, Iodides, and Bromides produce a deposit in the Bath; but the
solution, although weakened, may again be used after passing through a
filter.

Hyposulphites, Cyanides, and all fixing agents decompose Nitrate of Silver.

Organic matters, generally, reduce Nitrate of Silver, either with or
without the aid of light. Grape Sugar, Albumen, Serum of Milk containing
caseine, etc., blacken the Bath, even in the dark. Alcohol and Ether
act more slowly, and produce no injurious effect unless the liquid is
constantly exposed to light.

These facts indicate that the Nitrate Bath containing volatile organic
matters must be preserved in a dark place; also that it should be kept
exclusively for sensitizing the Collodion plates, and not used in floating
papers intended for the printing process.

_Changes in the Nitrate Bath by use._--The solution of Nitrate of Silver
employed in exciting the Collodion film gradually decreases in strength,
but not so quickly as the Bath used in sensitizing papers for printing. If
the amount of Nitrate be allowed to fall as low as twenty grains to the
ounce of water, the decomposition will be imperfect, and the film will be
pale and blue, even with a highly iodized Collodion.

A gradual accumulation of Ether and Alcohol also takes place in the Bath
after long use, in consequence of which the developing solutions flow
less readily upon the Collodionized plates, and oily stains are apt to be
produced.

Diminished sensitiveness of the Iodide film is sometimes traced to
impurities in the Bath, when it is very old, and has been much used. These
are probably of an organic nature and may often be partially removed
by agitation with kaolin, or animal charcoal. The latter however is
objectionable, being usually contaminated with _Carbonate of Lime_, which
makes the Bath alkaline; or (in the case of _purified_ animal charcoal)
with traces of Hydrochloric Acid, which liberate Nitric Acid in the Bath.
Even the kaolin may as a preliminary precaution be washed with dilute
Acetic Acid to remove Carbonate of Lime if any should be present.


SECTION III.

_The Conditions which affect the Formation and Development of the Latent
Image in the Collodion process._

It will be necessary to preface the observations contained in this Section
by defining two terms which are frequently confounded with each other, but
are in reality of distinct meaning. These terms are "Sensitiveness" and
"Intensity."

By Sensitiveness is meant a facility of receiving impression from very
feeble rays of light, or of receiving it quickly from brighter rays.

Intensity, on the other hand, relates to the appearance of the finished
Photograph, independently of the time taken to produce it,--_to the degree
of opacity of the image_, and the extent to which it obstructs transmitted
light.

It will be seen as we proceed that the conditions necessary to obtain
extreme sensitiveness of the Iodide film are different from, and often
opposed to, those which give the maximum intensity of image.


CAUSES WHICH INFLUENCE THE SENSITIVENESS OF IODIDE OF SILVER ON COLLODION.

Some of the most important are as follows:--

a. _The presence of free Nitrate of Silver._--When the sensitive film is
removed from the Nitrate Bath, the Iodide of Silver is left in contact
with excess of Nitrate of Silver. The presence of this compound is not
_essential_ to the action of the light, since, if it be removed by washing
in distilled water, the image may still be impressed. In such a case
however the effect is produced slowly, and a longer exposure in the Camera
is required.

The sensitiveness of the Iodide film does not increase uniformly with the
amount of the excess of Nitrate of Silver, as measured by the strength of
the Bath. It is found that no advantage in this respect can be gained by
using a proportion of Nitrate of Silver greater than 30 or 35 grains to
the ounce of water, although solutions of three times this strength have
been sometimes employed.

It has been asserted that a chemically pure Iodide of Silver, which is
unaffected in colour by the direct action of light, is also incapable of
receiving the invisible image in the Camera; and that the sensitiveness of
a washed Collodion film is due to a minute quantity of Nitrate of Silver
still remaining. Iodide of Silver in the state in which it is thrown down
on diluting with water a strong solution of the salt known as the double
Iodide of Potassium and Silver,--and which must, from the mode of its
preparation, be free from Nitrate of Silver,--is quite insensitive; but
this form of Iodide differs from the other in colour, and not only so, but
is likely to contain an excess of Iodide of Potassium. The application
of a solution of Nitrate of Silver to this compound at once renders it
sensitive to light.

b. _Free acids in the Nitrate Bath._--Strong oxidizing agents, such as
Nitric Acid, greatly diminish the sensibility of the film, and hence the
importance of removing the free acid often met with in commercial samples
of the Nitrate of Silver. The effect of even a single drop of strong
Nitric Acid in an eight-ounce Nitrate Bath will be appreciable; and when
the proportion is increased to one drop per ounce, it will be difficult to
obtain a rapid impression.

Acetic Acid has far less effect upon the sensitiveness than Nitric
Acid, and being found useful during the development of the image is
commonly employed; but when great rapidity is desired, it should be added
cautiously, and in a proportion very much less than that in the solution
known as the Aceto-Nitrate of Silver, which contains about one drop of the
glacial acid to each grain of Nitrate of Silver.

c. _Addition of certain organic matters._--It has long been remarked that
the use of bodies like Albumen, Gelatine, Caseine, etc., which combine
with Oxides of Silver, retard the action of light upon Iodide of Silver;
and the recent observations of the Author enable him to confirm this
statement. It is probable that one cause, amongst others, of the great
sensibility of the Collodion film is due to the fact that Pyroxyline is
a substance peculiarly indifferent to the Salts of Silver, exhibiting
no tendency to reduce them to the metallic state; and it is proved by
experiment that the addition of Grape Sugar, or of the resinous body,
Glycyrrhizine, which resembles Albumen in causing a white precipitate in
strong solution of Nitrate of Silver, renders necessary a longer exposure
in the Camera. Alkaline Citrates have a still more marked effect, as also
have Tartrates, Oxalates, etc.

d. _Impurities in the soluble Iodides._--Commercial Iodide of Potassium
often contains _Iodate_ of Potash, which is found to have a retarding
effect upon the action of light; also Carbonate of Potash, which, in
Collodion, produces Iodoform,[14] and in the paper processes, where
"Aceto-Nitrate" is used for sensitizing, forms Acetate of Silver. Iodoform
has a marked influence in diminishing the sensitiveness of Iodide of
Silver; Acetate of Silver may perhaps increase it a little by securing the
absence of free Nitric Acid (p. 117). Iodide of Potassium prepared by the
process in which Sulphuretted Hydrogen and Alcohol are used, and having
a smell of Garlic, contains probably Xanthate of Potash, and is nearly
useless for Photography.

[14] See the Vocabulary, Part III., Art. Iodoform.

Commercial Iodide of Cadmium is a purer salt than the Iodide of
Potassium, and may be advantageously substituted for it; but it possesses
the property of coagulating Albumen, and hence cannot be employed in
conjunction with that substance.

e. _Presence of free Iodine._--Both in the waxed paper and the Collodion
processes, the solutions often contain a small quantity of free Iodine.
This Iodine, in contact with the Nitrate of Silver of the Bath, produces
a mixed Iodide and _Iodate_ of Silver, and liberates Nitric Acid. It thus
retards the sensitiveness of the film in proportion to the quantity of
Iodine present. Collodion of a full yellow colour is perceptibly less
sensitive than the same rendered colourless; and when enough Iodine has
been liberated to give a red or brown tint, double the original exposure
will probably be required.

If brown Collodion be much used, the Nitrate Bath may by degrees become
sufficiently contaminated with free Nitric Acid to interfere with the
sensitiveness of the film; but if colourless or lemon-yellow tinted
Collodion be employed, this evil need not be anticipated.

Certain substances may be added to coloured Collodion which possess the
property of counteracting the retarding influence of the free Iodine,
such, for instance, as the Oils of Cloves, Cinnamon, etc.; they probably
act in virtue of their affinity for Oxygen, by preventing the formation
of Iodate of Silver. In colourless Collodion they produce little or no
effect, neither do they remove the insensitiveness of the film when
dependent upon a too acid condition of the Nitrate Bath.

f. _Addition of Bromide or Chloride to Collodion._--In the Daguerreotype
a very exalted state of sensibility is obtained by exposing the silvered
plate first to the vapour of Iodine, and afterwards to that of Bromine or
Chlorine; but this rule does not apply to the Collodion process, which
differs essentially in principle. Soluble Bromides added to Collodion
lessen its sensibility to an appreciable extent, as also do Chlorides.
This rule however may perhaps be liable to an exception when artificial
light is used, which contains a greater proportion of the rays of small
refrangibility, known to act more powerfully upon the Bromide than upon
the Iodide of Silver (p. 66).

g. _Density of the sensitive film._--When the proportion of soluble Iodide
in the iodizing solution is too great, the film is very dense, and the
Iodide of Silver is apt to burst out upon the surface, and fall away in
loose flakes into the Bath. This condition, which is highly unfavourable
to sensitiveness, is very common in Collodion, and constitutes what is
termed "over-iodizing." The Iodide, in fact, is formed in such a case too
much upon the surface, and consequently, when the fixing agent is applied,
the image not being retained by the film, is washed off and lost.

On the other hand, the sensibility of the film is not lessened by reducing
the amount of Iodide in Collodion to a minimum, if all the solutions are
neutral; but the pale blue films formed by a dilute Collodion, and which
almost rival the Daguerreotype itself in delicacy, are nearly useless in
practice; for if free Iodine or other bodies of a retarding nature are
present in any quantity, either in the Collodion or in the Bath, they
almost destroy the action of a weak light, producing a far more injurious
effect than if the film were more yellow and opaque.

h. _Impurities in Ether and Alcohol._--Pure Ether should be neutral to
test-paper, but the commercial samples of this article have usually either
an acid or an alkaline reaction. The frequent occurrence of a peculiar
oxidizing principle in Ether has also been pointed out (p. 85). Each of
these three conditions is injurious to sensitiveness; the first and last
by liberating Iodine when alkaline Iodides are used; and the second,
by producing Iodoform under the same circumstances. In this case the
Collodion remains colourless, but gives inferior results.

The Author has also observed that Ether which has been redistilled from
the residues of Collodion may contain a volatile principle (probably a
compound Ether?) which produces a retarding effect upon the action of
light.

Commercial Spirit of Wine is not always uniform in composition, as
sufficiently evidenced by the test of smell. It may contain "fusel oil" or
other volatile substances, which become milky on dilution with water, and
are believed to injure the quality of the spirit for Photographic use.

i. _Relative proportions of Ether and Alcohol in Collodion._--It was
shown at p. 84 that the addition of Alcohol to Collodion lessens the
contractility of the film, and renders it soft and gelatinous. This
condition is favourable to the formation of the invisible image in the
Camera, the play of affinities being promoted by the loose manner in which
the particles of Iodide are held together. It is therefore usual to add to
Collodion as much Alcohol as it will bear without becoming glutinous, or
leaving the glass; the exact quantity required varying with the strength
of the spirit or its freedom from dilution with water.

k. _Decomposition in the Collodion._--Collodion iodized with the metallic
Iodides generally, excepting the Iodide of Cadmium, becomes brown and
loses sensitiveness in the course of a few days or weeks. If the free
Iodine, the cause of the brown colour, be removed, the greater part,
but not the whole, of the sensitiveness is regained. The experiments of
the Author, and of others, have proved that a solution of Pyroxyline in
contact with an unstable iodide, slowly undergoes decomposition, the
result of which is that Iodine is set free, and an equivalent quantity of
the base remains in union with certain organic elements of the Collodion.

Decomposition also gradually ensues when iodized Collodion is placed in
contact with reducing agents, such as Proto-iodide of Iron, Gallic Acid,
Grape Sugar, Glycyrrhizine, etc., so that these combinations do not retain
a constant sensibility for any length of time. Even plain Collodion
uniodized cannot be preserved many months without a small but perceptible
amount of change.

l. _Decomposition in the Nitrate Bath._--A Collodion Nitrate Bath which
has been much used, often gives a less sensitive film than when newly
made. It is known also that many organic substances which reduce Nitrate
of Silver, if added to the Bath, produce a state which is favourable to
sensitiveness whilst the decomposition is taking place, but is eventually
unfavourable; hence the solution will be injured by adding either Gallic
or Pyrogallic Acid, and by organic matters generally if exposed to light.

_Recapitulation._--The conditions most favourable to extreme sensitiveness
of the Iodide of Silver on Collodion may be condensed as follows:--perfect
neutrality of the solutions employed; a soft, gelatinous state of the
film; absence of Chlorides and other salts which precipitate Nitrate of
Silver; an undecomposed Collodion, containing no organic matter of that
kind which is precipitated by basic Acetate of Lead, and combines with
oxides of Silver.


THE CONDITIONS WHICH AFFECT THE DEVELOPMENT OF THE LATENT IMAGE.

The general theory of the development of a latent image by means of a
reducing agent, having been simply explained in the third Chapter, may
now be more fully examined in its application to the Iodide of Silver on
Collodion.

a. _The presence of free Nitrate of Silver essential to the
development._--This subject has already been mentioned (p. 36). A
sensitive Collodion plate, carefully washed in distilled water, is still
capable of receiving the radiant impression in the Camera, but it does not
admit of development until it has been redipped in the Bath, or treated
with a reducing agent to which Nitrate of Silver has been added: and if
the proportion of free Nitrate of Silver on a Collodion film be too small,
the image will be feeble or altogether imperfect in parts, with patches of
green or blue, due to deficient reduction.

b. _Comparative strength of deducing Agents._--No increase of power in a
developer will suffice to bring out a perfect image on an under-exposed
plate, or upon a film containing too little Nitrate of Silver. But there
is considerable difference in the length of time which the various
developers require to act. Gallic Acid is the most feeble, and Pyrogallic
Acid the strongest, producing at least four times more effect than an
equal weight of the crystallized Protosulphate of Iron, and twenty times
more than the Protonitrate of Iron.

c. _The effect of free Acid upon the development._--Acids tend to retard
the reduction of the image as well as to diminish the sensibility of the
film to light. Nitric Acid especially does so, from its powerful oxidizing
and solvent properties. The effect of Nitric Acid is particularly seen
when the film of Iodide of Silver is very blue and transparent, and the
quantity of Nitrate of Silver retained upon its surface small. Under such
circumstances the proper development of the image may be suspended, and
spangles of metallic Silver separate. This indicates that the quantity of
the acid should be diminished, or the strength of the Nitrate Bath and of
the reducing agent be increased, as a counterpoise to the retarding action
of acid upon the development.

Acetic Acid also moderates the rapidity of development, but it has not
that tendency altogether to suspend it possessed by Nitric Acid. It is
therefore usefully employed, to enable the operator to cover the plate
evenly with liquid before the development commences, and to preserve the
white parts of the impression from any accidental deposit of metallic
Silver due to irregular action of the reducing agent.

On comparing the retarding effects of free acid upon the light's action,
and upon the development, we see that the former is the most marked,--that
a small quantity of Nitric Acid produces a more decided influence upon the
impression of the image in the Camera than upon the bringing out of that
image by means of a developer.

d. _Accelerating effect of certain organic matters._--Organic bodies,
like Albumen, Gelatine, Glycyrrhizine, etc., which combine chemically
with oxides of Silver, and were shown in the last Section to lessen the
sensitiveness of the Iodide film,--facilitate the development of the
image, producing often a dense deposit of a brown or black colour by
transmitted light.

In the same way, viz. by a retention of organic matter, may partly be
explained the fact, that the image developed by Pyrogallic Acid, although
proved by the application of tests to contain no more than an equal
quantity of Silver, possesses greater opacity by transmitted light, than
that resulting from the use of protosalts of Iron: and in the case of
the Collodion itself the same rule applied--if it be pure, it is liable
to give a less vigorous impression than when by long keeping a partial
decomposition has taken place, and products have been formed which
combine with reduced Oxide of Silver more easily than the unaltered
Pyroxyline.

e. _Molecular conditions affecting Intensity._--The physical structure
of the Collodion film is thought to exert an influence upon the mode in
which the reduced Silver is thrown down during the development. A short
and almost powdery state, such as Collodion iodized with the alkaline
iodides acquires by keeping, is considered favourable, and a glutinous,
coherent structure unfavourable, to density. This is certainly the case
when the film is allowed to dry before development, as in the process
with desiccated Collodion and, to some extent, in the Oxymel preservative
process.

The mode of conducting the development also affects the density; a rapid
action tending to produce an image of which the particles are finely
divided and offer a considerable resistance to the passage of light,
whilst a slow and prolonged development often leaves a metallic and almost
crystalline deposit, comparatively translucent and feeble.

The writer has observed, that with certain samples of Collodion the
image is much enfeebled by keeping the plate for a considerable time,--a
quarter of an hour or longer,--after sensitizing, but before development.
This effect is not the result of the Nitrate of Silver having partially
drained away, since a second dip in the Nitrate Bath immediately before
applying the Pyrogallic Acid, does not remedy it. An alteration of
molecular structure may therefore be the correct explanation, and if so, a
contractile Collodion would suffer more than one possessing less coherency.

The actinic power of the light at the time of taking the picture,
influences the appearance of the developed image; the most vigorous
impressions being produced by a strong light acting for a short time. On a
dull dark day, or in copying badly lighted interiors, the photograph will
often lack bloom and richness, and be blue and inky by transmitted light.

f. _Development of images upon Bromide and Chloride of Silver._--Of the
three principal Salts of Silver, the Iodide is the most sensitive to
light, but the Bromide and Chloride, under some conditions, are more
easily developed and give a darker image. In the Collodion process the
difference is principally seen when organic bodies, like Grape Sugar,
Glycyrrhizine, etc., are introduced in order to increase the intensity; a
far more decided effect being produced by adding both Glycyrrhizine and a
portion of Bromide or Chloride, than by using the Glycyrrhizine alone.[15]

[15] See the Author's Paper on the chemical composition of the
photographic image, in the eighth Chapter.

g. _The intensity of the image affected by the length of exposure._--This
point has been briefly alluded to in the third Chapter. If the exposure in
the Camera be prolonged beyond the proper time the development takes place
rapidly but without any intensity, the picture being pale and translucent.
The effects produced by over-action of the light are particularly seen
when the Nitrate Bath contains Nitrite of Silver, or Acetate of Silver;
the image being frequently in such a case dark by reflected light, and
red by transmitted light,--more nearly resembling in fact a photographic
print, developed on paper prepared with Chloride of Silver. When Collodion
plates are coated with honey without previously removing the free Nitrate
of Silver, a slow reducing action is set up, which may give rise to the
characteristic appearance above referred to, after development. Other
organic substances, such as biliary matter, etc., will act in the same way.

h. _Certain conditions of the Bath which affect development._--Attention
may be called to a peculiar state of the Nitrate Bath, in which the
Collodion image developes unusually slowly, and has a dull grey metallic
appearance, with an absence of intensity in the parts most acted on by
the light. This condition, which occurs only when using a newly mixed
solution, is thought by the Author to depend upon the presence of an
Oxide of Nitrogen retained by the Nitrate of Silver. It is removed
partially by neutralizing the Bath with an alkali, more perfectly so by
adding an excess of alkali followed by Acetic Acid; but most completely by
carefully _fusing_ the Nitrate of Silver before dissolving it.

Commercial Nitrate of Silver has sometimes a fragrant smell, similar to
that produced by pouring strong Nitric Acid upon Alcohol. When such is the
case, it contains organic matter, and produces a Bath which yields red and
misty pictures.

Nitrate of Silver which has been sufficiently strongly fused to decompose
the Salt, and produce a portion of the basic Nitrite of Silver exhibits
great peculiarity of development, the image coming out instantaneously and
with great force. This condition is exactly the reverse of that produced
by the presence of acids, in which the development is slow and gradual.

In summing up the different conditions of the Nitrate Bath which affect
the development of the image, as many as _four_ might be mentioned, each
of which gives a more rapid reduction than the one which precedes it.
These are--the acid Nitrate Bath, the neutral Bath, the Bath of strongly
fused Nitrate of Silver, and the Bath containing _Ammoniacal_ Nitrate of
Silver, which is quite unmanageable, and produces an instantaneous and
universal blackening of the film on the application of the developer.

Greater intensity of image is commonly obtained in a Nitrate Bath which
has been a long time in use, than in a newly mixed solution: this may be
due to minute quantities of organic matter dissolved out of the Collodion
film, which, having an affinity for Oxygen, partially reduce the Nitrate
of Silver; and also to the accumulation of Alcohol and Ether in an old
Bath producing a short and friable structure of the film.

i. _Effect of Temperature on Development._--Reduction of the oxides of
noble metals proceeds more rapidly in proportion as the temperature
rises. In cold weather it will be found that the development of the image
is slower than usual, and that greater strength of the reducing agent and
more free Nitrate of Silver is required to produce the effect.

On the other hand, if the heat of the atmosphere be excessive, the
tendency to rapid reduction will be greatly increased, the solutions
decomposing each other almost immediately on mixing. In this case the
remedy will be to use Acetic Acid _freely_ both in the Bath and in the
developer, at the same time lessening the quantity of Pyrogallic Acid, and
omitting the Nitrate of Silver which is sometimes added towards the end of
the development.

Also in the case of films which are to be kept for a long time in a
sensitive condition by means of honey, etc., the modifying influence of
temperature must be observed, and the quantity of free Nitrate of Silver
left upon the film be reduced to a minimum if the thermometer stands
higher than usual.


SECTION IV.

_On certain Irregularities in the Developing Process._

The characteristics of the proper development of a latent image are--that
the action of the reducing agent should cause a blackening of the Iodide
in the parts touched by light, but produce no effect upon those which have
remained in shadow.

In operating both on Collodion and paper however there is a liability to
failure in this respect; the film beginning, after the application of the
developer, to change in colour to a greater or less extent over the whole
surface.

There are two main causes which produce this state of things:--the first
being due to an irregularity in the action of the light; the second to a
faulty condition of the chemicals employed.

If from a defect in the construction of the instrument, or from other
causes which will be pointed out more particularly in the Second Part
of this work, diffused white light gains entrance into the Camera, it
produces indistinctness of the image by affecting the Iodide more or less
universally.

The luminous image of the Camera not being perfectly pure, mere
_over-exposure_ of the sensitive plate will usually have the same effect.
In such a case, when the developer is poured on, a faint image first
appears, and is followed by a general cloudiness.

The clearness of the developed Collodion picture is much influenced by the
condition of all the solutions employed, but particularly so by that of
the Nitrate Bath. If this liquid be in the state termed alkaline (p. 88),
it will be impossible to obtain a good picture; and even when neutral,
care and avoidance of all disturbing causes will be required to prevent
a deposition of Silver upon the shadows of the image: especially so when
Nitrite of Silver or Acetate of Silver are present, both of these salts
being more easily reduced than the Nitrate of Silver.

The use of _Acid_ is the principal resource in obviating cloudiness of the
image. Acids lessen the facility of reduction of the Salts of Silver by
developing agents (p. 98), and hence when they are present the metal is
deposited more slowly, and only on the parts where the action of the light
has so modified the particles of Iodide as to favour the decomposition:
whereas if acids be absent or present in insufficient quantity, the
equilibrium of the mixture of Nitrate of Silver and reducing agent which
constitutes the developer is so unstable, that any rough point or sharp
edge is likely to become a centre from which the chemical action, once
started, radiates to all parts of the plate.

Various acids have been employed, such as Acetic acid, Citric acid,
Tartaric acid, etc. Nitric acid is the most effectual of all, but is
seldom used, because, although the image can often be developed with great
clearness when the Bath contains a small quantity of Nitric acid, yet
such a condition is not favourable to _intensity_; on the other hand,
films which are prone to irregular reduction, such as those prepared in a
chemically neutral bath or a bath containing Acetate or Nitrite of Silver,
are likely to give the greatest vigour of impression. Hence, when this
quality is desired, the use of Nitric Acid will be adopted cautiously.

The state of the Collodion must be attended to as well as that of the
bath; it should be either acid or neutral, not alkaline. Colourless
Collodion may be used successfully as a rule, but sometimes a little
free Iodine is advantageously added. Care should be taken in introducing
organic substances, many of which dissolve out into the bath, and spoil it
for giving clear pictures. Glycyrrhizine, however, which is recommended
to produce intensity of Negatives, has no effect of that kind, and may be
employed with safety.

The condition of the developing agent is a point of importance in
producing clear and distinct pictures. The Acetic acid, which is advised
in the formulæ, cannot be omitted or even lessened in quantity without
danger. This is particularly the case in hot weather or under any other
condition which favours reduction, such as neutrality of the bath, etc.;
at all times, in fact, when the solutions of Pyrogallic acid and Nitrate
of Silver decompose each other with unusual rapidity.

In addition to the points now mentioned, viz. the state of the Bath,
of the Collodion, and of the developer, the reader should also study
the remarks made in the Third Section of Chapter III. on the effect
of _surface conditions_ in modifying the deposition of vapour and of
metallic Silver: he will then in all probability experience but little
difficulty in dealing with those numerous irregularities in the action
of the developing fluid, which often prove the greatest hindrance to the
successful practice of the Collodion process.




CHAPTER VII.

ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS.


The terms "Positive" and "Negative" occur so frequently in all works upon
the subject of Photography, that it will be impossible for the student to
make progress without thoroughly understanding their meaning.

A Positive may be defined to be a Photograph which gives a natural
representation of an object, as it appears to the eye.

A Negative Photograph, on the other hand, has the lights and shadows
reversed, so that the appearance of the object is changed or negatived.

In Photographs taken upon _Chloride of Silver_, either in the Camera or by
superposition, the effect must necessarily be Negative; the Chloride being
_darkened by luminous rays_, the lights are represented by shadows.

The following simple diagrams will make this obvious.

[Illustration: Fig. 1. Fig. 2. Fig. 3.]

Fig. 1 is an opaque image drawn upon a transparent ground; fig. 2
represents the effect produced by placing it in contact with a layer of
sensitive Chloride and exposing to light; and fig. 3 is the result of
copying this negative again on Chloride of Silver.

Fig. 3 therefore is a Positive copy of Fig. 1, obtained by means of a
Negative. By the first operation the tints are reversed; by the second,
being reversed again, they are made to correspond to the original. The
possession of a Negative therefore enables us to obtain Positive copies of
the object, indefinite in number and all precisely similar in appearance.
This capability of multiplying impressions is of the utmost importance,
and has rendered the production of good Negative Photographs of greater
consequence than any other branch of the Art.

The same Photograph may often be made to show either as a Positive or
as a Negative. For instance, supposing a piece of silver-leaf to be cut
into the shape of a cross and pasted on a square of glass, the appearance
presented by it would vary under different circumstances.

[Illustration: Fig. 1. Fig. 2.]

Fig. 1 represents it placed on a layer of black velvet; fig. 2 as held
up to the light. If we term it Positive in the first case, _i. e._
by reflected light, then it is Negative in the second, that is, by
transmitted light. The explanation is obvious.

Therefore to carry our original definition of Positives and Negatives
a little further, we may say, that the former are usually viewed by
reflected, and the latter by transmitted, light.

All Photographs however cannot be made to represent both Positives and
Negatives. In order to possess this capability, it is necessary that a
part of the image should be transparent, and the other opaque _but with a
bright surface_. These conditions are fulfilled when the Iodide of Silver
upon Collodion is employed, in conjunction with a developing agent.

Every Collodion picture is to a certain extent both Negative and Positive,
and hence the processes for obtaining both varieties of Photographs are
substantially the same. Although however the general characters of a
Positive and a Negative are similar, there are some points of difference.
A surface which appears perfectly opaque when looked down upon, becomes
somewhat translucent on being held up to the light; hence, to give the
same effect, the deposit of metal in a Negative must be proportionally
thicker than in a positive; otherwise the minor details of the image, will
be invisible, from not obstructing the light sufficiently.

With these preliminary remarks, we are prepared to investigate more
closely the _rationale_ of the processes for obtaining Collodion Positives
and Negatives. All that refers to paper Positives upon Chloride of Silver
will be treated in a subsequent Chapter.


SECTION I.

_On Collodion Positives._

Collodion Positives are sometimes termed _direct_, because obtained by a
single operation. The Chloride of Silver, _acted upon by light alone_,
is not adapted to yield direct Positives, the reduced surface being dark
and incapable of representing the lights of a picture. Hence a developing
agent is necessarily employed, and the Iodide of Silver substituted for
the Chloride, as being a more sensitive preparation. Collodion Positives
are closely allied in their nature to Daguerreotypes. The difference
between the two consists principally in the surface used to sustain the
sensitive layer, and the nature of the substance by which the invisible
image is developed.

In a Collodion Positive the lights are formed by a bright surface of
reduced Silver, and the shadows by a black background showing through the
transparent portions of the plate.

Two main points are to be attended to in the production of these
Photographs.

First, to obtain an image distinct in every part, _but of comparatively
small intensity_.--If the deposit of reduced metal be too thick, the
dark background is not seen to a sufficient extent, and the picture in
consequence is deficient in shadow.

Secondly, to _whiten_ the surface of the reduced metal as much as
possible, in order to produce a sufficient contrast of light and shade.
Iodide of Silver developed in the usual way presents a dull yellow
appearance which is sombre and unpleasing.

_The Collodion and Nitrate Bath for Positives._--Good Positives may be
obtained by diluting down a sample of Collodion with Ether and Alcohol
until it gives a pale bluish film in the Bath. The proportion of Iodide
of Silver being in that case small, the action of the high lights is less
violent, and the shadows are allowed more time to impress themselves. The
dilution lessens the amount of Pyroxyline in the Collodion at the same
time with the Iodide, which is an advantage, the slight and transparent
films always giving more sharpness and definition in the picture.

The employment of a very thin film for Positives is not however always a
successful process. The particles of the Iodide of Silver being closely
in contact with the glass, unusual care is required in cleaning the
plates in order to avoid stains; and the amount of free Nitrate of Silver
retained upon the surface of the film being small, circular patches of
imperfect development are liable to occur, unless the reducing agent be
scattered evenly and perfectly over the surface. Also if free Iodine or
organic substances which have a retarding effect on the action of light
are present to a considerable extent, the Collodion will not work well
with a small proportion of Iodide. The Author found in experimenting on
this subject that with perfectly pure Collodion and a _neutral_ Bath most
vigorous impressions were produced when the density of the film had been
so far reduced by dilution that scarcely anything could be seen upon the
glass; but with Collodion strongly tinted with Iodine, or with a Bath
containing Nitric Acid, it was necessary to stop the dilution at a certain
point or the film became absolutely insensitive to feeble radiations of
light, and the shadows could not be brought out by any amount of exposure.
In this case, by adding more Iodide a better effect was obtained.

A thicker Collodion may be used for positives if a little free Iodine be
added, for the purpose of diminishing intensity and keeping the shadows
clear during the development. This process is easier to practice than the
last, but does not always give the same perfect definition.

No organic substance of the class to which Glycyrrhizine and Sugars belong
should be added to Collodion which is to be used for Positives. By so
doing the image would be rendered intense, and the high lights liable to
solarization, _id est_, a dark appearance by reflected light.

_The Nitrate Bath._--If the materials are pure, the Nitrate Bath may
advantageously be diluted down at the same time with the Collodion, when
Positives are to be taken; but the employment of a very weak Nitrate
Bath (such as one of 20 grains to the ounce), although highly useful
in obviating excess of development, has some disadvantages; it becomes
necessary to exclude free Nitric Acid, and to avoid the employment of a
Collodion too highly tinted with Iodine. On the other hand, with a strong
Nitrate Bath, and a tolerably dense film of Iodide of Silver, a better
result is often secured by the use of Nitric Acid. The sensitiveness of
the plates is impaired, but at the same time the intensity is diminished,
and the picture shows well upon the surface of the glass.

A new Bath is better for taking Positives than one which has been a long
time in use. The latter often causes _haziness_ and irregular markings on
the film during the action of the developer. This is due partly to the
accumulation of Alcohol and Ether in the Bath, which causes the solution
of Sulphate of Iron to flow in an oily manner; and partly to a reduction
of the Nitrate of Silver by organic matter.

The presence of _Acetate of Silver_ is objectionable in a Positive Nitrate
Bath as producing solarization and intensity of image; hence those
precautions which obviate its formation must be adopted (p. 89).

If fused Nitrate of Silver be used for the Positive Nitrate Bath, it is
very important that the fusion should not be carried too far, or the
solution would contain a basic Nitrite of Silver, and yield an intense,
solarized, and misty image.

_The Developers for Collodion Positives._--Pyrogallic Acid when used with
Acetic Acid, as is usual for negative pictures, produces a surface which
is dull and yellow. This may be obviated by substituting Nitric Acid in
small _quantity_ for the Acetic. The surface produced by Pyrogallic Acid
with Nitric Acid is lustreless, but very white, if the solution be used
of the proper strength. On attempting to increase the amount of Nitric
Acid the deposit becomes metallic, and the half-tones of the picture are
injured; Pyrogallic Acid, although an active developer, does not allow
of the addition of mineral acid to the same extent as the Salts of Iron.
It requires also, when combined with Nitric Acid, a fair proportion of
Nitrate of Silver on the film, or the development will be imperfect in
parts of the plate.

_Sulphate of Iron._--The Protosalts of Iron were first employed in
Photography by Mr. Hunt. The Sulphate is a most energetic developer, and
often brings out a picture when others would fail. To produce by means of
it a dead white tint with absence of metallic lustre, it may be used in
conjunction with Acetic Acid, and in a somewhat concentrated condition, so
as to develope the picture quickly.

The addition of _Nitric Acid_ to Sulphate of Iron modifies the
development, making it more slow and gradual, and producing a bright
sparkling surface of reduced Silver. Too much of this acid however must
not be used, or the action will be irregular. The Nitrate Bath also must
be tolerably concentrated, in order to compensate for the retarding effect
of Nitric Acid upon the development. The blue and transparent films of
Iodide of Silver, formed in a very dilute Nitrate Bath, are not well
adapted for Positives to be developed in this way. They are injured by the
acid, and the development of the image becomes imperfect.

_Protonitrate of Iron._--This salt, first used by Dr. Diamond, is
remarkable as giving a surface of brilliant metallic lustre without any
addition of free acid. Theoretically, it may be considered as closely
corresponding to the Sulphate of Iron with Nitric Acid added. There are
however slight practical differences between them, which are perhaps in
favour of the Protonitrate.

The reducing powers of _Protoxide_ of Iron appear to be in inverse ratio
to the strength of the acid with which it is associated in its salts;
hence the _Nitrate_ is by far the most feeble developer of the Protosalts
of Iron.

The rules already given for the use of Sulphate of Iron acidified with
Nitric Acid, apply also to the Nitrate of Iron; the proportion of free
Nitrate of Silver must be large, and the film of Iodide of Silver not too
transparent.

In developing direct Positives either by Pyrogallic Acid or the Salts of
Iron, the colour of the image will be found liable to some variation;
the character of the light, whether bright or feeble, and the length of
exposure in the Camera, affecting the result.

_A Process for whitening the Positive Image by means of Bichloride of
Mercury._--In place of brightening the Positive image by modifying the
developer, it was proposed some time since by Mr. Archer to effect the
same object by the use of the salt known as _Corrosive Sublimate_, or
Bichloride of Mercury.

The image is first developed in the usual way, fixed, and washed. It is
then treated with the solution of Bichloride, the effect of which is to
produce almost immediately an interesting series of changes in colour. The
surface first _darkens considerably_, until it becomes of an ash-grey,
approaching to black; shortly it begins to get lighter, and assumes a
_pure white_ tint, or a white slightly inclining to blue. It is then
seen, on examination, that the whole substance of the deposit is entirely
converted into this white powder.

The _rationale_ of the reaction of Bichloride of Mercury appears to be,
that the Chlorine of the mercurial salt divides itself between the Mercury
and the Silver, a portion of it passing to the latter metal and converting
it into a Protochloride. The white powder is therefore probably a compound
salt, as is further evidenced by the effects produced on treating it with
various reagents.


SECTION II.

_On Collodion Negatives._

As in the case of a direct Positive we require an image which is _feeble_
though distinct, so, on the other hand, for a negative, it is necessary to
obtain one of considerable intensity. In the Chapter immediately following
the present, it will be shown that in using glass Negatives to produce
Positive copies upon Chloride of Silver paper, a good result cannot
be secured unless the Negative is sufficiently dark to obstruct light
strongly.

_The Collodion and Nitrate Bath for Negatives._--A Collodion containing
a very small portion of Iodide and yielding a blue transparent film in
the Bath is not well adapted for taking Negatives. Pale opalescent films
often give too little intensity in the high lights, and, unless the
Nitrate Bath be acid, do not admit of being exposed in the Camera for
the proper length of time without cloudiness and indistinctness of image
being produced under the action of the developer. The effect known as
"solarization of negatives," _i. e._ a red and translucent appearance
of the highest lights, is also more liable to occur when operating with
a very pale film. On the other hand, if the layer of Iodide be too
yellow and creamy, the half-tones of the image will often be imperfectly
developed, so that a middle point between these extremes is the best.

A pure and newly prepared Collodion, although highly sensitive to
light, does not always give, with one application of the developer, a
sufficiently vigorous image to serve as a negative matrix; and this
particularly in the most brightly illuminated parts, such as the sky in a
landscape photograph, or the white borders of an engraving. But on keeping
the Collodion for some weeks or months it becomes yellow, if iodized with
the alkaline iodides, and a decomposition takes place in it, as before
shown (p. 97), which lessens the rapidity of action, but adds to the
intensity of the negative.

Grape Sugar may be employed for the purpose of giving intensity to newly
mixed Collodion: also Glycyrrhizine, which is a resinous body extracted
from the root of Liquorice; but as both substances have an effect in
lessening the sensitiveness and keeping qualities of the fluid, they
should be used cautiously. In taking portraits in the open air, on bright
days, and with a Bath which has been mixed for a considerable time, it
will rarely be found that the intensity will be deficient; and especially
so if the developer be applied a second time to the film with a few drops
of solution of Nitrate of Silver added. In landscape Photography however,
or in copying engravings, where extreme sensitiveness is not an object,
the Glycyrrhizine may sometimes be added with advantage in order to obtain
perfect opacity of the blacks.

When the use of this substance is resorted to, the mode of iodizing the
Collodion appears to be of importance, the increase of intensity being
greater with the Iodide of Cadmium than with the Iodides of the Alkalies;
the latter probably exercising a decomposing action. An addition of a
Bromide or a Chloride to the Collodion in small quantity has also a marked
effect in adding to the intensity when Glycyrrhizine is used with alkaline
Iodides (p. 101).

Substances which produce intensity of the Collodion image have often,
if added in too large quantity, a tendency to lower the half-tone, and
prevent the darker parts of the picture from being sufficiently brought
out. The print from the Negative is then pale and white, or "chalky" as
it is termed, in the high lights. Collodion in this condition is often
preferred by the beginner, from the facility with which the Negatives
are obtained, but it does not give the finest results. An excess of
Glycyrrhizine in Collodion has also the effect of interfering with the
precipitation of the Iodide of Silver, producing a blue and smoky film
which is nearly useless for Negatives.

A judicious employment of free Iodine in Collodion which has been
previously intensified with Glycyrrhizine, has a remarkable effect in
improving the gradation of tone. The excessive opacity of the high lights
is diminished, and hence the operator is enabled by a longer exposure of
the sensitive plate to bring out the shadows and minor details of the
image with great distinctness. Collodion prepared in this manner is too
slow to be used for portraits, excepting in a strong light, but often
gives an image with great roundness and stereoscopic effect.

The Iodine and the liquorice sugar employed conjointly, tend also to
preserve the clearness of the plates under the influence of the developer,
and to give sharpness to the lines and dots of engravings, etc., which,
with a new and sensitive Collodion, are often imperfectly rendered. These
advantages will be appreciated by the operator who has failed from
working with a too feeble Collodion; but it must be borne in mind, that
all substances acting as intensifiers have a bad effect when the state of
the film is not such as to call for their employment.

The Proto-iodide of Iron has been recommended as an addition to Negative
Collodion. In the Nitrate Bath it forms, in addition to Iodide of Silver,
Protonitrate of Iron, an unstable substance and a developer. The use of
Iodide of Iron gives great sensibility, but it is difficult to preserve
it pure and unchanged. It also decomposes the Collodion in the course of
a few hours, becoming itself peroxidized, and producing an insensitive
condition of film. In addition to this, the negatives taken by the aid of
Iodide of Iron are commonly of an inferior kind, the reduction being too
marked in the high lights; so that its employment is of doubtful utility.

_The Nitrate Bath._--This should be prepared from Nitrate of Silver which
has been melted at a moderate heat (see pp. 13 and 101). If this point be
neglected, the best Collodion will sometimes fail in producing an intense
negative.

Acetic Acid must be added in minute quantity, to preserve the solution
from a too ready reduction by the Alcohol and Ether of the Collodion.
Also, unless the Nitrate of Silver be quite pure and free from organic
matter (p. 104), clear pictures will not be obtained without the use of
Acid.

Acetate of Silver has often been advised as an addition to the Negative
Nitrate Bath. It is produced by dropping into the solution an alkali, such
as Ammonia, followed by Acetic Acid in excess. The Negatives are rendered
blacker and more vigorous by this proceeding, but especially so when the
Bath is contaminated with Nitric Acid; which neutralizes itself at the
expense of the Acetate of Silver, thus:--

    Acetate of Silver + Nitric Acid
  = Nitrate of Silver + Acetic Acid.

As a rule, it will be better to avoid adding Acetate of Silver to the
Bath, since with, pure melted Nitrate of Silver no Nitric Acid can be
present, and perfect intensity is easily obtained. When the Bath is
saturated with Acetate of Silver, it is in a more reducible state, and
hence unless the glass plates are very perfectly cleaned, black lines
and markings, the results of irregular action, will be produced on the
application of the developer to the film (p. 104). Solarization, or
reddening by over-exposure, is also promoted by the presence of Acetate of
Silver.

_Developing solutions for Negatives._--The Protosalts of Iron are not
usually employed in developing Negative impressions. They are liable
to yield a violet-coloured image, which cannot easily be rendered more
intense by continuing the action.

Gallic Acid is too feeble for developing Collodion pictures. Pyrogallic
Acid is much superior, and may be used of any strength, according to
the effect desired. When the light is bad, the temperature low, and
the Negative developes slowly and appears blue and inky by transmitted
light, the proportion of the reducing agent should be increased. But with
an intense Collodion, on a clear summer's day, the finest gradation is
obtained with a weak solution, which does not begin to act until the plate
has been evenly covered. A strong developer might in such, a case produce
too much opacity in the highest lights, and would probably occasion stains
of irregular reduction.

_Modes of strengthening a finished impression which is too feeble to be
used as a Negative._--The ordinary plan of pushing the development cannot
be applied with advantage after the picture has been washed and dried. In
that case, if it is found to be too feeble to print well, its intensity
may be increased by one of the following methods.--

It must be premised however, that the same degree of excellence is not to
be expected in a Negative Photograph which has been improperly developed
in the first instance and more especially if the exposure to light was
too short. Any "instantaneous Positive" may be rendered sufficiently
intense for a Negative, but in that case the shadows are almost invariably
imperfect.

1. _Treatment of the image with Sulphuretted Hydrogen or Hydrosulphate of
Ammonia._--The object is to convert the metallic Silver into _Sulphuret
of Silver_, and if this could be done it would be of service. The mere
application of an Alkaline Sulphuret has however but little effect upon
the image, excepting to darken its surface and destroy the Positive
appearance by reflected light; the structure of the metallic deposit being
too dense to admit of the Sulphur reaching its interior.

Professor Donny ('Photographic Journal,' vol. i.) proposes to obviate this
by first converting the image into the white Salt of Mercury and Silver by
the application of Bichloride of Mercury, and afterwards treating it with
solution of Sulphuretted Hydrogen or Hydrosulphate of Ammonia. Negatives
produced in this way are of a brown-yellow colour by transmitted light,
and opaque to chemical rays to an extent which would not, _à priori_, have
been anticipated.

2. _MM. Barreswil and Davanne's process._--The image is converted into
Iodide of Silver by treating it with a saturated solution of Iodine in
water. It is then washed--to remove the excess of Iodine,--exposed to
the light, and a portion of the ordinary developing solution, mixed with
Nitrate of Silver, poured over it. The changes which ensue are precisely
the same as those already described; the whole object of the process being
to bring the metallic surface back again into the condition of Iodide of
Silver modified by light, that the developing action may be commenced
afresh, and more Silver deposited from the Nitrate in the usual way.

3. _The process with Bichloride of Mercury and Ammonia._--The image is
first converted into the usual white double Salt of Mercury and Silver
by the application of a solution of the Corrosive Sublimate. It is then
treated with Ammonia, the effect of which is to _blacken_ it intensely.
Probably the alkali acts by converting Chloride of Mercury into the black
Oxide of Mercury. In place of Ammonia, a dilute solution of Hyposulphite
of Soda or Cyanide of Potassium may be used, with very similar results.




CHAPTER VIII.

ON THE THEORY OF POSITIVE PRINTING.


The subject of Collodion Negatives having been explained in the previous
Chapter, we proceed to show how they may be made to yield an indefinite
number of copies with the lights and shadows correct as in nature.

Such copies are termed "Positives," or sometimes "Positive prints," to
distinguish them from direct Positives upon Collodion.

There are two distinct modes of obtaining photographic prints;--first by
development, or, as it is termed, _by the Negative process_, in which
a layer of Iodide or Chloride of Silver is employed, and the invisible
image developed by Gallic Acid; and second, by the direct action of light
upon a surface of Chloride of Silver, no developer being used. These
processes, involving chemical changes of great delicacy, require a careful
explanation.

The action of light upon Chloride of Silver was described in Chapter II.
It was shown that a gradual process of darkening took place, the compound
being reduced to the condition of a coloured _subsalt_; also, that the
rapidity and perfection of the change were increased by the presence of
excess of Nitrate of Silver, and of organic matters, such as Gelatine,
Albumen, etc.

We have now to suppose that a sensitive paper has been prepared in this
way, and that a Negative having been laid in contact with it, the
combination has been exposed to the agency of light for a sufficient
length of time. Upon removing the glass, a Positive representation of the
object will be found below, of great beauty and detail. Now if this image
were in its nature fixed and permanent, or if there were means of making
it so, without injury to the tint, the production of Paper Positives would
certainly be a simple department of the Photographic Art; for it will be
found that with almost any Negative, and with sensitive paper however
prepared, the picture will look tolerably well on its first removal from
the printing-frame. Immersion in the bath of Hyposulphite of Soda however,
which is essentially necessary in order to fix the picture, produces
an unfavourable effect upon the tint; decomposing the violet-coloured
Subchloride of Silver, and leaving behind a red substance which appears
to be united to the fibre of the paper, and, when tested, reacts in the
manner of a Suboxide of Silver.

Other chemical operations are therefore required to remove the
objectionable red colour of the print, and hence the consideration of the
subject is naturally divided into two parts; first, the means by which
the paper is rendered sensitive, and the image impressed upon it;--and
secondly, the subsequent fixing and _toning_, as it may be termed, of the
proof.

The present Chapter will also include, in two additional Sections, a
condensed account of the most important facts relating to the properties
and the mode of preservation of photographic prints.


SECTION I.

_The Preparation of the Sensitive Paper._

In this Section the general theory of the preparation of Positive paper,
in so far as it affects the tone and intensity of the print, will be
described; the reader being referred to the second division of the Work
for the formulæ required.

_The Preparation of the Sensitive Paper._--The conditions which are
required for producing a sharp and well defined print are--that an even
layer of Chloride of Silver should exist upon the very surface of the
paper, and that the particles of this Chloride should be in contact with
a sufficient excess of Nitrate of Silver. These points have been already
referred to at an early part of the Work (p. 19).

The material used for _sizing_ the paper is of importance. English papers
are usually sized with Gelatine, which is a photographic agent, and acts
chemically in forming the image. Foreign papers on the other hand being
sized with starch only, require an addition of Gelatine, Caseine, or
Albumen, to retain the Salt at the surface of the paper, and to assist
in producing the picture: if otherwise, the print will be flat and
"mealy," as it is termed. Albumen especially produces a beautifully smooth
surface, and is advantageously employed in printing small portraits and
stereoscopic subjects.

The uniform surface distribution of the Chloride of Silver is sometimes
interfered with by a faulty structure of the paper, causing it to absorb
liquids unevenly, and in consequence the pictures, when removed from the
printing frame, appear _spotted_. Another cause producing the same effect,
is the employment of too weak a solution of Nitrate of Silver, or the
removal of the sheet from the Nitrate bath before the Chloride of Ammonium
has been perfectly decomposed; it is thus rendered unequally sensitive at
different portions of the surface, and the prints have the characteristic
marbled appearance above referred to.

A sufficient excess of Nitrate of Silver being essential, it is important
to bear in mind, that the quantity of this salt eventually remaining in
the paper, is much influenced by the manner in which the solution is
applied. If it be laid on by _floating_, then the proportion of Nitrate to
that of Chloride of Sodium should be about as 3 to 1 (the atomic weights
are nearly as 5 to 2); but if the plan of brushing or spreading with a
glass rod be adopted, 7 to 1 or 8 to 1 will not be too much.

_The Darkening of the Sensitive Paper by Light._--The operator should be
familiar with the changes of colour which indicate the progress of the
reduction of the sensitive layer. Much in this respect depends upon the
kind of organic matter used, but there is always a regular sequence of
tints; in the case of a paper prepared simply with Chloride of Ammonium
and Nitrate of Silver, it is as follows: pale violet, violet-blue,
slate-blue, _bronze_ or copper-colour. When the _bronzed_ stage is
reached, there is no further change. On immersion in the fixing bath of
Hyposulphite, the violet tones due to Subchloride of Silver are destroyed,
and the print assumes a red or brown colour, which is deepest and most
intense in the parts where the light has acted longest.

Hence we see, that, to produce a good print, it is essential that the
Negative should possess considerable intensity in the dark parts. Pale
and feeble Negatives yield proofs which are wanting in vigour, and have
a flat and indistinct appearance. The combination cannot be exposed to
light for a sufficient length of time to bring about the requisite degree
of reduction of the Chloride of Silver; and hence the deepest shadows of
the resulting Positive are not sufficiently dark, and there is _a want of
contrast_ which is fatal to the effect.

A good Negative should be so opaque as to preserve the lights of the
printed image beneath clear, _until the darkest shades are about to pass
into the bronze or coppery condition_. If the amount of intensity be less
than this, the finest effect cannot be obtained.


CONDITIONS AFFECTING THE SENSITIVENESS OF THE PAPER AND THE INTENSITY OF
THE IMAGE.

Some of the principal of these are as follows:--

a. _The Strength of the Salting Bath._--The sensibility of the paper is
regulated up to a certain point by the amount of salt[16] used in the
preparation. The quantity of alkaline Chloride determines the amount of
Chloride of Silver; and with a proper excess of Nitrate of Silver, papers
are to a certain point more sensitive in proportion as they contain more
of the Chloride.

[16] The difference in the atomic weights of the various soluble Chlorides
used in salting must be borne in mind. Ten grains of Chloride of Ammonium
contain as much Chlorine as eleven of Chloride of Sodium, or twenty-two
grains of Chloride of Barium. (See the Vocabulary, Part III.)

Highly sensitized papers darken rapidly, and pass very completely into the
bronze stage. Those containing less Chloride darken more slowly, and do
not become bronzed with the same intensity of light. A Photographic print,
formed upon paper highly salted and sensitized, is usually vigorous, with
great contrast of light and shade; particularly so when the printing is
conducted in a strong light. Hence it will be an advantage, with a feeble
Negative, and in dull weather, to _double_ the ordinary quantity of Salt,
whereas in the case of an intense Negative, and with direct sunlight, the
deep shadows will be too much bronzed unless the quantity of Chloride and
Nitrate of Silver in the paper be kept low.

In proportion as Photographic papers are highly salted and sensitized,
they become more prone to change colour spontaneously in the dark.

b. _Proportion of Nitrate of Silver._--The compound on which a positive
print is formed is a Chloride, or an organic Salt of Silver, _with
an excess of Nitrate of Silver_. Nothing is gained by increasing the
proportion of Chloride of Sodium, unless at the same time an addition be
made to the quantity of free Nitrate in the sensitizing Bath.

A surface of Chloride of Silver with a bare excess of Nitrate, darkens on
exposure, but it does not reach the bronzed stage; the action appearing to
stop at a certain point. On placing the print in Hyposulphite of Soda, it
becomes very red and pale, and when tinted, looks cold and slaty, without
depth or intensity.

c. _The sensitiveness and intensity affected by substituting the Oxide
of Silver for the Nitrate._--Many operators employ a solution of Oxide
of Silver in Ammonia[17] or Nitrate of Ammonia, in preparing Chloride of
Silver paper. By doing so, a great increase of sensitiveness, and also of
intensity of image, is obtained. This will be understood if we remember
that the action of light in producing the print is of a reducing nature.
Hence the substitution of Oxide for Nitrate of Silver facilitates the
decomposition; just as _Ammonio-Nitrate_ of Silver is more readily reduced
by Gallic or Pyrogallic Acid than the simple Nitrate (see p. 31).

[17] The chemistry of Ammonio-Nitrate of Silver is explained in the
Vocabulary, Part III.

Ammonio-Nitrate paper has the disadvantage of soon _discolouring_ when
kept; but it is very serviceable in printing during the winter months.
The proportion of Chloride in the salting Bath may, if desired, be
considerably reduced; the intensity of action being greatly exalted by the
use of the Oxide of Silver.

d. _Employment of organic matters._--Those recommended in this work
are--Albumen, Gelatine, and Iceland Moss. Albumen adds much to the
sensibility of the paper, and gives very fine surface definition. A less
amount of Chloride is required than in the case of plain paper simply
salted, the glutinous character of Albuminous liquids causing more of
the fluid to be retained upon the surface of the paper, and the animal
matter assisting the reduction. By varying the proportion of salt, both
feeble and intense Negatives may be printed successfully upon albuminized
paper. No process gives better results, either as regards sensitiveness,
or in faithfully rendering all the finer details of the Negative, than the
process with Albumen.

Iceland Moss, when boiled in water, yields a mucilaginous liquid which is
conveniently employed as a vehicle for Chloride of Silver; it increases
the sensitiveness of the paper and gives additional power of bronzing,
by assisting to reduce the free Nitrate of Silver. Many other organic
matters, tending to absorb oxygen, would act in the same way.

_Gelatine_ is used in positive printing; it is analogous to Albumen in
composition, and, like it, forms a red compound with Suboxide of Silver.
It is serviceable in keeping the print at the surface of the paper, but
does not alter the sensibility or the general appearance of the finished
picture so greatly as Albumen.

e. _Impurities in Nitrate of Silver._--Nitrate of Silver used for
Photographic printing should be free from even a trace of Protonitrate of
Mercury, since it is known that the precipitation of Chloride of Mercury
prevents the darkening of Chloride of Silver by light.

The peculiar condition of Nitrate of Silver spoken of at page 101, in
which it is thought to contain Oxides of Nitrogen, is likely to interfere
with Photographic printing. This is probably the explanation of a
faulty state of the Nitrate solution, in which it yields red and feeble
positives, and does not darken in colour in exciting albuminized paper.
The remedy will be, to fuse the Nitrate of Silver at a moderate heat
before dissolving it.


THE COLOUR OF THE IMAGE INFLUENCED BY THE PREPARATION OF THE SENSITIVE
PAPER.

This subject should be studied by those who desire to print with taste.
By introducing a few simple modifications into the mode of preparing the
sensitive paper, almost any variety of tint may be obtained.

The tendency of the "toning" process, to which the print is afterwards to
be submitted, is to darken the colour, and, if gold be used, to give a
shade of _blue_. Hence, if the Positive be printed of a red tone, it will
change in the gold Bath to a purple; whereas if left, after exposure to
light and fixing, of a dark brown or sepia tint, it passes by toning into
a pure black.

The Positive should look warm and bright on its removal from the printing
frame; but the tint which remains after immersion in Hyposulphite of Soda
is the proper colour of the simply fixed print.

The following points may be mentioned as affecting the colour and general
appearance of the picture.

a. _The proportions of Salt and Nitrate of Silver._--Highly salted and
sensitized papers give a _darker_ image than those which, containing a
small proportion of Chloride of Silver, are less sensitive to light.
Hence in printing upon paper weakly sensitized, in order to bring out the
finer details of a highly intense negative, we find the image unusually
red after fixing, and of a brown or mulberry colour when toned. The above
remarks apply also in some degree to the strength of the Nitrate Bath, and
especially so when no organic matter excepting Gelatine is employed,--in
such a case the image will be _darker_ after fixing, if the proportion of
free Nitrate of Silver be large.

b. _Effect of Oxide of Silver on the colour._--Prints formed upon
Ammonio-Nitrate papers highly salted are of a sepia colour after fixing,
and usually of a pure black or a purple-black when toned. With the
increased facility of reduction by light afforded by use of _Oxide_ of
Silver, there is also less redness in the print. But if the quantity of
salt used in preparing the paper be reduced to a minimum (one grain to
the ounce or less), for the sake of economy or to improve the half-tone,
then the usual red colour returns, and the Positive is brown or purple
after toning, in place of black. Thus by employing a solution of Oxide of
Silver, the operator is enabled, without the addition of organic matter,
to print Positives of a pleasing variety of tint, combined with a peculiar
softness and delicacy, which cannot easily be obtained with the simple
Nitrate of Silver.

c. _The colour affected by organic matter._--Albumen is coagulated by
Nitrate of Silver, and forms a permanent gloss upon the paper. The
sensitive albuminized paper darkens in the sun to a chocolate-brown
colour, which becomes very red on immersion in the Hyposulphite. The
finished prints are clear and transparent; usually of a brown tone, or
with a shade of purple when the gold Bath is newly made and active; pure
blacks are not easily obtained.

Iceland Moss affects the colour of the proof to a certain extent, but less
than Albumen; the finished prints are nearly black if the paper is highly
salted.

The Gelatinous sizing used for the English papers, and obtained by boiling
hides in water, and hardening the product by an admixture of Alum, has
a _reddening_ influence upon reduced Silver salts, analogous to that of
Albumen, or of Caseine, the characteristic animal principle of milk.
Positives printed upon English paper, commonly assume some shade of brown
more or less removed from black; the darker tones being more readily
obtained upon the foreign papers.

Citrates and Tartrates have a marked effect upon the colour of prints.
Paper prepared with Citrate, in addition to Chloride of Silver, darkens to
a fine purple colour which changes to brick-red in the fixing Bath. The
Positives, when toned, are usually of a violet-purple or of a bistre tint,
with a general aspect of warmth and transparency.


SECTION II.

_The Processes for Fixing and Toning the Proof._

This part of the operation is one to which great attention should be
paid, in order to secure bright and lasting colours: it involves more of
delicate chemical change than perhaps any other department of the Art.

The first point requiring explanation is the process of fixing; to which
(p. 41) brief reference has already been made. The methods adopted to
improve the tint of the finished picture will then be described.


CONDITIONS OF A PROPER FIXING OF THE PROOF.

This subject is not always understood by operators, and consequently they
have no certain guide as to how long the prints should remain in the
fixing Bath.

The time occupied in fixing will of course vary with the strength of
the solution employed; but there are simple rules which may be usefully
followed. In the act of dissolving the unaltered Chloride of Silver
in the proof, the fixing solution of Hyposulphite of Soda converts it
into Hyposulphite of Silver (p. 43), which is soluble in an _excess_ of
Hyposulphite of Soda. But if there be an insufficient excess,--that is, if
the Bath be too weak, or the print removed from it too speedily,--then the
Hyposulphite of Silver is not perfectly dissolved, and begins by degrees
to _decompose_, producing a brown deposit in the tissue of the paper.
This deposit, which has the appearance of yellow spots and patches, is
not usually seen upon the surface of the print, but becomes very evident
when it is held up to the light, or if it be split in half, which can be
readily done by gluing it between two flat surfaces of deal, and then
forcing them asunder.

_The reaction of Hyposulphite of Soda with Nitrate of Silver._--In order
to understand more fully how _decomposition_ of Hyposulphite of Silver may
affect the process of fixing, the peculiar properties of this salt should
be studied. With this view Nitrate of Silver and Hyposulphite of Soda may
be mixed in equivalent proportions, viz. about twenty-one grains of the
former salt to sixteen grains of the latter, first dissolving each in
separate vessels in half an ounce of distilled water. These solutions are
to be added to each other and well agitated; immediately a dense deposit
forms, which is Hyposulphite of Silver.

At this point a curious series of changes commences. The precipitate, at
first white and curdy, soon alters in colour: it becomes canary-yellow,
then of a rich orange-yellow, afterwards liver-colour, and finally black.
The _rationale_ of these changes is explained to a certain extent by
studying the composition of the Hyposulphite of Silver. The formula for
this substance is as follows:--

  AgO S{2}O{2}.

But AgO S{2}O{2} plainly equals AgS, or Sulphuret of Silver, and SO{3}, or
Sulphuric Acid. The acid reaction assumed by the supernatant liquid is due
therefore to Sulphuric Acid, and the black substance formed is Sulphuret
of Silver. The yellow and orange-yellow compounds are earlier stages of
the decomposition, but their exact nature is uncertain.

The instability of Hyposulphite of Silver is principally seen when it is
in an isolated state: the presence of an excess of Hyposulphite of Soda
renders it more permanent, by forming a double salt, as already described.

In fixing Photographic prints, this brown deposit of Sulphuret of Silver
is very liable to form in the Bath and upon the picture; particularly
so when the _temperature_ is high. To obviate it, observe the following
directions:--It is especially in the reaction between _Nitrate of Silver_
and Hyposulphite of Soda that the blackening is seen; the Chloride and
other _insoluble_ Salts of Silver being dissolved, even to saturation,
without any decomposition of the Hyposulphite formed. Hence if the
print be washed in water to remove the soluble Nitrate, a very much
weaker fixing Bath than usual may be employed. But if the proofs are
taken at once from the printing frame and immersed in a dilute Bath of
Hyposulphite (one part of the salt to six or eight of water), _a shade of
brown_ may often be observed to pass over the surface of the print, and
a large deposit of Sulphuret of Silver soon forms as the result of the
decomposition. On the other hand, with a strong Hyposulphite Bath there is
little or no discoloration, and the black deposit is absent.

The print must also be left for a sufficient time in the fixing bath,
or some appearance of brown patches,[18] visible by transmitted light,
may occur. Each atom of Nitrate of Silver requires _three_ atoms of
Hyposulphite of Soda to form the _sweet and soluble double salt_, and
hence, if the action be not continued sufficiently long, another compound
will be formed almost tasteless and insoluble (p. 44). Even immersion in
a new Bath of Hyposulphite of Soda does not fix the print when once the
yellow stage of decomposition has been established. This yellow salt is
insoluble in Hyposulphite of Soda, and consequently remains in the paper.

[18] The writer has noticed that when sensitive paper is _kept for some
time_ before being used for printing, these yellow patches of imperfect
fixation are very liable to occur. The Nitrate of Silver appears gradually
to enter into combination with the organic matter of the size of the
paper, and cannot then be so easily extracted by the fixing bath.

In fixing prints by Ammonia the Author has found that the same rule may be
applied as in the case of Hyposulphite of Soda, viz. that if the process
be not properly performed, the white parts of the print will appear
_spotted_ when held up to the light, from a portion of insoluble Silver
Salt remaining in the paper. Prints imperfectly fixed by Ammonia are also
usually brown and discoloured upon the surface of the paper.

More exact directions as to the strength of the fixing bath and the time
occupied in the process, will be given in the Second Part of the Work;
at present it may be noticed only that _Albuminized_ paper, from the
horny nature of its surface-coating, requires a longer treatment with the
Hyposulphite than the plain paper.


THE SALTS OF GOLD AS TONING AGENTS FOR PHOTOGRAPHIC PRINTS.

The Salts of Gold have been successfully applied to the improvement of the
tones obtained by simply fixing the proof in Hyposulphite of Soda. The
following are the principal modes followed:--

_M. Le Grey's Process._--The print, having been exposed to light until
it becomes very much darker than it is intended to remain, is washed in
water to remove the excess of Nitrate of Silver. It is then immersed in a
dilute solution of Chloride of Gold, acidified by Hydrochloric Acid. The
effect is to reduce the intensity considerably, and at the same time to
change the dark shades to a violet or bluish tint. After a second washing
with water, the proof is placed in plain Hyposulphite of Soda, which fixes
it and alters the tone to a pure black or a blue-black, according to the
manner of preparing the paper and the time of exposure to light.

The _rationale_ of the process appears to be as follows:-- the Chlorine,
previously combined with Gold, passes to the reduced Silver Salt; it
bleaches the lightest shades, by converting them again into white
Protochloride of Silver, and gives to the others a violet tint more or
less intense according to the reduction. At the same time metallic Gold
is deposited, the effect of which is not visible at this stage, since the
same violet tint is perceived when a solution of _Chlorine_ is substituted
for Chloride of Gold.

The Hyposulphite of Soda subsequently employed, decomposes the violet
Subchloride of Silver, and leaves the surface of a black tint, due to the
Gold and the reduced Silver Salt.

M. Le Grey's process is objectionable on account of the excessive
over-printing required. This however is to a great extent obviated by a
modification of the process in which an _alkaline_ instead of an acid
solution of the Chloride is employed; one grain of Chloride of Gold is
dissolved in about six ounces of water, to which are added twenty to
thirty grains of the common Carbonate of Soda. The alkali moderates the
violence of the action, so that the print washed with water and immersed
in the Gold Bath, is less reduced in intensity, and does not acquire the
same _inky_ blueness. On subsequent fixing in the Hyposulphite, the tint
changes from violet to a dark chocolate-brown, which is permanent.

_The Tetrathionate and Hyposulphite of Gold employed in toning._--After
the discovery of Le Grey's mode, it was proposed, as an improvement,
to add Chloride of Gold to the fixing solution, so as to obviate the
necessity of using two Baths. The print, in that case, although darkened
considerably, is less reduced in intensity, and the same amount of
over-printing is not required. The chemical changes which ensue are
different from before: they may be described as follows:--

Chloride of Gold, added to Hyposulphite of Soda, is converted into
Hyposulphite of Gold, Tetrathionate of Gold, and (if the Chloride of Gold
be free from excess of acid) a red compound, containing more of the metal
than, either of the others, but the exact nature of which is uncertain.
Each of these three Gold Salts possesses the property of darkening the
print, but not to the same extent. The activity is less as the stability
of the salt is greater, and hence the red compound, which is so highly
unstable that it cannot be preserved many hours without decomposing and
precipitating metallic Gold, is far more active than the Hyposulphite of
Gold, which, when associated with an excess of Hyposulphite of Soda, is
comparatively permanent.

When rapidity of colouring is an object it will therefore be advisable to
add Chloride of Gold to the fixing Bath of Hyposulphite rather than an
equivalent quantity of Sel d'or; and by dropping a little Ammonia into the
Chloride of Gold so as to precipitate "fulminating gold"[19] (a compound
which dissolves in Hyposulphite of Soda with considerable formation of the
unstable red salt), the activity of the Bath will be promoted.

[19] Read the observations on the Explosive Properties of Fulminating Gold
in the Vocabulary, Part III.

The Author explains the action of these Salts of Gold upon the Positive
print as follows:--they are unstable, and contain an excess of Sulphur
loosely combined; hence, when placed in contact with the image, which
has an affinity for Sulphur, the existing compound is broken up, and
Sulphuret of Silver, Sulphuric Acid, and metallic Gold are the results.
That a minute proportion of Sulphuret of Silver is formed seems certain;
but the change must be superficial, as the stability of the print is very
little lessened when the process is properly performed.

_Sel Or employed as a toning agent._--This process, which was communicated
to the 'Photographic Journal' by Mr. Sutton of Jersey, has been found
serviceable.

The prints are first washed in water, to which is added a little Chloride
of Sodium, to decompose the free Nitrate of Silver. They are then immersed
in a dilute solution of "Sel d'or," or double Hyposulphite of Gold and
Soda, which quickly changes the tint from red to purple without destroying
any of the details or lighter shades. Lastly, the Hyposulphite of Soda is
employed to fix the print in the usual way.

This process differs theoretically from the last in some important
particulars. The toning solution is applied to the print _before fixing_,
which experience proves to have an important influence upon the result,
it having been found that when the print is previously acted upon by
Hyposulphite of Soda, the rapidity of deposition of the Gold is interfered
with;--thus, a dilute solution of Sel d'or colours a print rapidly, but if
to this same liquid a few crystals of Hyposulphite of Soda be added, the
picture becomes red and may be kept in the Bath for comparatively a long
time without acquiring the purple tones.

As Hyposulphite of Soda in excess lessens the action of the Sel d'or, so
on the other hand the addition of an acid increases it. The acid does
not precipitate _Sulphur_, as might be expected from a knowledge of the
reaction of Hyposulphite with acid bodies (p. 137), but it favours the
reduction of metallic Gold. Hence it is usual to add a little Hydrochloric
Acid to the toning solution of Sel d'or, to increase the rapidity and
perfection of the colouring process.


THE CONDITIONS WHICH AFFECT THE ACTION OF THE FIXING AND TONING BATH OF
GOLD AND HYPOSULPHITE OF SODA.

Although the process of toning Positives by Sel d'or is very certain in
its results and gives good tints, yet, as involving a somewhat greater
expenditure of time and trouble, it is not at present universally adopted.
The ordinary plan of fixing and toning in one bath has been proved to
yield permanent prints if the proper precautions are observed, but it is
quite necessary, in order to ensure success, that the conditions by which
its action is modified should be understood. The more important of these
are as follows:--

a. _The_ AGE _of the Bath._--When Chloride of Gold is added to
Hyposulphite of Soda, several unstable salts are produced, which decompose
by keeping. Hence the solution is very active during the first few days
after mixing; but at the expiration of some weeks or months, if not used,
it becomes almost inert, a reddish deposit of Gold first forming, and
eventually a mixture of black Sulphuret of Silver and Sulphur, the former
of which often adheres to the sides of the bottle in dense shining laminæ.

When the Bath is constantly kept in use there is a loss of Gold, which,
although it is less perceived than it otherwise would be, from the fact
that sulphuretting principles are formed (see next page) capable of
replacing the Gold as toning agents--yet makes the Bath work more slowly,
and hence over-printing is required.

b. _Presence of free Nitrate of Silver upon the surface of the
proof._--This produces an accelerating effect, as may be shown by soaking
the print in salt and water, to convert the Nitrate into Chloride of
Silver; the action then takes place more slowly.

The free Nitrate of Silver increases the instability of the Gold salts;
but if present in too great an excess, it is apt to cause a decomposition
of Hyposulphite of Silver, and consequent yellowness in the white parts
of the proof. It is therefore particularly recommended to wash the print
in water before immersing it in the fixing and toning Bath.

c. _Temperature of the solution._--In cold weather, the thermometer
standing at 32° to 40°, the Bath works more slowly than usual; whereas
in the height of summer, and especially in hot climates, it occasionally
becomes quite unmanageable. The best temperature for operating
successfully appears to be about 60° to 65° Fahrenheit; if higher than
this the solutions must be employed more dilute.

d. _Addition of Iodide of Silver._--Some operators associate Iodide with
Chloride in the preparation of sensitive paper for printing. Another
source of the same salts is the admixture of a portion of the fixing Bath
used for Negatives with the Positive toning solution. The presence of
Iodides in the fixing and toning Bath is injurious: when in large excess,
they dissolve the image, or produce yellow patches of Iodide of Silver on
the lights; in smaller quantity, the deposition of the Gold is hindered,
and the action proceeds more slowly. Bromides and Chlorides have not the
same effect.

e. _Mode of preparing the paper._--The rapidity of toning varies with
causes independent of the Bath: thus, plain paper prints are toned more
quickly than prints upon albuminized paper, and the use of English paper
sized with Gelatine retards the action. Foreign papers rendered sensitive
with Ammonio-Nitrate tone the most quickly.

_On certain states of the fixing and toning Bath which are injurious to
the proofs._--The object of using the Hyposulphite Bath is to fix the
proof and to tone it by means of Gold. But it is a fact familiar to the
photographic chemist, that Positives can also be toned by a sulphuretting
action, and that the colours so obtained are not very different from
those which follow the employment of Gold.[20] Now the Hyposulphite of
Soda is a substance which can be very readily made to yield up Sulphur
to any bodies which possess an affinity for that element, and as the
reduced Silver compound in the print has such an affinity, there is
always a tendency to absorption of Sulphur when the proofs are immersed
in the Bath. Consequently in many cases a sulphur toning-process is
set up, and as the picture is improved by it in appearance, losing its
brick-red colour and assuming a purple shade, it was at first adopted
by Photographers. Experience however has shown that colours brightened
in this way are less permanent than others, and are liable to fade
unless kept perfectly dry. Hence the process will be discarded by all
careful operators, and the object will be to avoid sulphuration as far
as possible. This can be done to a great extent, and, when the Bath is
properly managed, the prints will be toned almost entirely by Gold, and
will, with care, be permanent.

[20] For a more detailed account of the toning process by Sulphur, see the
Third Section of this Chapter, page 145. The instability of sulphuretted
prints is shown in the fourth Section.

Some of the conditions which facilitate a sulphuretting action upon the
proof are as follows:--

a. _The addition of an Acid to the Bath._--It was at one time common to
add a few drops of Acetic Acid to the fixing Bath of Hyposulphite of
Soda, immediately before immersing the proofs. The Bath then assumes an
opalescent appearance in the course of a few minutes, and, when this
milkiness is perceptible, the print begins to _tone_ rapidly and becomes
nearly black.

The chemical changes produced in a Hyposulphite Bath by addition of acid,
may be explained thus:--The acid first displaces the feeble Hyposulphurous
acid from its combination with Soda.

  Acetic Acid + Hyposulphite Soda.
  = Acetate Soda + Hyposulphurous Acid.

Then the Hyposulphurous Acid, _not being a stable substance when
isolated_, begins spontaneously to decompose, and splits up into
Sulphurous Acid--which remains dissolved in the liquid, communicating the
characteristic odour of burning Sulphur--and _Sulphur_, which separates in
a finely divided state and forms a milky deposit.[21]

[21] From the Vocabulary, Part III., it will be seen that commercial
Chloride of Gold usually contains _free Hydrochloric Acid_; hence
a considerable deposit of Sulphur takes place on adding it to the
Hyposulphite solution, and the liquid must not be used immediately.

Observe therefore that free acids of all kinds must be excluded from the
fixing Bath, or, if inadvertently added, the liquid must be set aside for
some hours until the Hyposulphurous Acid has decomposed, and, the Sulphur
having settled to the bottom, the Bath has regained its original neutral
condition.[22]

[22] The chemical reader will understand the decomposition of free
Hyposulphurous Acid by the following equation:--S{2}O{2} = SO{2} and S.

b. _Decomposition of the Bath by constant use._--It has long been known
that a solution of Hyposulphite of Soda undergoes a peculiar change in
properties when much used in fixing. When first prepared it leaves the
image of a red tone, the characteristic colour of the reduced Silver
Salt, but soon acquires the property of darkening this red colour by a
subsequent communication of Sulphur. Hence a simple fixing Bath becomes at
last an active toning bath, without any addition of Gold.

This change of properties will be found more fully explained in the
abstract of the Author's researches given in the next Section (p. 156). At
present we remark only that it is due principally to a reaction between
Nitrate of Silver and Hyposulphite of Soda, attended with decomposition
of Hyposulphite of Silver (p. 130); and hence, if the prints are washed
in water before immersion in the Bath, the solution will be less quickly
liable to change.

Many operators state that the toning Bath having at first been prepared
with Chloride of Gold, no further addition of this substance will be
required. This no doubt is correct, but in such case the proofs will at
last be toned by Sulphur more than by Gold, and will not possess the
same stability; the Bath will also, after long use, be found to acquire
a distinct _acid_ reaction to test-paper, the acidity being due to a
peculiar principle generated by decomposing Hyposulphite of Silver, and
which is shown to have an injurious action upon the print (p. 158). To
avoid this the solution should be kept _neutral to test-paper_ by means of
a drop of Ammonia, if required; and when it begins to be exhausted, and
does not tone (quickly) a print from which the free Nitrate of Silver has
been removed by washing, a fresh quantity of Chloride of Gold should be
added.

c. _Tetrathionate in the Hyposulphite Bath._--The Author has shown that
the Tetrathionates, which are analogous to the Hyposulphites, have an
active sulphuretting action upon Positive prints (see the papers in the
next Section). Very fine colours can be obtained in this way; but toning
by Sulphur having been proved to be wrong in principle, the formulæ given
in the first two editions of this Work have been omitted.[23]

[23] The preparation of a toning bath by Tetrathionate, without Gold, is
described in the next Section, but it is not recommended for practical use.

The bodies which produce Tetrathionate when added to a solution of
Hyposulphite of Soda, and hence are inadmissible in the toning process,
are as follows:--Free Iodine, Perchloride of Iron, Chloride of Copper,
Acids of all kinds (in the latter case the acid first produces Sulphurous
Acid, and the Sulphurous Acid, if present in any quantity, by reacting
upon Hyposulphite of Soda, forms Tetrathionate and Trithionate of Soda).

Chloride of Gold also produces a mixed Tetrathionate of Gold and Soda when
added to the fixing Bath (p. 133); but as the quantity of Chloride used
is small, the prints are far less sulphuretted than in the case of toning
Baths prepared by Tetrathionate without Gold.


SECTION III.

_The Author's Researches in Photographic Printing._

Having been long engaged in conducting experiments upon the composition
and properties of the reduced material forming the Photographic image, and
especially with a view of determining the exact conditions under which the
picture may be considered permanent, the Author has thought it advisable
to give the results of these researches in the form of an abstract of the
original papers read at the meetings of the Photographic Society.

A previous perusal of these papers will put the reader in possession of
the principal facts upon which are founded the precautions advised in the
next Section for the preservation of Photographic prints. In order to keep
the Work as nearly as possible within its original limits, and also for
the purpose of distinguishing the present Section from the others, as one
referring principally to scientific details, the type has been reduced to
the size of that used in the Appendix.


ON THE CHEMICAL COMPOSITION OF THE PHOTOGRAPHIC IMAGE.

The determination of the chemical nature of the Photographic image in
its various forms is a point of much importance, both as indicating the
conditions required for the preservation of works of art of that class,
and also as a guide to the experimenter in selecting bodies likely to have
an effect as chemical agents in Photography.

It has been stated by some who have given attention to the subject,
that the image is formed in all cases of pure metallic Silver, and that
any observable variations in its colour and properties, are due to a
difference in the molecular arrangement of the particles. But this
hypothesis, although involving much that is correct, yet does not contain
the whole truth, for it is evident that the chemical properties of the
Photographic image often bear no resemblance to those of a metal. One
Photograph may also differ essentially from another, so that we are led
to infer the existence of two varieties, the first of which is less of a
metallic nature than the second.

In investigating the subject, the principal point appeared to be to
examine the action of light upon Chloride of Silver, and afterwards
to associate the Chloride with organic matter in order to imitate the
conditions under which Photographs are obtained.

The following is an epitome of the conclusions arrived at:--

_Action of Light upon Chloride of Silver._--The process is accompanied
by a separation of Chlorine, but its product is not a mere mixture of
Chloride of Silver and Metallic Silver; if it were so, we cannot suppose
that the darkening would take place beneath the surface of Nitric Acid,
which it is found to do. A definite Subchloride of Silver seems to be
formed, the most important property of which is its decomposition by
fixing agents, such as Ammonia, and Hyposulphite of Soda, both of which
destroy the violet colour, dissolving out Protochloride of Silver, and
leaving a small quantity of a grey residue of metallic Silver.

Inasmuch therefore as all Photographic pictures require fixing, we may
conclude that if they could be produced upon pure and isolated Chloride
of Silver (which however is not the case), they would consist solely of
metallic Silver.

_Decomposition of organic Salts of Silver by Light._--Compounds of Oxide
of Silver with organic bodies, are as a rule darkened by exposure to
light, but the process does not always consist in a simple reduction to
the metallic state. This assertion is proved by the employment of the
following tests.

a. _Mercury._--Little or no amalgamation takes place on triturating the
darkened salt with this metal.

b. _Ammonia and fixing agents._--These usually produce only a limited
amount of action. Thus, the Albuminate of Protoxide of Silver is perfectly
soluble in Ammonia; but after having been reddened by exposure to light,
it is little or not at all affected.

c. _Potash._--Animal matters coagulated by Nitrate of Silver, and reduced
by the sun's rays, are dissolved by boiling Potash, the solution being
clear and of a blood-red colour. Metallic Silver, it is presumed, if
present, would remain insoluble.

d. _Boiling Water._--Gelatine treated with Nitrate of Silver and exposed
to light, loses its characteristic property of dissolving in hot water.
This experiment is conclusive.

The above facts justify us in supposing the existence of combinations of
organic matter with a low Oxide of Silver; and analysis indicates further
that the relative proportion of each constituent in these compounds may
vary. For instance, when Citrate of Silver is reduced by light, and acted
on with Ammonia, a black powder remains, which was found to contain as
much as 95 per cent, real Silver; but Albuminate of Silver treated in the
same way yields on analysis less of metallic Silver, and more volatile and
carbonaceous matter.

The use of _Ammonio_-Nitrate of Silver in preparing the salt tends also
to increase the relative quantity of metal left in the compound after
reduction and fixing. The length of time during which the light has acted,
has also a modifying effect of the same kind,--the product of reduction by
a powerful light being more nearly in the state of metal, and containing
less both of Oxygen and organic matter.

_Action of Light upon Chloride of Silver associated with organic
matter._--Photographs formed on Chloride of Silver alone, would, after
fixing, consist of metallic Silver, but such a process could not be
carried out in practice. The addition of organic matter is absolutely
necessary in order to increase the sensitiveness, and to prevent the
image from being dissolved in the Bath of Hyposulphite of Soda. The blue
Subchloride of Silver is decomposed by fixing, a very scanty proportion of
grey metallic Silver remaining insoluble; but the red compound of Suboxide
of Silver with organic matter is almost unaffected by Hyposulphite of
Soda, or Ammonia.

The increase of sensitiveness and intensity produced by the use of organic
matter is accompanied also by a change in the composition of the picture;
the image losing the metallic character which it possesses when formed on
pure Chloride of Silver, and resembling in every respect the product of
the action of light upon organic Salts of Silver.

There are certain characteristic tests which may usefully be employed in
distinguishing the metallic image from what may be termed the organic or
non-metallic image. One of these tests is Cyanide of Potassium. An image
formed upon pure Chloride of Silver, although pale and feeble, may, after
fixing, be immersed ill dilute solution of Cyanide of Potassium without
injury. But a photograph on Chloride of Silver supported by an organic
basis, is much acted upon by Cyanide of Potassium, quickly losing its
finer details.

A second test is the Hydrosulphate of Ammonia. If no organic matter be
employed, the image becomes darker and more intense by treatment with a
soluble Sulphuret; whilst the non-metallic image, formed on an organic
surface, is quickly bleached and faded. The action of Sulphur upon the
image is indeed a mode of determining the real quantity of Silver present.
When existing in a very finely divided layer, Sulphuret of Silver often
appears yellow; but in a thicker layer it is black. Hence the colour
of the Photograph, after treatment with Sulphuretted Hydrogen, is an
indication of the proportion of metal present, and the reason of the
organic image becoming so perfectly faded is because it contains a minimum
of Silver in relation to the intensity. We see, therefore, that the
addition of organic matter to Chloride of Silver does not so much increase
the actual quantity of Silver reduced by light, as it adds to its opacity
by associating other elements with the Silver, and altogether modifying
the composition of the image.

The employment of _oxidizing agents_ shows also that in an ordinary
Photographic process by the direct action of light, other elements besides
Silver assist in forming the image: the pictures being found to be easily
susceptible of oxidation, whereas the metallic image formed on pure
Chloride of Silver resists oxidation.

_Composition of_ developed _images._--By exposing sensitive
layers of the Iodide, the Bromide, and the Chloride of Silver to the light
for a short time only, and subsequently developing with Gallic Acid,
Pyrogallic Acid, and the protosalts of Iron, a variety of images may be
obtained, which differ from each other materially in every important
particular, and a comparison of which assists the determination of the
disputed point.

The appearance and properties of the developed Photograph are found to
vary with the existence of the following conditions.

1st. _The surface used to sustain the sensitive layer._--There is a
peculiarity in the image formed on _Collodion_. Collodion contains
Pyroxyline, a substance which behaves towards the salts of Silver in a
manner different from that of most organic bodies, exhibiting no tendency
to assist their reduction by light. Hence Chloride of Silver on Collodion
darkens far more slowly than the same salt upon Albumen, and the image,
after fixing, is feeble and metallic. Iodide of Silver on Collodion,
exposed and developed, gives usually a more metallic image, with less
intensity, than Iodide of Silver upon Albumen, or on paper sized with
Gelatine. By adding to the Collodion a body which has an affinity for low
oxides of Silver, such for instance as Glycyrrhizine, the opacity of the
developed image is increased.

2nd. _The nature of the sensitive salt._--When Iodide of Silver is used
to receive the latent impression, the image after development, although
lacking intensity of colour by reflected light, is more nearly in the
condition of metallic Silver than if Bromide or Chloride of Silver
be substituted; and of the three salts, the Chloride gives the most
intensity, with the least quantity of metallic Silver. This rule applies
especially when organic matters, Gelatine, Glycyrrhizine, etc., are
present.

3rd. _The developing agent employed._--An organic developing agent like
Pyrogallic Acid may be expected to produce a Collodion image more intense,
but less metallic, than an inorganic developer, such as the Protosulphate
of Iron.

4th. _The length of time during which the light has acted._--Over-action
of the light favours the production of an image which is dark by
reflection and brown or red by transmission, corresponding in these
particulars to what may be termed the non-metallic image containing an
oxide of Silver.

5th. _The stage of the development._--The red image first formed on the
application of the developer to a gelatinized or albuminized surface of
Iodide of Silver is less metallic, and more easily injured by destructive
tests, than the black image, which is the result of prolonging the action.
Developed photographs which are of a bright red colour after fixing,
correspond in properties to images obtained by the direct action of light
on paper prepared with Chloride of Silver, more nearly than to Collodion,
or even to fully developed Talbotype Negatives.

To conclude the Paper, the following may be offered in the way of
recapitulation:--An image consisting of metallic silver, as a rule,
reflects white light, and shows as a positive when laid on black velvet;
but a non-metallic organic image is dark, and represents the shadows of a
picture. Collodion positives developed with protosalts of Iron are nearly
or quite metallic. Photographs on Albumen or Gelatine less so than those
on Collodion. Developed Photographs contain more Silver than others, if
the development has been prolonged. The half shadows of the image in a
Positive Print are especially liable to suffer under injurious conditions,
since they contain the Silver in a less perfect state of reduction.[24]

[24] The Author omits, in this place, all mention of molecular conditions
affecting intensity, inasmuch as at the present time nothing positive
has been determined with regard to them. It is however known that in the
use of the protosalts of Iron as developing agents, the appearance of
the image is much influenced by the rapidity with which the reduction is
effected--the particles of Silver being larger and more metallic when the
development is conducted slowly. The process of electro-plating and other
chemical operations of a similar kind prove that the physical properties
of metals precipitated from solutions of their salts, vary greatly with
the degree of fineness and arrangement of their particles.


ON THE VARIOUS AGENCIES DESTRUCTIVE TO PHOTOGRAPHIC PRINTS.

_Action of Sulphuretting Compounds upon Positive Prints._--It was first
noticed by Mr. T. A. Malone, that the most intense Photograph might be
destroyed by acting upon it with solution of Sulphuretted Hydrogen or a
soluble Sulphuret, for a sufficient length of time.

The changes produced by a sulphuretting compound acting upon the red image
of a simply fixed print are these:--the colour is first darkened, and a
degree of brilliancy imparted to it; this is the effect termed "toning."
Then the warm tint by degrees alters to a colder shade, the _intensity_ of
the whole image is lessened, and the half-tones turn yellow. Lastly, the
full shadows pass also from black to yellow, and the print fades.

Now in this peculiar reaction we notice the following points of interest.
If at that particular stage at which the print has reached its maximum of
blackness, it be raised partially out of the liquid and allowed to project
into the air, the part so treated becomes yellow before that which remains
immersed. Again, if a print toned by Sulphur be placed in a pan of water
to wash, after the lapse of several hours it is apt to assume a faded
appearance in the half-tones. The full shadows, in which the reduced
Silver salt is thicker and more abundant, retain their black colour for
a longer time, but if the action of the sulphuretting Bath be continued,
every portion of the print becomes yellow.

These facts prove that _Oxygen_ has an influence in accelerating the
destructive action of the Sulphur compounds upon Positive prints; and this
idea is borne out by the results of further experiments, for it is found
that moist Sulphuretted Hydrogen has little or no effect in darkening the
colour when every trace of air is excluded. When prints are washed in
water they are exposed to the influence of the dissolved air which water
always contains, and hence the change from black to yellow is produced.[25]

[25] Further remarks upon the action of damp air upon Positives toned by
Sulphur are given at p. 153.

There are some substances which facilitate the yellow degeneration of
Positives toned by Sulphur, a knowledge of which will be useful: they
are--1st, powerful oxidizers, such as Chlorine, Permanganate of Potash,
and Chromic Acid; these, even when highly diluted, act with great
rapidity: 2nd, bodies which dissolve Oxide of Silver, as soluble Cyanides,
Hyposulphites, Ammonia; also _acids_ of various kinds, and hence the
frequency of yellow finger impressions upon old sulphuretted prints, which
are probably caused by a trace of organic (Lactic?) Acid left by contact
with the warm hand.

It was at one time supposed that the Photograph in the stage at which it
appears _blackened_ by Sulphur, consisted of Sulphuret of Silver, and that
this black Sulphuret became yellow by absorption of Oxygen and conversion
into Sulphate. MM. Davanne and Girard, who examined the subject, thought
that there might be two isomeric forms of Sulphuret of Silver, a black
and a yellow form; the former of which passing gradually into the latter
produced the fading of the impression. But neither of these views are
correct; for it is proved by careful experiment, that the Sulphuret of
Silver is a highly stable compound, not prone to oxidize, and, further,
that the change of colour from black to yellow has no reference to a
modification of this salt. The truth appears to be that the image whilst
in the black stage contains other elements besides Sulphur and Silver, but
when it has become yellow by the continued action of the sulphuretting
compound, it is then a true Sulphuret.

_Comparative permanence of Photographs under the action of
Sulphur._--_Developed_ Positives, as a rule, stand better than those
printed by direct exposure to light; but much depends upon the nature of
the negative process followed; and hence no general statement can be made
which will not be liable to many exceptions. The mode of conducting the
development must not be overlooked. The prints, which become very red in
the Hyposulphite fixing Bath from the action of the developer having been
stopped at too early a period, are often sulphuretted and destroyed even
more readily than a vigorous sun-print obtained by direct exposure to
light.

A point of even greater importance is _the nature of the sensitive
surface_ which receives the latent image. It is the print _developed upon
Iodide of Silver_ which especially resists sulphuration. In that case,
not only is the preliminary toning effect of the Sulphur more slow than
usual, but the impression cannot be made to fade by any continuance of the
action. It loses much of its brilliancy, and is reduced in intensity, but
it is not so completely destroyed as to be useless. The reason of this, as
shown in the last paper, depends upon the fact that the Talbotype proofs
contain the largest amount of Silver in the image.

The employment of Gold in toning does not render an ordinary sun-print
as permanent as a Positive developed upon Iodide of Silver. The deep
shadows of the picture are protected by the Gold, but the lighter shades
not so perfectly. Hence after the Sulphur has acted, in place of the
universal yellow and faded aspect presented by the simple untoned print,
the Positive fully toned by Gold has black shadows with yellow half-tones.
Therefore, whilst recommending the use of Gold as a toning agent, it does
not seem advisable to lay too much stress upon it as a preservative from
the destructive action of Sulphur.

_Exposure of Positive Prints to a Sulphuretting Atmosphere._--In testing
the action of a solution of Sulphuretted Hydrogen upon paper Positives,
it did not appear that the conditions under which the prints were placed
bore a sufficiently close resemblance to the case of Positives exposed
to an atmosphere contaminated with _minute traces_ of the gas; and this
more particularly because it is known that _dry_ Sulphuretted Hydrogen has
comparatively little effect upon Photographic Prints.

The experiments were therefore repeated in a somewhat different form.
A number of Positives (about three dozen) printed in various ways,
were suspended in a glass case, measuring 2-1/2 feet by 21 inches,
and containing 7-1/2 cubic feet of air; into which was introduced,
occasionally, a few bubbles of Sulphuretted Hydrogen, just sufficient to
keep the air of the chamber smelling perceptibly of the gas. A polished
Daguerreotype plate was hung up in the centre, to serve as a guide to the
progress of the sulphuretting action.

By the second day the metal plate had acquired a faint yellow hue,
not easily seen except in certain positions; but the Positives were
unaffected. At the expiration of three days the majority of the pictures
exhibited no signs of change, but a few untoned prints of a pale red
colour, some of which had been printed by development, and others by
direct exposure to light, had perceptibly darkened.

After the eighth day, the action, appearing to progress more slowly
than at first, was stopped, and the prints removed. The general results
obtained were as follows:--

The Daguerreotype plate had become strongly tarnished with a film of
Sulphuret of Silver, which appeared yellowish-brown in some parts and
steel-blue in others. The Positives were, as a rule, toned to a slightly
colder shade, but many of them had scarcely changed.

No obvious difference was observed between prints developed on paper
prepared with Chloride of Silver, and others printed by direct exposure to
light; but in all cases the prints obtained by those methods which give a
very red image after fixing, were the first to show the change of colour
due to sulphuration, the proofs submitted to the test having all been
previously toned with Gold.

Effect of Oxidizing Agents upon Positive Prints.--It appeared of
importance to ascertain to what extent Photographic Prints are susceptible
of oxidation; on account of the atmospheric influences to which they are
necessarily exposed. In experimenting upon this subject the following
results have been obtained.

Powerful oxidizers destroy Positive Prints rapidly; the action usually
commencing at the corners and edges of the paper, or at any isolated
point, such as a metallic speck or particle of extraneous matter, which
can serve as a centre of chemical action. This same fact is often noticed
in the fading of Positives by long keeping, and therefore since other
destructive actions (with the exception of that of Chlorine) do not appear
to follow the same rule, it is an argument in addition to others which can
be adduced, that Photographic Prints are frequently destroyed by oxidation.

Air which has been _Ozonized_ by Phosphorus, and in which blue
litmus-paper becomes reddened, quickly bleaches the Positive image. Oxygen
gas, obtained by voltaic decomposition of acidified water and which should
contain Ozone, did not appear to have an equal amount of effect, the
action being comparatively slight, or altogether wanting.

_Peroxide of Hydrogen_ obtained in solution, and in conjunction with
Acetate of Baryta, by adding Peroxide of Barium to dilute Acetic Acid,[26]
bleaches darkened Positive paper; but the effect is slow, and does not
take place to a very perceptible extent if the liquid be kept alkaline to
test-paper.

[26] Hydrochloric Acid, which is usually recommended in place of Acetic
Acid, cannot be employed in this experiment; it seems to cause a
liberation of free Chlorine, which bleaches the print instantly.

Nitric Acid applied in a concentrated form acts immediately upon the
darkened surface, bleaching every part of the print with the exception
of the bronzed shadows, which usually retain a slight residual colour. A
solution of Chromic Acid is still more active. This liquid may usefully be
applied to distinguish prints toned by Sulphur from others toned by Gold;
the presence of metallic Gold protecting the shadows of the picture in
some measure from the action of the acid. The solution should be prepared
as follows:--

  Bichromate of Potash                  6 grains.
  Strong Sulphuric Acid                 4 minims.
  Water                                12 ounces.

A solution of Permanganate of Potash is an energetic destroyer of paper
positives; and, as it is a neutral substance, may conveniently be employed
in testing the relative capability of withstanding oxidation possessed
by different Photographic Prints. The solution should be dilute, of a
pale pink hue, and the Positives must be moved occasionally, as the first
effect is to decolorize a great portion of the liquid, the Permanganate
oxidizing the size and organic tissue of the paper. After an immersion
of twenty minutes to half an hour, varying with the degree of dilution,
the half-tones of the picture begin to die out, and the full shadows
become darker in colour; the bronzed portions of the print withstand the
action longer, but at length the whole is changed to a yellow image much
resembling in appearance the Photograph faded by Sulphur.

_Comparative permanence of Photographs treated with Permanganate of
Potash._--Developed prints prepared by a Negative process withstand the
action better than others. But to this rule there are exceptions; much
depending upon the time of exposure to light, and the extent to which
the development is carried. Those prints which, being exposed for a
short time, and afterwards strongly developed, become dark in colour and
vigorous in outline, are more permanent than others which having been
over-exposed and under-developed, lose their dark colour and become red
and comparatively faint in the Hyposulphite fixing Bath.

Positives developed upon a surface of _Chloride_ of Silver on plain paper
do not resist the oxidizing action so perfectly as those on Iodide of
Silver. Prints developed upon paper prepared with Serum of Milk containing
Caseine stand better than those on plain paper.

Of prints obtained by the ordinary process of direct exposure to light,
those on plain paper are the first to fade, the oxidizing action being
most seen upon the _half-tones_. The use of _Albumen_ gives a great
advantage. Developed prints on Albumen stand far better than the same
upon plain paper; and even the Albuminized sun prints are less injured by
the Permanganate than the best of the Negative prints prepared without
Albumen. Caseine has the same effect, but to a less extent; and as Serum
of Milk almost invariably contains uncoagulated Caseine, its efficacy is
thus explained.

The manner of toning the print is a point of importance; previous
sulphuration in an old Hyposulphite Bath always facilitating the oxidizing
action.

_Action of Chlorine upon Positive Prints._--Aqueous solution of Chlorine
destroys the Photographic image, changing it first to a violet tint
(probably Subchloride), and subsequently obliterating it by conversion
into white Chloride of Silver. The impression, although invisible,
remains in the paper, and may be developed in the form of yellow or brown
Sulphuret of Silver by the action of Sulphuretted Hydrogen. It also
becomes visible on exposure to light, and assumes considerable intensity
if the paper be previously brushed with free Nitrate of Silver. Sulphate
of Iron produces no effect upon the invisible image of Chloride of Silver;
but Gallic or Pyrogallic Acid, rendered alkaline by Potash, converts it
into a black deposit.

The Action of Chlorine water usually commences at the edges and corners of
the print, in the same manner as that of oxidizing agents. The proofs upon
Albumen are the least readily injured, and next, those developed on Iodide
of Silver.

_Hydrochloric Acid._--The liquid acid of sp. gr. ·116, even when free from
Chlorine, acts immediately upon the half-tones of a positive print, and
destroys the full shadows in the course of a few hours; a slight residual
colour however usually remains in the darkest parts. The prints developed
on Iodide of Silver are the most permanent.

_Sulphuric, Acetic Acids, etc._--Acids of all kinds appear to exert an
injurious influence upon Positive prints, and especially so upon the
half-tones of the image, the effect varying with the strength of the acid
and the degree of dilution with water. Even a vegetable acid like Acetic
gradually darkens the colour and destroys partially or entirely the faint
outlines of the picture.

_Bichloride of Mercury._--The most important particulars relating to
the action of this test upon Photographs are well known. The image is
ultimately converted into a white powder, and hence, in the case of a
Positive print, it becomes invisible; immersion in Ammonia or Hyposulphite
of Soda however restores it in a form often resembling in tint the
original impression. A point worthy of note is the protective effect of a
deposit of Gold, which is very marked, the proof, after toning, resisting
the action of the Bichloride for comparatively a long time.

_Ammonia._--The effect of Ammonia upon a print is rather to _redden_ the
image than to destroy it; the half-tones become pale and faint, but they
do not disappear. Toning with Gold enables the proof to resist the action
of the strongest solution of Ammonia, and hence Ammonia may safely be
employed as a fixing agent after the use of the Sel d'or Bath.

_Hyposulphite of Soda._--A concentrated solution of Hyposulphite of Soda
exercises a gradual solvent action upon the image of Photographic Prints,
at the same time tending to communicate Sulphur and to darken the colour
of the impression. A faint yellow outline of Sulphuret of Silver usually
remains after the solution of the image is completed.

Developed prints of all kinds, but in particular the Talbotype proofs
upon Iodide of Silver, are less readily dissolved by Hyposulphite of Soda
than those obtained by the direct action of light. There is also a slight
difference between plain and Albuminized prints, which is in favour of the
former, the albuminized paper always losing somewhat more by immersion in
the Hyposulphite Bath than plain Chloride paper sensitized by Nitrate of
Silver.

_Cyanide of Potassium._--The solvent action of Cyanide of Potassium is
most energetic upon Photographs formed on paper. These images, whether
developed or not, do not withstand the test so well as the impressions on
Collodion. Albuminized proofs are also somewhat more easily affected than
prints on simple Chloride paper sensitized with Nitrate or Ammonio-Nitrate
of Silver.

_Heat, moist and dry._--Long-continued boiling in distilled water has
a reddening action upon Positive Prints. The image becomes at length
pale and faint, resembling a print treated with Ammonia before toning.
A deposit of Gold upon the image lessens, but does not altogether
neutralize, the effect of the hot water. If the boiling be long continued,
the violet-purple tone often imparted by the Gold invariably gives place
to a chocolate-brown, which appears to be the most permanent colour.
Prints _developed_ by Gallic Acid upon paper prepared with Serum of Milk
or with a Citrate, suffer as much as others obtained by direct action of
light. Ammonio-Nitrate prints on highly salted paper, which become nearly
black when toned with Gold, retain their original appearance the most
perfectly; a slight diminution of brightness being the only observable
difference after long boiling in water. Albumen proofs, and prints on
English papers, or foreign papers prepared with Serum of Milk, Citrates,
Tartrates, or any of those bodies which _redden_ the reduced Salt, are,
as a rule, rendered lighter in colour, and pass from purple to brown when
boiled in water.

Dry heat has an opposite effect to that of hot water, usually _darkening_
the colour of the image. On exposing a plain paper print simply fixed, and
thoroughly freed from Hyposulphite of Soda by washing, to a current of
heated air, it changes gradually from red to dark brown, in which state
it continues until the temperature rises to the point at which the paper
begins to char, when it resumes its original red tone, becoming at the
same time faint and indistinct.

_The Products of Combustion of Coal-gas a cause of Fading._--Coal-gas
contains Sulphur compounds, which in combustion are oxidized into
Sulphurous and Sulphuric Acids; other substances of a deleterious nature
may also be present. A plate of polished silver suspended in a glass tube,
through which was directed the current of heated air rising from a small
gas jet, became tarnished with a white film in the course of twenty-four
hours. Positive prints exposed to the same, absorbed moisture and faded;
the action resembling that of oxidation, in being preceded by a general
darkening in colour. Of four prints exposed, an Iodide-developed print was
the least injured, and next, a print upon Albuminized paper.


ON THE ACTION OF DAMP AIR UPON POSITIVE PRINTS.

In order to ascertain this point, more than six dozen Positives, printed
on every variety of paper, were mounted in new and perfectly clean
stoppered glass bottles, at the bottom of each of which was placed a
little distilled water, to keep the contained air always moist. They were
removed at the expiration of three months, having been kept during that
time, some in the dark, and others exposed to the light. As the prints
were prepared by various methods, toned in different ways, and mounted
with or without substances likely to exercise a deleterious action, this
series of experiments will possess considerable value in determining some
of the intrinsic causes of fading of Positives.[27]

[27] For a more detailed account of the experiments, see the original
paper in the 'Photographic Journal,' vol. iii.

The general results obtained were as follows:--Positives which had been
_simply fixed_ in Hyposulphite of Soda remained quite uninjured. Whether
developed by Gallic Acid on either of the three Salts of Silver usually
employed, or printed by direct action of light, the result was the same.
Hence we may infer that the darkened material which forms the image of
Photographic Prints does not readily oxidize in a damp atmosphere.

_Toned_ Positives were found in many cases to be less permanent than
Positives simply fixed. This was especially the case when the toning had
been effected by _Sulphur_; all the sulphuretted prints, fixed in solution
of Hyposulphite which had been long used, became yellow in the half-tones
when exposed to moisture. Positives fixed and toned in Hyposulphite
containing Gold were variously affected; some prepared when the solution
was in an active state being unchanged, others losing a little half-tone,
and others, again, fading badly. These latter were prepared in a Bath
which had lost Gold and acquired sulphuretting properties; and it was
noticed that they were more injured by the action of boiling water than
those Positives which proved to be permanent under the influence of the
moisture.

Toning by means of Chloride of Gold appeared to be highly satisfactory,
but the number of prints operated upon was small. The Sel d'or process
also did not injure the integrity of the image, no commencing yellowness
or bleaching of half-tones being visible after exposure to the moist air.

This series of experiments confirmed the statement made in a former paper,
that some tints obtained in Positive printing are more permanent than
others. Violet tones produced by Sulphur invariably passed into a dull
brown by the action of the moist air; and even when Gold was employed
in toning, these same purple colours were usually _reddened_. This was
especially the case when English papers were used, or foreign papers
re-sized with Serum of Milk containing Caseine. The chocolate-brown tints
which best stand the action of boiling water, and in particular those upon
Ammonio-Nitrate paper, were least affected by the damp air; and indeed it
was evident that the two agents, viz. moist air and hot water, acted alike
in tending to _redden_ the print, although the latter did so in the most
marked manner.

It seemed also, from the results of these experiments, to be a point of
great importance that the size should be removed from the print in order
to render it indestructible by damp air. This was evidently seen in two
cases where Positives, toned in an old Hyposulphite and Gold Bath, were
divided into halves, one of which was treated with a strong solution of
Ammonia. The result was that the halves in which the size was allowed
to remain, faded, whilst the others were comparatively uninjured. The
Albumen proofs especially suffered when the size was left in the paper, a
destructive mouldiness forming, and fading the picture. The use of boiling
water obviated this, and the prints so treated remained clean and bright.
A partial decomposition of Albumen however occurred in some cases even
when hot water was used, the gloss disappearing from the paper in isolated
patches. With _Caseine_ substituted for Albumen there was also a loss of
half-tone; thus seeming to indicate that both these animal principles,
although stable under ordinary conditions, will, even when coagulated by
Nitrate of Silver, decompose if kept long in a moist state.

The use of improper substances for mounting proved to be another
determining cause of fading by oxidation. Those bodies which combine
with Oxide of Silver, are likely upon theoretical grounds to destroy the
half-tones of the image; and it was found, that if the picture were left
in contact with Alum, Acetic Acid, etc., or with the substances which
generate an acid by fermentation, such as paste or starch, it invariably
faded.

The supposed accelerating influence of _Light_ upon the fading of
Positives was not confirmed by these experiments, as far as they extended.
Many of the bottles containing the Photographs were placed outside the
window of a house with a southern aspect during the whole of the three
months with the exception of two or three weeks, but no difference
whatever could be detected between Positives so treated and others kept
in total darkness. It will be proper however that this part of the
investigation should be repeated, allowing a longer time.

An examination of the various modes employed for coating Positives, in
order to exclude the atmosphere, showed that many of them were not fitted
to fulfil the purpose intended. Waxed prints faded quite as much when
exposed to moisture as others not waxed. White wax is a substance often
adulterated, and Oil of Turpentine has been shown to contain a body
resembling Ozone in properties, and possessing the power of bleaching a
dilute solution of Sulphate of Indigo. Spirit varnish applied to the
surface of the picture after re-sizing with Gelatine was plainly superior
to white wax, but nevertheless it did not obviate the fading effect of the
moisture upon an unstable Positive which had been toned by sulphuration.
Its protective influence is therefore limited.


ON THE CHANGE IN COMPOSITION WHICH HYPOSULPHITE OF SODA EXPERIENCES BY USE
IN FIXING PAPER PROOFS.[28]

[28] These observations are condensed and re-arranged from the papers
published by the Author in the 'Photographic Journal' for September and
October, 1854.

It was remarked by Photographers at an early period that the properties of
the Fixing Bath of Hyposulphite of Soda became altered by constant use;
that it gradually acquired the power of _darkening_ the colour of the
Positive image. This change was at first referred to the accumulation of
_Salts of Silver_ in the Bath, and hence directions were given to dissolve
a portion of blackened Chloride of Silver in the Hyposulphite in preparing
a new solution.

Careful experiments performed by the Author convinced him that an error
had been entertained; since it was found that the simple solution of
Chloride of Silver in Hyposulphite of Soda had no power of yielding
the black tones. But it afterwards appeared that if the fixing Bath,
containing dissolved Silver Salts, were set aside for a few weeks, a
_decomposition_ occurred in it, evidenced by the formation of a black
deposit of Sulphuret of Silver; and _then_ it became active in toning the
proofs.

The presence of this deposit of Sulphuret of Silver indicated that a
portion of Hyposulphite of Silver had spontaneously decomposed, and,
knowing the products which are generated by the spontaneous decomposition
of this salt, a clue to the difficulty was afforded. One atom of
Hyposulphite of Silver includes the elements of one of Sulphuret of Silver
and one of Sulphuric Acid. Sulphuric Acid in contact with Hyposulphite of
Soda produces _Sulphurous Acid_ by a process of displacement; and Plessy
has shown that Sulphurous Acid reacts upon an excess of Hyposulphite
of Soda, forming two of that interesting series of Sulphur compounds
designated by Berzelius the "Polythionic Acids."

It appeared therefore probable, upon theoretical grounds, that the Penta-,
Tetra-, and Trithionates might produce some effect in the Hyposulphite
fixing Bath. Upon making the trial these expectations were verified; and
it was found that Tetrathionate of Soda added to Hyposulphite of Soda
yielded a fixing and toning Bath quite equal in activity to that produced
by means of Chloride of Gold.

It may be useful to review for an instant the composition of the
Polythionic series of acids; it is thus represented:--

                                   Sulphur.  Oxygen.  Formulæ.
  Dithionic or Hyposulphuric Acid  2 atoms   5 atoms  S{2}O{5}
  Trithionic Acid                  3   "     5   "    S{3}O{5}
  Tetrathionic Acid                4   "     5   "    S{4}O{5}
  Pentathionic Acid                5   "     5   "    S{5}O{5}

The amount of _Oxygen_ in all is the same, that of the other element
increases progressively; hence it is at once evident that the highest
member of the series might _by losing Sulphur_ descend gradually until it
reached the condition of the lowest.

This transition is not only theoretically possible, but there is an actual
tendency to it, all the acids being unstable with the exception of the
Hyposulphuric. The Alkaline Salts of these acids are more unstable than
the acids themselves; a solution of Tetrathionate of Soda becomes milky in
the course of a few days from deposition of Sulphur, and, if tested, is
then found to contain _Tri_thionate and eventually _Di_thionate of Soda.

The cause of the change in properties of the fixing Bath being thus
clearly traced to a decomposition of Hyposulphite of Silver, and a
consequent generation of unstable principles capable of imparting Sulphur
to the immersed proofs, it seemed desirable to continue the experiments.--

There is a peculiar _acid condition_ commonly assumed by old fixing
Baths, which could not be satisfactorily explained, since it was known
that acids do not exist long in a free state in solution of Hyposulphite
of Soda, but tend to neutralize themselves by displacing _Hyposulphurous
Acid_ spontaneously decomposable into Sulphurous Acid and Sulphur. This
point is set at rest by the discovery of a peculiar reaction which takes
place between certain salts of the Polythionic acids and Hyposulphite of
Soda. A solution of Tetrathionate of Soda may be preserved for many hours
unchanged; but if a few crystals of Hyposulphite of Soda be dropped in, it
begins very shortly to deposit Sulphur, and continues to do so for several
days. At the same time the liquid acquires an acid reaction to test-paper,
and produces effervescence on the addition of Carbonate of Lime.

It is evident that a Sulphur acid exists which has not hitherto been
described, and that this acid is formed as one of the products of the
decomposition of the Hyposulphite of Silver contained in the fixing Bath.
The subject is an important one to Photographers, because it is found that
Hyposulphite Baths which have acquired the acid reaction, although toning
quickly, yield Positives which fade on keeping. The acid may perhaps
combine with the reduced Silver Salt, which, if the image be allowed to
contain Suboxide of Silver, is theoretically probable.

The experiments were next directed towards ascertaining more carefully the
effect of the acid fixing Bath upon the Positive proofs. Tetrathionate of
Soda added to solution of Hyposulphite of Soda produces, at the expiration
of twelve hours, a liquid which, when filtered from the deposited Sulphur,
reddens blue litmus-paper slowly. Positive prints immersed in the Bath
pass from red to black, dissolving in the half-tones, and becoming yellow
and faded if the action be too long continued. On adding Carbonate of Soda
in quantity sufficient to remove the acid reaction, the power of toning
is much diminished, but dark colours can still be obtained by continuing
the action. The solvent effect upon the half-tones, evidently caused in
great measure by the acid, is lessened; whilst the tendency to yellowness
in the white parts of the proof, almost disappears. These effects are
more particularly manifested when the prints are immersed in the Bath
immediately on their removal from the printing frame; and it is found
almost impossible to preserve the whites of the impression clear, in the
acid Bath, unless the Nitrate of Silver has been washed away.

Solution of half-tones and yellowness in the lights, both a source of
annoyance to the operator, are thus traced in great measure to an acid
condition of the fixing and toning Bath; and the remedy is obvious.

The Author's experiments upon the Tetrathionates and their reaction with
Hyposulphite of Soda likewise elicited the important fact that _alkalies_
decompose the unstable sulphuretted principle. If the Bath be treated
with Potash or Carbonate of Soda, an alkaline _Sulphuret_ appears to be
gradually formed, which precipitates Sulphuret of Silver, and in the
course of a few days the liquid returns to its original condition and
ceases to act as a toning agent upon the proof. The same effect takes
place to a great extent when the solution is set aside for several weeks
or months; a process of spontaneous change going forward, which issues in
a deposition of Sulphur and Sulphuret of Silver, and a partial loss of
sulphuretting properties in the liquid.

It may be interesting to the scientific investigator to describe the mode
of preparing a fixing and toning Bath, illustrating the above remarks:--

  Take of Nitrate of Silver                3 drachms.
  Hyposulphite of Soda                     4 ounces.
  Water                                    8 ounces.

Dissolve the Nitrate of Silver in 2 ounces of the water, then from the
total quantity of Hyposulphite of Soda, weigh out

  Hyposulphite of Soda                     2 drachms;

dissolve this likewise in 2 ounces of water, and the remainder of the
Hyposulphite in the other 4 ounces. Then, having the three solutions in
separate vessels, pour the Nitrate of Silver at once into the 2-ounce
solution of Hyposulphite, agitating the precipitated Hyposulphite of
Silver rapidly. In a short time it will begin to decompose, passing from
white to canary-yellow, and then to orange-yellow. _When the orange-yellow
begins to verge towards brown_, add the 4-ounce concentrated solution of
Hyposulphite, which will at once complete the decomposition, a part of the
precipitate dissolving and the remainder becoming perfectly black. After
filtering out the black Sulphuret of Silver, the solution is ready for use.

A Bath prepared by this formula is not usually very active, but it shows
clearly the process by which an ordinary fixing Bath may be converted into
a toning Bath by the immersion of positives having free Nitrate of Silver
upon the surface.

The following formula is more economical and gives a better result, but
it cannot be used for "Ammonio-Nitrate" prints; the addition of an alkali
precipitating Sulphuret of Iron.

  Strong solution of Perchloride of Iron     6 fluid drachms.
  Hyposulphite of Soda                       4 ounces.
  Water                                      8 ounces.
  Nitrate of Silver                         30 grains.

Dissolve the Hyposulphite of Soda in seven ounces of the water, the
Nitrate of Silver in the remaining one ounce; then pour the Perchloride of
Iron into the solution of Hyposulphite, by degrees, stirring all the time.
The addition of the Iron Salt strikes a fine purple colour, but this soon
disappears. When the liquid has become again colourless, which it does
in a few minutes, add the Nitrate of Silver, stirring briskly. Perfect
solution will take place without any formation of black Sulphuret.

A toning Bath prepared with Chloride of Iron will be ready for use twelve
hours after mixing, but it will be more active at the expiration of a
week. The solution is acid to test-paper, and _milky_ from a deposit of
Sulphur, which must be filtered out.

The Perchloride of Iron should be prepared by boiling Peroxide of
Iron with Hydrochloric Acid, in preference to dissolving Iron wire in
Aqua-Regia.

The addition of the Nitrate of Silver is made in order to produce a
portion of Hyposulphite of Silver in the bath; the presence of a Silver
Salt having been found to modify the tint of the Positives, and to prevent
their quickly turning yellow.


SECTION IV.

_On the Fading of Photographic Prints._

For many years subsequent to the discovery of the process of Photographic
Printing by Mr. Fox Talbot, it was not generally known that pictures so
produced were easily susceptible of injury from various causes, and in
particular from traces of the _fixing-agent_ remaining in the paper.
Hence, due care not being taken in the proper cleansing and preservation
of the proofs, the majority of them faded.

This matter became at last one of such importance that the Council of the
Photographic Society decided upon forming a Committee for the purpose of
examining the subject. The Author was honoured by being placed upon this
Committee, and the researches of which an abstract has been given in the
previous Section, were undertaken at the request of the Society.

The present Section is intended to explain practically and in a
concise manner the causes of the fading of Photographic Prints, and
the precautions which should be taken to ensure their permanency. The
chemistry of the subject having been fully explained in the last Section,
it will suffice to refer the reader to its pages for more detailed
information.

_Historical evidence of the permanence of Photographs._--It is a point of
interest to collect information as to the existence of old Photographs
which have remained many years unchanged. There are numerous instances
of Positives printed more than ten years ago, which have not perceptibly
altered up to the present time. These prints are mostly on plain paper,
Albumen not having come into use at so early a date. The general
impression of practical operators however is, that fading has occurred
less frequently since the introduction of Albuminized paper.

Positives printed by development on paper prepared by Talbot's method
seem as a rule to have stood remarkably well, and instances of Talbotype
Negatives having faded are rare.

Of the prints which have proved to be permanent, some are red or brown in
colour, but many, being of a dark or purple shade, have evidently been
toned, although not with Gold, the use of which was unknown to the earlier
Photographers.

It is plain from data thus collected, that Photographs do not necessarily
fade by time; and the fact that in one and the same portfolio are
constantly seen prints which appear permanent, and others in an advanced
state of change, cannot but lead to the inference that the main causes of
deterioration are intrinsic, depending upon some injurious matters left in
the paper; which is confirmed by experiment.

_Causes of fading._--The Author believes that the fading of Photographic
Prints may almost invariably be referred to one or other of the following
conditions:--

a. _Imperfect washing._--This is perhaps the most important of all,
and the most frequent. When Hyposulphite of Soda is allowed to remain
in the paper, even in minute quantity, it gradually decomposes, with
liberation of Sulphur, and destroys the print in the same way and quite
as effectually as a solution of Sulphuretted Hydrogen or an alkaline
sulphuret.

Imperfect washing may be suspected, if the Photograph, within a few months
from the date of its preparation, _begins to get darker in colour_: the
_half-tints_, which are the first to show the action, afterwards passing
into the yellow stage, whilst the dark shadows remain black or brown for a
longer time.

The proper mode of washing Photographs is sometimes misunderstood. The
length of time during which the print lies in the water is a point of
less importance, than that the water should be continually changed. When
a number of Positives are placed together in a pan, and a tap turned upon
them, the circulation of fluid does not necessarily extend to the bottom.
This is proved by the addition of a little colouring matter, which shows
that the stream flows actively above, but at the lower part of the vessel,
and between the prints, there is a stationary layer of water which is of
little use in washing out the Hyposulphite. Care should therefore be taken
that the pictures are kept as far as possible separate from each other,
and when running water cannot be had, that they are frequently moved and
turned over, fresh water being constantly added. When this is done, and
especially if the _size_ be removed from the paper in the manner presently
to be advised, _four or five hours_ washing will be sufficient. It is a
mistake to allow the pictures to remain in the water for several days;
which produces no good effect, and may tend to encourage a putrefactive
fermentation, or the formation of a white deposit upon the image when the
water contains Carbonate of Lime.

b. _Acid matters left in the Paper._--Upon examining collections of old
Photographs, it is not uncommon to find prints which are stated to have
remained unaltered for a long time after their first production, but
in the course of time to have lost their brilliancy, and become pale
and indistinct. This kind of fading often commences at the corners and
edges of the paper, and works inwards towards the centre. The Author's
experiments have shown that it is principally caused by a slow process of
_oxidation_.

The Photographic Image does not appear readily susceptible of oxidation
unless it be previously darkened by the action of Sulphur, or placed in
contact with acids or bodies which act as solvents of Oxide of Silver
(p. 146). The materials often used in sizing papers, such as Alum and
Resin, being of an acid nature, are directly injurious to the image;
and the removal of the size, which may easily be effected by means of a
dilute alkali or an alkaline carbonate, without injury to the tint, has
the additional advantage of carrying out the last traces of Hyposulphite
of Soda, and also the germs of _fungi_, which if allowed to remain would
vegetate and produce a destructive mouldiness on exposure to damp (Chap.
III. Part II.).

The fact that acids facilitate oxidation of the image is likewise a hint
that Photographic Prints should not be handled too frequently, or touched
with the finger more than is necessary; the warm hand may leave behind a
trace of acid[29] which would tend in time to produce a yellow mark.

[29] The writer has seen blue litmus-paper immediately reddened by being
laid upon the arm of a person suffering from acute Rheumatism. This acid
is probably Lactic Acid!

c. _Moisture as a cause of fading._--Although. Photographs properly
printed are not readily injured by damp air (p. 153), yet as there are
_impurities_ of various kinds constantly floating in the atmosphere,
a state of comparative dryness may be said to be essential to the
preservation of all Photographs. In collecting evidence upon the subject,
"wet" and "damp" are frequently alleged as having been causes of
fading--the prints were hung against a damp wall during frosty weather,
in a room without a fire: or the rain had been allowed to penetrate the
frame! No pictures will long survive such treatment, and Photographs, like
engravings and water-colour paintings, require common care to be exercised
in their preservation.

d. _The modes of Mounting the Proof._--This subject has been alluded to in
the abstract of the Author's papers at p. 155. All cements which are of an
acid nature, or which are liable to become _sour_ by acetous fermentation,
should be avoided. Flour paste is especially injurious, and many cases
of fading have been traced to this cause. The addition of Bichloride of
Mercury, which is often made to prevent the paste from becoming mouldy,
would still more unfit it for Photographic use (p. 151). Starch is not
much preferable. No substance appears better than Gelatine, which does not
readily decompose, and shows no tendency to absorb atmospheric moisture.
The _deliquescent_ nature of many bodies is a point which should be
borne in mind in mounting Photographs, and hence the use of a salt like
_Carbonate of Potash_, which the writer has known to be added to paste to
prevent the formation of acid, would be unadvisable.

e. _The effect of Imperfect Fixation._--The earlier Photographers did not
always succeed in properly fixing their prints, since old Photographs are
often found thickly studded with spots and blotches in the tissue of the
paper. These prints however are not invariably faded upon the surface,
and hence it cannot be said that imperfect fixation will certainly end
in the total destruction of the picture. Still a notice of the subject
may properly be introduced in this place, and the attention of the reader
be once more drawn to the importance of washing the print in water on
removing it from the printing frame; a decomposition invariably occurring
when paper Positives _saturated with free Nitrate of Silver_ are plunged
in a dilute solution of Hyposulphite of Soda, containing an insufficient
quantity of the salt to dissolve away the Hyposulphite of Silver before it
begins to undergo spontaneous change.

f. _Exposure to an impure Atmosphere as a cause of Fading._--The five
causes of fading which precede, have mostly reference to an intrinsically
faulty condition of the print. This, the sixth, explains the mode in which
a Photograph carefully prepared may yet suffer injury from deleterious
matters often present in the atmosphere. The air of large cities, and
particularly that emanating from sewers and drains, contains Sulphuretted
Hydrogen, and hence articles of silver-plate become tarnished unless
placed beneath glass. The injury which a print sustains by exposure to
air contaminated with Sulphuretted Hydrogen, is less than the tarnish
produced upon the bright surface of a silver plate (see p. 148); but it
is recommended as a precautionary measure, that Photographic Pictures be
protected by glass or kept in a portfolio, and that they be not exposed
too freely to the air.

The products of the combustion of coal-gas are probably more likely than
the cause last named, to be a source of injury to Photographs suspended
without any covering. The sulphur compounds in gas burn into Sulphurous
and Sulphuric Acids, the latter of which, in combination with Ammonia,
produces the sparkling crystals often observed upon the shop windows.

The question as to the manner in which the Photographic Image may best be
protected from these extraneous causes of fading has been mooted, and many
plans of coating prints with some impervious material have been devised.
If the pictures are to be glazed or kept in a portfolio, this of itself
will be sufficient, but in other cases it may perhaps be useful to apply a
layer of spirit or gutta-percha varnish. The use of wax, resin, and such
bodies is likely, by introducing impurities, to act injuriously rather
than otherwise.

g. _Decomposition of Pyroxyline a source of Injury to Collodion
Photographs._--Collodion Positives and Negatives are usually esteemed
permanent; but some have been exhibited which, having been put away in a
damp place, gradually became pale and indistinct. The change commences at
rough edges and isolated points, leaving the centre, as a rule, the last
affected. On examination, numerous cracks are often visible, thus seeming
to indicate that the Collodion film has undergone decomposition. The
result of this would be the liberation of corrosive Oxides of Nitrogen,
which destroy the image. Substitution compounds containing Peroxide of
Nitrogen are known to be liable to spontaneous change. The bitter resin
produced by acting upon white sugar with Nitro-Sulphuric Acid, if not kept
perfectly dry, will sometimes evolve enough gas to destroy the cork of the
bottle in which it is kept; the solution of the resin has then a strong
acid reaction, and rapidly fades an ordinary Positive Print.

These facts are interesting, and indicate that Collodion Pictures,
containing in themselves the elements of their destruction, should be
protected from moisture by a coating of varnish.

_Comparative Permanence of Photographic Prints._--There is every reason
to think that the Photographic Image, however formed, is permanent, if
certain injurious conditions are avoided;--in other words, that prints do
not necessarily fade, in the same manner as fugitive colours, by a simple
exposure to light and air. But supposing a case, which is the common
one, of injurious influences which cannot altogether be removed, it may
be useful to inquire what mode of printing gives the greatest amount of
stability.

Positives produced by a short exposure to light and subsequent
development with Gallic Acid, may be expected to be more permanent than
ordinary sun-prints; not that there is any reason to suppose the chemical
composition of a developed image to be peculiar, but that the use of the
Gallic Acid enables us to increase the intensity of the red picture first
formed, and to add to its stability by precipitating fresh Silver upon
it. This point has not always been attended to. It has been recommended
to remove the print from the developing solution whilst in the _red_ and
early stage of development, and to produce the dark tones subsequently by
means of gold; but this plan, although giving very good results as regards
colour and gradation of tone, appears to lessen the advantage which would
otherwise accrue from the adoption of a Negative process, and to leave the
picture, as regards permanency, much in the condition of an ordinary print
obtained by direct action of light.

The original Talbotype process, in which the latent image is formed upon
Iodide of Silver, produces, next to Collodion, the most stable image; but
the difficulty of obtaining bright and warm tints on Iodide of Silver,
will stand in the way of its adoption.

The _toning_ of Paper Positives is the part of the process which is likely
to injure their stability; inasmuch as the finest results cannot easily be
obtained without incurring _sulphuration_, and the action of Sulphur, if
carried to any extent, has been shown to be detrimental. The point to be
kept in view, is to alter the original structure of the image as little as
possible in toning; and it is best to use Gold in preference to Sulphur
as the colouring agent. On theoretical grounds, toning by an alkaline
solution of Chloride of Gold (p. 132), and fixing by Ammonia, is the best
process; but the employment of Sel d'or, which gives a more agreeable
colour and has not been found practically to injure the image, will be
generally preferred. In using _a single fixing and toning Bath_ the same
object of working by Gold rather than by Sulphur may be best attained by
maintaining the activity of the Bath by constant additions of Chloride of
Gold.

The prints which are _least stable_ are such as have been toned in _acid
Hyposulphite Baths, without Gold;_ and the difficulty of preserving such
pictures from becoming yellow in the half-tones is very great. Possibly
a portion of the Sulphuretted Acid may unite with the Suboxide of Silver
and cannot be removed by washing (see p. 158); but even if this be not
the case, it is certain that no ordinary amount of care will obviate the
occasional occurrence of fading, unless the Hyposulphite Bath be kept
_neutral to test-paper_. And all those plans of toning in which Acetic or
Hydrochloric Acid is mixed with Hyposulphite of Soda, and the Positive
immersed whilst the liquid is in a milky state from precipitation of
Sulphur, ought studiously to be avoided.

It will be well also to avoid pushing the action of the fixing and toning
Bath to its utmost limits, since practice and theory both teach us that
the Positives which have been long in the Hyposulphite, and consequently
show a tendency to yellowness in the light parts, are most liable to
lose their half-tones on keeping. Photographic Prints are found often to
_darken_ slightly in the course of years; and therefore by suspending the
toning action at an earlier stage a margin is left for what some have
termed "an improvement by time."

The use of _Albuminized_ in preference to plain paper gives an advantage
in protecting the image from oxidation; but if constantly exposed to
moisture, a putrefactive decomposition of the animal matter may occur. The
proper colour of the Albumen image being a _pale red_, the black tones
should not be sought for on that variety of paper: their production,
if Hyposulphite of Soda were used in toning, would probably imply an
amount of Sulphuration which would more than counterbalance any advantage
otherwise derivable from the Albumen.

Permanent Positives of a black colour may easily be obtained by
sensitizing plain paper, free from animal matters, with Oxide of Silver
in place of Nitrate. The simply fixed image being in that case of a
_sepia tint_, requires a less amount of toning to change it to black.
An impression was at one time prevalent that Ammonio-Nitrate prints
were unstable; but so far from such being the case, they are proved
to withstand the action of all destructive tests better than pictures
prepared upon the same kind of paper sensitized with plain Nitrate of
Silver.

_Mode of testing the permanence of Positives._--The tests for Hyposulphite
of Soda are not sufficiently delicate to indicate with certainty when the
process of washing has been properly performed. The quantity of that salt
left in the paper is usually so small and so much mixed up with organic
matter, that the application of Protonitrate of Mercury or of Nitrate of
Silver to the liquid which drains from the corner of the print, would
probably mislead the operator.

A dilute solution of Permanganate of Potash, prepared by dissolving from
half a grain or two grains of the salt, according to its purity, in one
gallon of distilled water, affords a convenient mode of testing Positives
as regards their power of resisting oxidation; and to an experienced
eye it will prove the presence or absence of Hyposulphite of Soda, the
smallest trace of which is sufficient to remove the pink colour of the
Permanganate.

The most available and simple plan of testing permanence is to enclose
the pictures in a stoppered glass bottle with a small quantity of water.
If they retain their half-tones after a course of three months of this
treatment, and do not become mouldy, the mode of printing followed is
satisfactory.

Boiling water will also be found useful in distinguishing the unstable
colours produced by Sulphur from those following the judicious employment
of Gold; in all cases the image will at first be reddened by the hot
water, but if toned without Sulphur it will, as a rule, recover much of
its dark colour on drying.

The characteristic appearance of prints which have been I much
sulphuretted in the toning Bath, and are very liable to fade, should be
known. A yellow colour in the lights is a bad sign; and if the half-tones
are at all faint and indistinct, with an aspect of commencing yellowness,
it is almost certain that the Positive will not last for any considerable
length of time.




CHAPTER IX.

ON THE THEORY OF THE DAGUERREOTYPE AND TALBOTYPE PROCESSES, ETC.


SECTION I.

_The Daguerreotype._

It was not the original intention of the Author to include a description
of the Daguerreotype Process within the limits of the present Work. The
Daguerreotype is a branch of the Photographic Art so distinct from the
others, that, in manipulatory details, it bears very little analogy to
them; a slight sketch of the theory of the process may not however be
unacceptable.

All necessary remarks will fall under three heads:--The preparation
of the Daguerreotype film;--the means by which the latent image is
developed;--and the strengthening of the image by Hyposulphite of Gold.

_The Preparation of the Daguerreotype Film._--The sensitive film of the
Daguerreotypist is in many respects different from that of the Calotype or
Collodiotype. The latter may be termed wet processes, in contradistinction
to the former, where aqueous solutions are not employed. The Daguerreotype
film is a pure and isolated Iodide of Silver, formed by the direct action
of Iodine upon the metal. Hence it lacks one element of sensitiveness
possessed by the others, viz. the presence of soluble Nitrate of Silver in
contact with the particles of Iodide of Silver.

It is important to remember that the Iodide of Silver prepared by
acting with vapour of Iodine upon metallic Silver, is different in its
Photographic action from the yellow salt obtained by double decomposition
between Iodide of Potassium and Nitrate of Silver. A Daguerreotype film,
when exposed to a bright light, first darkens to an ash-grey colour and
then becomes nearly white; the solubility in Hyposulphite of Soda being
at the same time lessened. A Collodion film, on the other hand, if the
excess of Nitrate of Silver be washed off, although it is still capable of
receiving the radiant impression in the Camera, does not alter either in
colour or in solubility, by exposure even to the sun's rays.

_Details of the process for preparing a Daguerreotype Plate._--A copper
plate of moderate thickness is coated upon the surface with a layer of
pure Silver, either by the electrotype or in any other convenient manner.
It is then polished with great care, until the surface assumes a brilliant
metallic lustre. This preliminary operation of polishing is one of great
practical importance, and the troublesome details attending it constitute
one of the main difficulties to be overcome.

After the polishing is complete, the plate is ready to receive the
sensitive coating. This part of the process is conducted in a peculiar
manner. A simple piece of cardboard or a thin sheet of wood, previously
soaked in solution of Iodine, evolves enough of the vapour to attack the
silver plate; which being placed immediately above, and allowed to remain
for a short time, acquires a pale violet hue, due to the formation of
_an excessively delicate layer_ of Iodide of Silver. By prolonging the
action of the Iodine the violet tint disappears and a variety of prismatic
colours are produced, much in the same way as when light is decomposed
by thin plates of mica or the surface of mother-of-pearl. From violet
the plate becomes of a straw-yellow, then rose-colour, and afterwards
steel-grey. By continuing the exposure, the same sequence of tints is
repeated; the steel-grey disappears, and the yellow and rose-colours
recur. The deposit of Iodide of Silver gradually increases in thickness
during these changes; but to the end it remains excessively thin and
delicate. In this respect it contrasts strongly with the dense and creamy
layer often employed in the Collodion process, and shows that a large
proportion of the Iodide of Silver must in such a case be superfluous, as
far as any influence produced by the light is concerned. An inspection
of a sensitive Daguerreotype plate reveals the microscopic nature of the
actinic changes involved in the Photographic Art, and teaches a useful
lesson.

_Increase of sensibility obtained by combining the joint action of Bromine
and Iodine._--The original process of Daguerre was conducted with the
vapour of Iodine only; but in the year 1840 it was discovered by Mr. John
Goddard that the sensibility of the plate was greatly promoted by exposing
it to the vapours of Iodine and Bromine in succession,--the proper time
for each being regulated by the tints assumed.

The composition of this Bromo-Iodide of Silver, so called, is uncertain,
and has not been proved to bear any analogy to that of the mixed salt
obtained by decomposing a solution of Iodide and Bromide of Potassium with
Nitrate of Silver. Observe also that the Bromo-Iodide of Silver is more
sensitive than the simple Iodide _only token the vapour of Mercury is
employed as a developer_. M. Claudet proves that if the image be formed by
the direct action of light alone (see page 174), the usual condition is
reversed, and that the use of Bromine under such circumstances retards the
effect.

_The Development and Properties of the Image._--The latent image of the
Daguerreotype is developed in a manner different from that of the humid
processes generally,--viz. by the action of Mercurial vapour. Mercury,
or Quicksilver, is a metallic fluid which boils at 662° Fahrenheit. We
are not however to suppose that the iodized plate is subjected to the
vapour of Mercury at a temperature at all approaching to 662°. The cup
containing the Quicksilver is previously heated by means of a spirit-lamp
to about 140°, a temperature easily borne by the hand, in most cases,
without inconvenience. The amount of Mercurial vapour evolved at 140° is
very small, but it is sufficient for the purpose, and after continuing the
action for a short time the image is perfectly developed.

There are few questions which have given rise to greater discussion
amongst chemists than the nature of the Daguerreotype image.
Unfortunately, the quantity of material to be operated on is so small,
that it becomes almost impossible to ascertain its composition by direct
analysis. Some suppose it to consist of Mercury alone. Others have thought
that the Mercury is in combination with metallic Silver. The presence
of the former metal is certain, since M. Claudet shows that, by the
application of a strong heat, it can actually be volatilized from the
image in sufficient quantity to develope a second impression immediately
superimposed.

It is a remarkable fact that an image more or less resembling that
developed by Mercury can be obtained by _the prolonged action_ of light
alone upon the iodized plate. The substance so formed is a white powder,
insoluble in solution of Hyposulphite of Soda; amorphous to the eye,
but presenting the appearance of minute reflecting crystals when highly
magnified. Its composition is uncertain.

For all practical purposes the production of the Daguerreotype image by
light alone is useless, on account of the length of time required to
effect it. This was alluded to in the third Chapter, where it was shown
that in the case of the Bromo-Iodide of Silver an intensity of light 3000
times greater is required, if the use of the Mercurial vapour be omitted.

_M. Ed. Becquerel's discovery of the continuing action of rays of
yellow light._--Pure homogeneous yellow light has no action upon the
Daguerreotype plate; but if the iodized surface be first exposed to
white light for a sufficient time to impress a latent image, and then
_afterwards_ to the yellow light, the action already commenced is
_continued_, and even to the extent of forming the peculiar white deposit,
insoluble in Hyposulphite of Soda, already alluded to.

Yellow light may therefore in this sense be spoken of as a _developing_
agent, since it produces the same effect as the Mercurial vapour in
bringing out to view the latent image.

A singular anomaly however requires notice, viz. that if the plate be
prepared with the mixed vapours of Bromine and Iodine, in place of Iodine
alone, then the yellow light cannot be made to develope the image. In
fact, the same coloured ray which continues the action of white light upon
a surface of _Iodide_ of Silver, actually _destroys_ it, and restores the
particles to their original condition, with a surface of _Bromo_-Iodide of
Silver.

These facts, although not of great practical importance, are interesting
in illustration of the delicate and complex nature of the chemical changes
produced by light.

_The Strengthening of the Daguerreotype Image by means of Hyposulphite of
Gold._--The use of the Hyposulphite of Gold to whiten the Daguerreotype
image, and render it more lasting and indestructible, was introduced by M.
Fizeau, subsequent to the original discovery of the process.

After removal of the unaltered Iodide of Silver by means of Hyposulphite
of Soda, the plate is placed upon a levelling stand and covered with a
solution of Hyposulphite of Gold, containing about one part of the salt
dissolved in 500 parts of water. The flame of a spirit-lamp is then
applied until the liquid begins to boil. Shortly a change is seen to take
place in the appearance of the image; it becomes whiter than before, and
acquires great force. This fact seems to prove conclusively that metallic
Mercury enters into its composition, since a surface of Silver--such, for
instance, as that of the Collodion image--is _darkened_ by Hyposulphite of
Gold.

The difference in the action of the gilding solution upon the image and
the pure Silver surrounding it illustrates the same fact. This Silver,
which appears of a dark colour, and forms the shadows of the image, is
rendered still darker; a very delicate crust of metallic Gold _gradually_
forming upon it, whereas with the image the whitening effect is immediate
and striking.


SECTION II.

_Theory of the Talbotype and Albumen Processes._

_The Talbotype or Calotype._--This process, as practiced by many at the
present time, is almost identical with that originally described by Mr.
Fox Talbot. The object is to obtain an even and finely divided layer of
Iodide of Silver upon the surface of a sheet of paper; the particles of
the Iodide being left in contact with an excess of Nitrate of Silver,
and usually with a small proportion of Gallic Acid, to heighten, still
further, the sensibility to light.

The English papers sized with Gelatine are commonly used for the Calotype
process; they retain the film more perfectly at the surface, and the
Gelatine in all probability assists in forming the image. With a foreign
starch-paper, unless it be re-sized with some organic substance, the
solutions sink in too deeply, and the picture is wanting in clearness and
definition.

There are two modes of iodizing and sensitizing the sheets: first, by
floating alternately upon Iodide of Potassium and Nitrate of Silver, in
the same manner as in the preparation of papers for Positive Printing;
and second, by what is termed "the single wash," which is thought by many
to give superior results as regards sensitiveness and intensity of image.
To iodize by this mode, the yellow Iodide of Silver, prepared by mixing
solutions of Iodide of Potassium and Nitrate of Silver, is dissolved in
a strong solution of Iodide of Potassium; the sheets are floated for an
instant upon this liquid and dried; they are then removed to a dish of
water, by the action of which the Iodide of Silver is precipitated upon
the surface of the paper in a finely divided state.

The properties of a solution of Iodide of Silver in Iodide of Potassium,
or of the double Iodide of Potassium and Silver, are described at page 43,
a reference to which will show that the double salt is decomposed by a
large quantity of water, with precipitation of the Iodide of Silver, this
substance being insoluble in a dilute solution of Iodide of Potassium,
although soluble in a strong solution.

Paper coated with Iodide of Silver by this mode, after proper washing in
water to remove soluble salts (which if allowed to remain would attract
damp), will keep good for a long time. The layer of Iodide appears of a
pale primrose colour, and is perfectly insensitive to light. Even exposure
to the sun's rays produces no change, thus indicating that an excess of
Nitrate of Silver is essential to the visible darkening of Iodide of
Silver by light. The paper is also insensitive to the reception of an
invisible image, differing in this respect from the washed Collodion
plate, which receives an impression in the Camera, although apparently
freed from Nitrate of Silver.

To render Calotype paper sensitive to light, it is brushed with a solution
of Nitrate of Silver containing both Acetic and Gallic Acids, termed
"Aceto-Nitrate" and "Gallo-Nitrate" solution. The Gallic Acid lessens
the keeping qualities of the paper, but increases the sensitiveness.
The Acetic Acid prevents the paper from blackening all over during
the development, and preserves the clearness of the white parts; its
employment is indispensable.

The paper is commonly excited upon the morning of the day upon which it
is intended to be used; and the longer it is kept, the less active and
certain it becomes. An exposure of five to eight minutes in the Camera is
the average time with an ordinary view lens.

The picture is developed with a saturated solution of Gallic Acid, to
which a portion of Aceto-Nitrate of Silver is added to heighten the
intensity. Both Sulphate of Iron, and Pyrogallic Acid have also been used,
but they are unnecessarily strong, the invisible image being more easily
developed upon paper than upon Collodion (see page 143).

After fixing the Negative by removing the unaltered Iodide of Silver with
Hyposulphite of Soda, it is well washed and dried. White wax is then
melted in with a hot iron, so as to render the paper transparent, and to
facilitate the after-process of printing.

The Calotype cannot be compared with the Collodion process for
sensitiveness and delicacy of detail, but it possesses advantages for
tourists and those who do not wish to be encumbered with large glass
plates. The principal difficulty appears to be in obtaining a uniformly
good paper, many samples giving a speckled appearance in the black parts
of the Negative.

_The Waxed Paper process of Le Grey._--This is a useful modification
of the Talbotype introduced by M. Le Grey. The paper is waxed before
iodizing, by which, without involving any additional operation, a very
fine surface layer of Iodide of Silver can be obtained. The Waxed Paper
Process is well adapted for tourists, from its extreme simplicity and the
length of time which the film may be kept in a sensitive condition.

Both English and foreign papers are employed: but the former take the
wax with difficulty. Mr. Crookes, who has devoted his attention to this
process, gives clear directions for waxing paper; it is essential that
pure white wax should be obtained direct from the bleachers, since the
flat cakes sold in the shops are commonly adulterated. The _temperature_
must also be carefully kept below that point at which decomposition of
the wax takes place; the use of too hot an iron being a common source of
failure (see 'Photographic Journal,' vol. ii. p. 231).

The sheets of paper, having been properly waxed, are soaked for _two
hours_ in a solution containing Iodide and Bromide of Potassium, with,
enough free Iodine to tinge the liquid of a port-wine colour. The greasy
nature of wax impedes the entry of liquids, and hence a long immersion
is required. The iodizing formulæ of the French Photographers have been
encumbered by the addition of a variety of substances which appear to
introduce complications without giving proportional advantage, and Mr.
Townshend has done the art a service by proving that the Iodide and
Bromide of Potassium, with free Iodine, are sufficient. This latter
ingredient was first used by Mr. Crookes; it seems to add to the clearness
and sharpness of the Negatives; and as the papers are _coloured_ by the
Iodine, air-bubbles cannot escape detection. The process of exciting with
Nitrate of Silver is also rendered more certain by the employment of free
Iodine, the action of the Bath being continued until the purple colour
gives place to the characteristic yellow tint of the Iodide of Silver.

Waxed Paper is rendered sensitive by immersion in a Bath of Nitrate of
Silver containing Acetic Acid; the quantity of which latter ingredient
should be increased when the papers are to be long kept. As the excess of
Nitrate is subsequently removed, the solution may be used weaker than in
the Calotype or Collodion process.

After exciting, the papers are washed with water to reduce the amount of
free Nitrate of Silver to a minimum. This lessens the sensitiveness, but
greatly increases the keeping qualities, and the paper will often remain
good for ten days or longer.

It is a very important point, in operating with Waxed Paper, to keep
the developing dishes clean. The development is conducted by immersion
in a Bath of Gallic Acid containing Acetic Acid and Nitrate of Silver;
and being retarded by the superficial coating of wax, there is always a
tendency to an irregular reduction of Silver upon the white portions of
the Negative. When the developer becomes brown and discoloured, this is
almost sure to happen; and it is well known to chemists that the length
of time during which Gallic Acid and Nitrate of Silver may remain mixed
without decomposing, is much lessened by using vessels which are dirty
from having been before employed for a similar purpose. The black deposit
of Silver exercises a _catalytic_ (καταλυσις, decomposition by contact)
action upon the freshly-mixed portion, and hastens its discoloration.

The Waxed Paper process is exceedingly simple and inexpensive,--very
suitable for tourists, as requiring but little experience, and a minimum
of apparatus. It is however slow and tedious in all its stages, the
sensitive papers frequently taking an exposure of twenty minutes in the
Camera, and the development extending over an hour or an hour and a
half. Several Negatives however may be developed at the same time; and
as the removal of the free Nitrate of Silver gives the process a great
advantage during hot weather, it will in all probability continue to be
extensively followed. The prints which have been sent to the Exhibition
of the Photographic Society, show that waxed paper in the hands of a
skilful operator may be made to delineate architectural subjects with
great fidelity, and also to give the details of foliage and landscape
Photography with distinctness.

_The Albumen process upon Glass._--The process with Albumen originated in
a desire to obtain a more even surface layer of Iodide of Silver than the
coarse structure of the tissue of paper will allow. It is conducted with
simple Albumen, or "white of eggs," diluted with a convenient quantity of
water. In this glutinous liquid Iodide of Potassium is dissolved; and the
solution, having been thoroughly shaken, is set aside, the upper portion
being drawn off for use, in the same manner as in the preparation of
Albuminized paper for printing.

The glasses are coated with the Iodized Albumen, and are then placed
horizontally in a box to dry. This part of the process is considered the
most troublesome, the moist Albumen easily attracting particles of dust,
and being apt to blister and separate from the glass. If an even layer of
the dried and Iodized material can be obtained, the chief difficulty of
the process has been overcome.

The plates are rendered sensitive by immersion in a Bath of Nitrate of
Silver with Acetic Acid added, and are then washed in water and dried.
They may be kept for a long time in an excited state.

The exposure in the Camera must be unusually long; the free Nitrate of
Silver having been removed by washing, and the Albumen exercising a direct
retarding influence upon the sensitiveness of Iodide of Silver.

The development is conducted in the ordinary way by a mixture of Gallic
Acid and Nitrate of Silver, with Acetic Acid added to preserve the
clearness of the lights. It usually requires one hour or more, but may be
accelerated by the gentle application of heat.

Albumen pictures are remarkable for elaborate distinctness in the
shadows and minor details, and are admirably adapted for viewing in the
Stereoscope; but they do not often possess the peculiar and characteristic
_softness_ of the Photograph upon Collodion. The process is well adapted
for hot climates, being very little prone to the cloudiness and irregular
reduction of Silver which are often complained of with moist Collodion
under such circumstances.

_M. Taupenot's Collodio-Albumen process._--This is a recent discovery
which seems to involve a new principle in the Art, and gives promise of
great utility.

One of the greatest objections to the Albumen process has been its want
of sensitiveness; but M. Taupenot found that this was obviated to a great
extent by pouring the Albumen upon a plate _previously coated with Iodide
of Silver_. In this way two layers of that sensitive salt are formed, and
the sensibility of the surface layer, which alone receives the image, is
promoted by its resting upon a substratum of Iodide rather than upon the
inert surface of the glass. In this view, if the theory be correct, the
lower particle of Iodide of Silver promotes the molecular disturbance of
the upper, itself remaining unchanged.

Other experimenters, pursuing the subject further, have asserted that
a successful result may be obtained by coating the plate with plain
Collodion and subsequently with Iodized Albumen. If this observation
should prove correct, the process will be simplified and its utility
increased.

In the sixth Chapter of Part II. the practical details of the
Collodio-Albumen process will be described.


END OF PART I.




PART II.

PRACTICAL DETAILS OF THE COLLODION PROCESS.




PRACTICAL DETAILS OF THE COLLODION PROCESS.

[Illustration]




CHAPTER I.

PREPARATION OF COLLODION.


This includes--the soluble Paper;--the Alcohol and Ether;--and the
iodizing compounds.

The formulæ for Negative and Positive Collodion, and for the Nitrate Bath
and developing fluids, are given in the second Chapter.

THE SOLUBLE PAPER.

Pyroxyline may be prepared either from cotton wool or from Swedish
Filtering-paper. Most operators prefer the latter, from its giving a
product of constant solubility, and yielding a fluid solution.[30] The
Cotton Wool however is better adapted for use with the Sulphuric Acid and
Nitre, since the Paper, from its closeness of texture, requires a longer
immersion in the mixture.

[30] Swedish filtering-paper may be procured at the operative chemists',
at about five shillings the quire. Each half-sheet has the water-mark "J.
H. Munktell."

Preparation of a Nitro-Sulphuric Acid of the proper strength.--There are
two modes of preparing the Nitro-Sulphuric Acid: first, by mixing the
acids; second, by the Oil of Vitriol and Nitre Process. The former is the
best in cases where large quantities of the material are operated on, but
the amateur is recommended to begin by trying the Nitre Process (p. 190)
as the most simple.


PREPARATION OF NITRO-SULPHURIC ACID BY THE MIXED ACIDS.

The operator may proceed in either of two ways; first, by taking the
strength of each sample of acid, and mixing according to fixed rule;
second, by a more ready plan, which may be used when the exact strength of
the acids is not known. Each of these will be described in succession.

a. _Directions for mixing according to fixed rule._--This process is given
from Mr. Hadow's original paper in the 'Quarterly Journal of the Chemical
Society.' It is certain in its results if the strength of both acids be
accurately determined.

A very perfect process for taking the strength of Nitric Acid is by means
of powdered Marble or Carbonate of Lime, as described in various works on
practical Chemistry. Sulphuric Acid may be estimated by precipitating with
Nitrate of Baryta, and weighing the insoluble Sulphate with the proper
precautions.

The specific gravity is not a criterion of strength to be perfectly relied
on, but if it be adopted as a test, the following points must be attended
to.

1st. That the temperature of the acid be at or near 60° Fahrenheit; the
density of Sulphuric Acid especially is, from its small specific heat,
greatly influenced a change of temperature.

2nd. The sample of Nitric Acid must be free from Peroxide of Nitrogen, or
only slightly coloured by it. This substance, when present, increases the
specific gravity of the acid without adding to its available properties.
A yellow sample of Nitric Acid will therefore be somewhat weaker than is
indicated by the specific gravity.

3rd. The Oil of Vitriol should yield no solid residue on evaporation.
Sulphate of Lead and Bisulphate of Potash are often found in the
commercial acid, and add much to its density. Oil of Vitriol containing
Sulphate of Lead becomes milky on dilution.

The formula for a definite Nitro-Sulphuric Acid, of the proper strength
for making the soluble Pyroxyline, may be stated thus:--

  HO NO{5}, 2 (HO SO{3}) + 3-1/2 HO

or

                         Atoms.      Atomic weight.
  Nitric Acid              1      |     54
  Sulphuric Acid           2      |     80
  Water                    6-1/2  |     58
                                  |    ---
                                  |    192

Having found the percentage of real acid which is present,[31] the
following calculation will give the relative weights of the ingredients
required to produce the formula:--

  Let { _a_ = percentage of real Nitric Acid,
      { _b_ =      "       "     Sulphuric Acid,

     then 5400/_a_          =  quantity of Nitric Acid,

          8000/_b_          =       "      Sulphuric Acid,

  192 - 5400/_a_ - 8000/_b_ =  "      Water.

[31] Tables are given in the Appendix for calculation by specific gravity;
but direct analysis of the acids is the most certain.

Observe that the numbers in the calculation correspond to the atomic
weights recently given; and that the amount of water is derived from the
_total atomic weight_, viz. 192, _minus_ the sum of the weights of both
acids.

Hence if the samples of acid employed are too weak for the purpose, the
formula for the water gives a negative quantity.

The weight of mixed acids produced by the formula is 192 grains, which
would measure somewhere about two fluid drachms. Ten times this quantity
forms a convenient bulk of liquid, in which about 50 or 60 grains of Paper
may be immersed.

In weighing corrosive liquids, such as Sulphuric and Nitric Acid, a small
glass may be counterbalanced in the scale-pan, and the acid poured in
carefully. If too much is added, the excess can be removed by a glass rod,
or by "the pipette" commonly employed for such a purpose.

The following example of a calculation similar to the above may be given:--

  100 parts of the Oil of Vitriol   = 76·65 real acid.
   "   "           Nitric Acid      = 65·4 real acid.

  therefore   8000/76·65 = 104·3 grains of Oil of Vitriol.
              5400/65·4  =  82·5    "      Nitric Acid
  192 - 104·3 - 82·5     =   5·2    "      Water.

Multiplying these weights ten times, we have

  Oil of Vitriol           1043 grains.
  Nitric Acid               825   "
  Water                      52   "
                           ----
  Total weight of the    } 1920 grains.
    Nitro-Sulphuric Acid }

Having prepared the acid mixture of a definite strength by the above
formula, the paper must be immersed according to directions given at page
191.

b. _Process for mixing Nitro-Sulphuric Acid, the strength of the two acids
not having been previously determined._--Take a strong sample of Nitric
Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with
Oil of Vitriol as follows:--

  Sulphuric Acid     10 fluid drachms,
  Nitric Acid        10      "

Now immerse a thermometer and note the temperature;[32] it should be
from 130° Fahr. to 150°. If it sinks below 120°, place the mixture in a
capsule, and float upon boiling water for a few minutes.

[32] In the preparation of soluble cotton, and indeed in all Photographic
manipulations, a thermometer is almost indispensable. Instruments of
sufficient delicacy for common purposes are sold in Hatton Garden and
elsewhere, at a low price. The bulb should be uncovered, to admit of being
dipped in acids, etc., without injury to the scale.

A preliminary experiment with a small tuft of Cotton Wool (cotton shows
it better than paper) will then indicate the actual strength of the
Nitro-Sulphuric Acid. Stir the tuft in the mixture for five minutes.
Remove with a glass rod, and wash with water for a short time, until no
acid taste can be perceived. If the Wool becomes _matted_, and gelatinizes
slightly on its first immersion in the acid, or if, in the subsequent
washing, the fibres appear to adhere and to be disintegrated by the action
of the water, _the Nitro-Sulphuric Acid is too weak_. In that case add to
the acid mixture.

  Oil of Vitriol, 3 drachms.

If the cotton was actually _dissolved_ in the first trial, an addition of
half of a fluid ounce of Oil of Vitriol may be required.

Supposing the cotton not to be gelatinized and to wash well, then wring
it out very dry, pull out the fibres, and treat it in a test-tube with
rectified Ether,[33] to which a few drops of Alcohol have been added. If
it be _insoluble_, dry it by a gentle heat and apply a flame: a brisk
explosion indicates that the Nitro-Sulphuric Acid employed is _too
strong_. In that case, add to the twenty drachms of mixed acids, one
drachm of water, and test again, repeating the process until a soluble
product is obtained.

[33] Observe that the Ether be pure; if it contains too much water and
Alcohol, it will not dissolve the Pyroxyline, or will yield an opalescent
solution.

There is a third condition of Pyroxyline, different from either of the
above, which may be puzzling:--the fibres of the Cotton mat together very
slightly or not at all on immersion, and the washing proceeds tolerably
well; the compound formed is scarcely explosive, and dissolves imperfectly
in Ether, leaving little nodules or hard lumps. The ethereal solution
yields, on evaporation, a film which is opaque instead of transparent. In
this case (presuming the Ether to be good) the acid mixture is slightly
too weak, or the temperature is too low, being probably about 90°, instead
of 130° to 140° (?).

When the acid mixture has been brought to the proper strength by a few
preliminary trials, proceed according to the directions given at the next
page.


PREPARATION OF NITRO-SULPHURIC ACID BY OIL OF VITRIOL AND NITRE.

This process is recommended, in preference to the other, to the amateur
who is unable to obtain Nitric Acid of convenient strength. The common Oil
of Vitriol sold in the shops is often very good for Photographic purposes;
but it is best, if possible, to take the specific gravity, when any doubt
exists of its genuineness. At a temperature of 58° to 60°, specific
gravity 1·833 is the usual strength, and if it falls below this, it should
be rejected. (See Part III. for 'Impurities of Commercial Sulphuric Acid.')

The Nitre must be the purest sample which can be obtained. Commercial
Nitre often contains a large quantity of _Chloride of Potassium_, detected
on dissolving the Nitre in distilled water, and adding a drop or two of
solution of Nitrate of Silver. If a milkiness and subsequent curdy deposit
is formed. Chlorides are present. These Chlorides are injurious; after
the Oil of Vitriol is added, they destroy a portion of Nitric Acid by
converting it into brown fumes of Peroxide of Nitrogen, and so alter the
strength of the solution.

_Nitrate of Potash is an anhydrous salt_,--it contains simply Nitric Acid
and Potash, without any water of crystallization; still, in many cases,
a little water is retained mechanically between the interstices of the
crystals, and therefore it is better to dry it before use. This may be
done by laying it in a state of fine powder upon blotting-paper, close to
a fire, or upon a heated metallic plate.

The sample must also be reduced to a fine powder before adding the Oil of
Vitriol; otherwise portions of the salt escape decomposition.

These preliminaries having been properly observed, weigh out

  Pure Nitre, powdered and dried, 600 grains.

This quantity is equivalent to 1-1/4 ounce Troy or Apothecaries'
weight;--and to 1-1/4 ounce Avoirdupois weight _plus_ 54 grains. Place
this in a teacup or any other convenient vessel, and pour upon it.

             Water                        1-1/2 fluid drachms
  mixed with Oil of Vitriol              12          "

Stir well with a glass rod for two or three minutes, until all
effervescence has ceased, and an even, pasty mixture, free from lumps, is
obtained.

During the whole process, abundance of dense fumes of Nitric Acid will be
given off, which must be allowed to escape up the flue or into the open
air.

_A modification of the formula._--The above formula will invariably
succeed with a good sample of acid and pure Nitre. When tried however with
Oil of Vitriol rather weaker than ordinary, and _commercial_ Nitre, it may
fail, the cotton being gelatinized and dissolved. When such is the case,
the addition of water must be omitted or the quantity reduced from one
drachm and a half to half a drachm.


GENERAL DIRECTIONS FOR IMMERSING, WASHING, AND DRYING THE PYROXYLINE.

The mixture of Sulphuric Acid and Nitre requires to be used immediately
after its preparation, as it solidifies into a stiff mass on cooling; but
the mixed acids may be kept for any length of time in a stoppered bottle.

When Cotton is used, the fibres should be well pulled out, and small tufts
added one by one to the acid mixture, stirring with a glass rod in order
to keep up a constant change of particles. The Paper is cut into squares
or strips, which are introduced singly.

In either case the quantity must not be too great, or some portions will
be imperfectly acted upon; about 20 grains to each fluid ounce of the
mixture will be sufficient.

The _time of immersion required_ varies from ten minutes with Cotton, to
twenty minutes or even half an hour with the Paper. When an unusually
large proportion of Sulphuric Acid is used, as in the case of a weak
sample of Nitric Acid, the Cotton should be removed at the expiration of
six or seven minutes, as there is a tendency to partial solution of the
Pyroxyline in the acid mixture under those circumstances.

It is an advantage in some cases to prepare the material at a high
temperature, but unless the proportions of the Acids are strictly
according to Mr. Hadow's formula, solution of the Cotton may take place if
the thermometer indicates more than 140°.

After the action is complete, the Nitro-Sulphuric Acid is left weaker than
before, from addition of various atoms of water necessarily formed during
the change. Hence, if the same portion be used more than once, an addition
of Sulphuric Acid will be required.

_Directions for Washing._--In removing the Pyroxyline from the
Nitro-Sulphuric Acid, press out as much of the liquid as possible, and
wash it rapidly in a large quantity of cold water, using a glass rod to
preserve the fingers from injury. If it were simply thrown into a small
quantity of water and allowed to remain, the rise in temperature and
weakening of the acid mixture might do mischief.

The washing should be continued for at least a quarter of an hour, or
longer in the case of Paper, as it is essential to get rid of every
trace of acid. When the Nitre plan has been adopted, a portion of the
_Bisulphate of Potash_ formed adheres to the fibres, and if not carefully
washed out, an opalescent appearance is seen in the Collodion, resulting
from the insolubility of this salt in the ethereal mixture.

If no acid taste can be perceived, and a piece of blue litmus-paper
remains in contact with the fibres for five minutes without changing in
colour, the product is thoroughly washed. It is however a safe plan to
place the Pyroxyline in running water and allow it to remain for several
hours.

Lastly, wring it out in a cloth, pull out the fibres, and dry slowly, by
a moderate heat. After drying, it may be kept for any length of time in a
stoppered bottle.


RECAPITULATION OF THE GENERAL CHARACTERS OF PYROXYLINE PREPARED IN
NITRO-SULPHURIC ACID OF VARIOUS DEGREES OF CONCENTRATION.

_The acid mixture too strong._--The appearance of the cotton is not much
altered on its first immersion in the mixture. It washes well, without
any disintegration. On drying, it is found to be strong in texture, and
produces a peculiar crackling sensation between the fingers, like starch.
It explodes on the application of flame, without leaving any ash. It is
insoluble in the mixture of Ether and Alcohol, but dissolves if treated
with Acetic Ether.

_The acid mixture of the proper strength._--No agglutination of the fibres
of the cotton on immersion, and the product washes well; soluble in the
ethereal mixture, and yields a _transparent_ film on evaporation.

_The acid mixture too weak._--The fibres of the cotton agglutinate, and
the Pyroxyline is washed with difficulty. On drying, the texture is found
to be short and rotten. It does not explode on being heated, but either
burns quietly with a flame, leaving behind a black ash--in which case it
consists simply of unaltered cotton,--or is only slightly combustible,
and not explosive. It dissolves more or less perfectly in glacial Acetic
Acid. When treated with the ethereal mixture, it is acted on _partially_,
leaving behind lumps of unchanged cotton; the solution does not form an
even transparent layer on evaporation, but becomes opaque and cloudy as it
dries. This opacity however may be seen to a small extent with any sample
of Pyroxyline, if the solvents contain too much water.

In using Swedish Paper in place of Cotton, the Pyroxyline formed in too
weak a Nitro-Sulphuric Acid is usually insoluble in Ether and Alcohol, and
burns slowly like unchanged paper.

By studying these characters, and at the same time bearing in mind that _a
drachm and a half of water_ in the quantities of acid given in the formula
(p. 188) will suffice to cause the difference, the operator will overcome
all difficulties.


PURIFICATION OF THE SOLVENTS REQUIRED FOR COLLODION.

The purity of the Ether employed is a matter of as much importance in the
manufacture of a good Collodion as that of any other ingredient; this
point must be attended to in order to secure a good result.

There are four kinds of Ether sold by manufacturing chemists; first,
ordinary rectified Sulphuric Ether, containing a certain percentage of
Alcohol and of water; specific gravity about ·750. Second, the washed
Ether, which is the same agitated with an equal bulk of water, to remove
the Alcohol: by this proceeding the specific gravity of the fluid is
reduced considerably. Third, Ether both washed and re-rectified from a
caustic alkali, so as to contain neither Alcohol nor water; in this case
the specific gravity should not be higher than ·720. Fourth, "Methylated"
Ether, manufactured at a lower price than the others.

Rectified Ether of 750° is not to be depended on, inasmuch as the
specific gravity is often made up by adding water instead of alcohol.
Methylated Ether should be used only when economy is an object, as it is
prone to acidity and less certain in its properties.

Some of the qualities which render Ether unfit for Photographic purposes,
are as follows:--a peculiar and disagreeable smell, either of some
essential oil, or of Acetic Ether; an acid reaction to test-paper; a
property of turning alcoholic solution of Iodide of Potassium brown with
unusual rapidity; an alkaline reaction to test-paper; a high specific
gravity, from superabundance of Alcohol and water.

The Ether which has been both washed and redistilled is always the most
uniform in composition, and especially so if the second distillation be
conducted from Quicklime, Carbonate of Potash, or Caustic Potash. These
Alkaline substances retain the impurities, which are often of an acid
nature, and leave the Ether in a fit state for use.

The redistillation of Ether is a simple process: in dealing with this
fluid however the greatest caution must be exercised, on account of its
inflammable nature. Even in pouring Ether from one bottle into another, if
a light of any kind be near, the vapour is apt to take fire; and severe
injuries have been occasioned from this cause.

_Purification of Ether by redistillation from a caustic or carbonated
alkali._--Take ordinary rectified Sulphuric Ether, and agitate it with an
equal bulk of water to wash out the Alcohol; stand for a few minutes until
the contents of the bottle separate into two distinct strata, the lower of
which--_id est_, the watery stratum--is to be drawn off and rejected. Then
introduce Caustic Potash, finely powdered, in the proportion of about one
ounce to a pint of the washed Ether; shake the bottle again many times,
in order that the water--a small portion of which is still present in
solution in the Ether--may be thoroughly absorbed. Afterwards set aside
for twenty-four hours (not longer, or the Potash may begin to decompose
the Ether), when it will probably be observed that the liquid has become
yellow, and that a flocculent deposit has formed in small quantity.
Transfer to a retort of moderate capacity, supported in a saucepan of warm
water, and properly connected with a condenser. On applying a gentle heat,
the Ether distils over quietly, and condenses with very little loss; care
must be taken that none of the alkaline liquid contained in the body of
the retort finds its way, by projection or otherwise, into the neck, so as
to run down and contaminate the distilled fluid.

A more economical plan of purifying Ether is, without previous washing
with water, to agitate with Carbonate of Potash or with Quicklime, and
redistil at a moderate temperature.

In order to preserve Ether from decomposition, it must be kept in
stoppered bottles, nearly full, and in a dark place. The stoppers
should be tied over with bladder and luted, or a considerable amount of
evaporation will take place, unless the neck of the bottle has been ground
with unusual care. After the lapse of some months, probably a certain
amount of decomposition, evidenced by the liberation of Iodine on adding
Iodide of Potassium, will be found to have taken place. This however is
small in amount, and not of a character to injure the fluid.

Rectification of Spirits of Wine from Carbonate of Potash.--The object of
this operation is to remove a portion of water from the spirit, and so to
increase its strength. Alcohol thus purified may be added to Collodion
almost to any extent, without producing glutinosity and rottenness of film.

The salt termed Carbonate of Potash is a deliquescent salt,--that is, it
has a great attraction for water; consequently when Spirits of Wine are
agitated with Carbonate of Potash, a portion of water is removed, the salt
dissolving in it and forming a dense liquid, which refuses to mix with the
Alcohol, and sinks to the bottom. At the expiration of two or three days,
if the bottle has been shaken frequently, the action is complete, and the
lower stratum of fluid may be drawn off and rejected. _Pure_ Carbonate
of Potash is an expensive salt, and a commoner variety may be taken. It
should be well dried on a heated metal plate, and powdered, before use.

The quantity may be about two ounces to a pint of spirit; or more, if an
unusually concentrated Alcohol is required.

After the distillation is complete, a fluid is obtained containing about
90 per cent, of absolute Alcohol, the remaining 10 per cent, being water.
The specific gravity at 60° Fahrenheit should be from ·815 to ·825;
commercial Spirit of Wine being ·836 to ·840.


PREPARATION OF THE IODIZING COMPOUNDS IN A STATE OF PURITY.

These are the Iodides of Potassium, Ammonium, and Cadmium. The properties
of each are more fully described in Part III.

a. _The Iodide of Potassium._--Iodide of Potassium, as sold in the
shops, is often contaminated with various impurities. The first and most
remarkable is _Carbonate of Potash_. When a sample of Iodide of Potassium
contains much Carbonate of Potash, it forms small and imperfect crystals,
which are strongly alkaline to test-paper, and become moist on exposure to
the air, from the deliquescent nature of the Alkaline Carbonate. _Sulphate
of Potash_ is also a common impurity; it may be detected by Chloride of
Barium.

A third impurity of Iodide of Potassium is _Chloride_ of Potassium; it is
detected as follows:--Precipitate the salt by an equal weight of Nitrate
of Silver, and treat the yellow mass with solution of Ammonia; if any
Chloride of Silver is present, it dissolves in the Ammonia, and, after
filtration, is precipitated in white curds by the addition of an excess of
pure Nitric Acid. If the Nitric Acid employed is not pure, but contains
traces of free Chlorine, the Iodide of Silver must be well washed with
distilled water before treating it with Ammonia, or the excess of free
Nitrate of Silver dissolving in the Ammonia would, on neutralizing,
produce Chloride of Silver, and so cause an error.

_Iodate of Potash_ is a fourth impurity often found in Iodide of
Potassium: to detect it, add a drop of dilute Sulphuric Acid, or a crystal
of Citric Acid, to the solution of the Iodide; when, if much Iodate be
present, the liquid will become yellow from liberation of free Iodine. The
rationale of this reaction is as follows:--The Sulphuric Acid unites with
the base of the salt, and liberates Hydriodic Acid (HI), _a colourless
compound;_ but if Iodic Acid (IO{5}) be also present, it decomposes the
Hydriodic Acid first formed, oxidizing the Hydrogen into Water (HO), and
setting free the Iodine. The immediate production of a yellow colour
on adding a weak acid to aqueous solution of Iodide of Potassium, is
therefore a proof of the presence of an Iodate. As Iodate of Potash
renders Collodion insensitive, this point should be attended to.

Iodide of Potassium may be rendered very pure by recrystallizing from
Spirit, or by dissolving in strong Alcohol of sp. gr. ·823, in which
Sulphate, Carbonate, and Iodate of Potash are insoluble. The proportion of
Iodide of Potassium contained in saturated Alcoholic solutions varies with
the strength of the spirit (_vide_ Part III., article Iodide of Potassium).

Solution of Chloride of Barium is commonly used to detect impurities in
Iodide of Potassium; it forms a white precipitate if Carbonate, Iodate,
or Sulphate be present. In the two former cases the precipitate dissolves
on the addition of pure dilute Nitric Acid, but in the latter it is
insoluble. The commercial Iodide however is rarely so pure as to remain
quite clear on the addition of Chloride of Barium.

b. _The Iodide of Ammonium._--This salt may be prepared by adding
Carbonate of Ammonia to Iodide of Iron, but more easily by the following
process:--A strong solution of Hydrosulphate of Ammonia is first made, by
passing Sulphuretted Hydrogen gas into Liquor Ammoniæ. To this liquid,
Iodine is added until the whole of the Sulphuret of Ammonium has been
converted into Iodide. When this point is reached, the solution at once
colours brown from solution of free Iodine. On the first addition of
the Iodine, an escape of Sulphuretted Hydrogen gas and a dense deposit
of Sulphur take place. After the decomposition of the Hydrosulphate of
Ammonia is complete, a portion of Hydriodic Acid--formed by the mutual
reaction of Sulphuretted Hydrogen and Iodine--attacks any Carbonate
of Ammonia which may be present, and causes an effervescence. The
effervescence being over, the liquid is still acid to test-paper, from
excess of Hydriodic Acid; it is to be cautiously neutralized with Ammonia,
and evaporated by the heat of a water-bath to the crystallizing point.

The crystals should be thoroughly dried over a dish of Sulphuric Acid, and
then sealed in tubes; by this means it will be preserved colourless.

Iodide of Ammonium is very soluble in Alcohol, but it is not advisable
to keep it in solution, from the rapidity with which it decomposes and
becomes brown.

The most common impurity of commercial Iodide of Ammonium is Sulphate of
Ammonia; it is detected by its sparing solubility in Alcohol. Carbonate
of Ammonia is also frequently present to a large extent, in which case
an alkaline Collodion and eventually an alkaline Nitrate Bath will be
produced.

e. _Iodide of Cadmium._--This salt is formed by heating filings of
metallic Cadmium with Iodine, or by mixing the two together with addition
of water.

Iodide of Cadmium is very soluble both in Alcohol and Water; the solution
yielding on evaporation large six-sided tables of a pearly lustre, which
are permanent in the air. The commercial Iodide is sometimes contaminated
with Iodide of Zinc; the crystals being imperfectly formed and slowly
liberating Iodine when dissolved in Ether and Alcohol. Pure Iodide of
Cadmium remains nearly or quite colourless in Collodion, if the fluid be
kept in a cool and dark place.




CHAPTER II.

FORMULÆ FOR SOLUTIONS REQUIRED IN THE COLLODION PROCESS.

  Section I.--Solutions for direct Positives.
  Section II.--Solutions for Negative Photographs.


SECTION I.

_Formulæ for Solutions for direct Positives._

The solutions are taken in the following order;--The Collodion.--The
Nitrate Bath.--Developing fluids.--Fixing liquids.--Whitening solution.


THE COLLODION.

_Formula No. 1._

  Purified Ether, sp. gr. ·720             5 fluid drachms.
  Purified Alcohol, sp. gr. ·825           3   "     "
  Pyroxyline                               3 to 5 grains.
  Pure Iodide of Cadmium or Ammonium       4 grains.

_Formula No. 2._

  Rectified Ether, sp. gr. ·750            6 fluid drachms.
  Spirits of Wine, sp. gr. ·836            2   "     "
  Pyroxyline                               2 to 4 grains.
  Iodide of Potassium or Ammonium          3 to 4   "

If the operator wishes to prepare a stock of the plain Collodion, and to
iodize as required, the last formula will stand thus:--

  Rectified Ether, ·750                    3 fluid ounces.
  Alcohol of ·836                          2 fluid drachms.
  Pyroxyline                               8 to 14 grains.

Dissolve the Pyroxyline, and let the fluid stand for forty-eight hours to
subside, then draw off clear, with a siphon.

To each fluid ounce of this plain Collodion add about two fluid drachms of
the following iodizing mixture:--

  Alcohol, sp. gr. ·836                    1 fluid ounce.
  Iodide of Potassium                     16 grains.

Of the two formulæ above given, the first is considered the best, but the
second may be substituted for it when highly rectified spirits cannot be
obtained. Iodide of Ammonium chemically pure is perhaps superior to any
other Iodide for preparing a portrait Collodion, but Iodide of Cadmium,
with addition of free Iodine, possesses better keeping properties, and
gives very good results. A mixture of the two Iodides may also be used
advantageously, or Iodide of _Potassium_ may be combined with Iodide of
Cadmium: this preparation has been much recommended, but the Collodion
will be liable to produce a spotted film unless the salts are quite pure.

The exact quantity of Pyroxyline will vary with the temperature at which
the preparation was made. The Collodion should flow smoothly on the glass
and remain free from crapy lines on setting. When Iodide of Cadmium is
used, the tendency to glutinosity will be a little greater than usual,
which must be obviated by the directions given at page 83.

The film, after dipping in the Bath, should appear opalescent and not too
yellow and creamy. Pale-blue films yield very good Positives, but with
more liability to failure than thicker films (p. 109).

If the Positives are not perfectly clear and transparent in the
shadows, dissolve 5 grains of Iodine in an ounce of Spirits of Wine
(not methylated), and add a few drops until the Collodion assumes a
golden-yellow colour.

In hot weather advantage will be gained by somewhat increasing the
quantity of Alcohol in Collodion; the evaporation of the solvents
being retarded, and the film rendered less liable to become dry before
development. _Anhydrous_ Alcohol of Sp. Gr. ·796, may be mixed with pure
Ether of ·715, even to the extent of equal parts; but this is the extreme
limit, and with the strongest spirit ordinarily obtainable, the Collodion
will often become somewhat glutinous if the proportions (by measure) of 5
parts of Ether to 3 of Alcohol be exceeded.

Collodion prepared by Formula No. 1, and iodized with Iodide of Cadmium,
may be kept for weeks or months without much loss of sensitiveness; but
when Alkaline Iodides are employed as in the second Formula, Iodine is
liberated, and the fluid becomes at last brown and insensitive.


THE NITRATE BATH.

  Nitrate of Silver                       30 grains.
  Nitric Acid 1/20 minim, or Acetic
    Acid (glacial)                       1/6 minim.
  Alcohol                                 15 minims.
  Distilled water                          1 fluid ounce.

Nitrate of Silver which has been melted, in order to expel Oxides of
Nitrogen, is always the most certain in its action: but the heat must not
be raised too high or the salt will be contaminated with _Nitrite_ of
Silver.

In the Vocabulary (see Part III.) directions are given for the preparation
and purification of Nitrate of Silver; also for the testing of distilled
water, and the best substitutes when it cannot be obtained.

The Bath must be saturated with Iodide of Silver, and Nitric Acid
neutralized if it be present. Nitrate of Silver however which has
undergone fusion is free from Nitric Acid.

Weigh out the total quantity of crystals of Nitrate required for the Bath,
and dissolve in about two parts of water. Then take a quarter of a grain
of Iodide of Potassium to each 100 grains of Nitrate, dissolve in half
a drachm of water, and add to the strong solution; a yellow deposit of
Iodide of Silver first forms, but on stirring is completely re-dissolved.
When the liquid is clear, test for free Nitric Acid by dropping in a
piece of blue litmus-paper. If at the expiration of two minutes the paper
appears _reddened_, Nitric Acid is present, to neutralize which, add
solution of Potash or Carbonate of Soda (not Ammonia) until a distinct
turbidity, remaining after agitation, is produced (an excess does no
harm). Then dilute down the concentrated solution with the remaining
portion of the water, stirring all the time, and filter out the milky
deposit. If the liquid does not at first run clear, it will probably do so
on passing it again through the same filter.

Lastly, add the Acetic Acid (previously tested for impurities, see Part
III.) and the Alcohol to the filtered liquid.

As the bulk of the Bath becomes lessened by use, fill it up with a
solution containing 40 grains of Nitrate to the ounce, which will be found
sufficient to maintain the strength nearly at the original point.

The common practice of occasionally dropping Ammonia or Potash into the
solution, to remove Nitric Acid liberated by free Iodine in the Collodion,
is not recommended (see p. 89).

When the Bath becomes old, and yields Positives which are highly intense
or stained, and slightly foggy, with a deficiency of half-tone, it will be
advisable to precipitate it with a Chloride and prepare a new one.


THE DEVELOPING FLUIDS.

Either of the three following formulæ may be used, according to the taste
of the operator:--

FORMULA No. 1.

  Sulphate of Iron, recrystallized        12 to 20 grains.
  Acetic Acid (glacial)                   20 minims.
  Alcohol                                 10 minims.
  Water                                    1 fluid ounce.

FORMULA No. 2.

  Pyrogallic Acid                          2 grains.
  Nitric Acid                              1 drop.
  Water                                    1 fluid ounce.

FORMULA No. 3.

  Solution of Protonitrate of Iron         1 fluid ounce.
  Alcohol                                 20 minims.

In all these formulæ, if distilled water is not at hand, read the
directions in the Vocabulary, Part III., Article "Water," for the best
substitute.

_Remarks upon these Formulæ._--_Formula No. 1_ is the most simple, since
the solution can be used _as a Bath_, the same portion being employed
many times successively. If it acts too rapidly, lessen the proportion of
Sulphate of Iron. An addition of Nitric Acid, half a minim to the ounce,
makes the image whiter and more metallic; but if too much is used, the
development proceeds irregularly, and spangles of Silver are formed.

The Alcohol and Acetic Acid render the development uniform by causing
the solution of Protosulphate to combine more readily with the film.
The latter also has an effect in whitening the image and increasing its
brightness.

Solution of Sulphate of Iron becomes red on keeping, from a gradual
formation of _per_salt. When it is too weak, add more of the
Protosulphate. The muddy deposit which settles to the bottom of the Bath
is metallic Silver, reduced from the soluble Nitrate upon the plates.

Some operators add pure Nitrate of Potash to this developing solution, to
form a _small portion_ of Protonitrate of Iron. It is said to improve the
colour slightly. The proportions are 10 grains of Nitrate of Potash to
about 14 or 15 grains of Protosulphate of Iron.

_Formula No. 2._--In this formula, if the colour of the image is not
sufficiently white, try the effect of increasing the amount of Nitric Acid
slightly. On the other hand, if the development is imperfect in parts,
and patches of a green colour are seen, use _three grains_ of Pyrogallic
Acid to the ounce, with less Nitric Acid. A few drops of Nitrate of Silver
solution added to the Pyrogallic, immediately before use, will augment the
energy of development when blue and green spots occur.

_Formula No. 3_, or Protonitrate of Iron, does not require any addition
of Acid; but it will be advisable, in some cases, to add to it a few
drops of Nitrate of Silver immediately before developing. It gives a
bright metallic image, resembling that obtained by adding Nitric Acid to
Protosulphate of Iron.

The following process is commonly followed for preparing Protonitrate of
Iron:--

Take of Nitrate of Baryta 300 grains;--powder and dissolve by the aid of
heat in three ounces of water. Then throw in by degrees, with constant
stirring, crystallized Sulphate of Iron, _powdered_, 320 grains. Continue
to stir for about five or ten minutes. Allow to cool, and filter from the
white deposit, which is the insoluble Sulphate of Baryta.

In place of Nitrate of Baryta, the Nitrate of Lead may be used (Sulphate
of Lead being an insoluble salt), but the quantity required will be
different. The atomic weights of Nitrate of Baryta and Nitrate of Lead are
as 131 to 166; consequently 300 grains of the former are equivalent to 380
grains of the latter.


THE FIXING SOLUTION.

  Cyanide of Potassium                     2 to 12 grains.
  Common Water                             1 fluid ounce.

Cyanide of Potassium is usually preferred to Hyposulphite of Soda for
fixing direct Positives; it is less liable to injure the purity of the
white colour. The percentage of _Carbonate of Potash_ in commercial
Cyanide of Potassium is so variable that no exact directions can be given
for the formula. It is best however to use it rather dilute--of such a
strength that the plate is cleared gradually in from half a minute to a
minute.

The solution of Cyanide of Potassium decomposes slowly on keeping, but
it will usually retain its solvent power for several weeks. In order to
escape inconvenience from the pungent odour evolved by this salt, many
employ a vertical Bath to hold the solution; but in that case the plates
must be carefully washed before fixing, as the Iron salts hasten the
decomposition of the Cyanide.


THE WHITENING SOLUTION.

  Bichloride of Mercury                   30 grains.
  Distilled Water                          1 fluid ounce.

By a gentle application of heat the corrosive sublimate dissolves and
forms a solution as nearly as possible saturated at common temperatures.
The addition of a portion of Muriatic Acid enables the water to take up a
larger quantity of Bichloride; but this concentrated solution, at the same
time that it whitens more quickly than the other, is apt to act unequally
upon different parts of the image.

Before applying the Bichloride, the image is to be fixed and the plate
well washed. Either the Protosulphate of Iron or the Pyrogallic Acid with
Acetic (p. 223) may be used for the development; but the whitening process
is more rapid and uniform in the latter case.


SECTION II.

_Formulæ, etc., for Negative Solutions._[34]

[34] The same Collodion and Nitrate Bath may be used both for Positives
and Negatives if required; but there are a few minor points of difference
which are included in the following remarks.

THE COLLODION.

FORMULA No. 1.

  Purified Ether, sp. gr. ·720             5 fluid drachms.
  Purified Alcohol, sp. gr. ·825           3 fluid drachms.
  Soluble Pyroxyline                       4 to 8 grains.
  Pure Iodide of Cadmium or Ammonium       4 to 5 grains.

FORMULA No. 2.

  Rectified Ether, sp. gr. ·750            6 fluid drachms.
  Alcohol, sp. gr. ·836                    2 fluid drachms.
  Soluble Pyroxyline                       4 to 8 grains.
  Iodide of Potassium or Ammonium          4 grains.

When the Collodion and Iodizing mixture are kept separate, the second
formula will stand thus:--

  Rectified Ether ·750                     3 fluid ounces.
  Alcohol of ·836                          2 fluid drachms.
  Pyroxyline                              15 to 30 grains.

To each fluid ounce of this plain Collodion add 2 fluid drachms of the
following Iodizing solution:--

  Alcohol, sp. gr. ·836                    1 fluid ounce.
  Iodide of Potassium                     20 grains.

When the temperature of the Nitro-Sulphuric Acid used in making the
Pyroxyline is high (140° to 155°), it often happens that the Collodion is
too fluid with 4 grains of soluble paper to the ounce, and forms a blue
transparent film of Iodide on dipping the plate in the Bath. In that
case, increase the quantity of Pyroxyline from 4 grains to 6, or even to 8
grains to each ounce.

If the Collodion is glutinous, and produces a wavy surface, with less than
4 grains of Pyroxyline to the ounce, it is probable that the Alcohol is
too weak, or that the soluble Cotton is badly made.

If flakes of Iodide of Silver are seen loose upon the surface of the film,
and falling away into the Bath, the Collodion is over-iodized, and it will
be impossible to obtain a good picture.

After the Collodion has been employed to coat a number of plates, the
relative proportions of Alcohol and Ether contained in it become changed,
from the superior volatility of the latter fluid: when it ceases to flow
readily, and gives a more dense film than usual, thin it down by the
addition of a little rectified Ether.

In dissolving the Pyroxyline, any fibrous or flocculent matter which
resists the action of the Ether, must be allowed to subside, the clear
portion being decanted for use. The Iodide of Potassium is to be finely
powdered, and digested with the spirit until dissolved; it is better not
to apply any heat. Both Iodide of Ammonium and Iodide of Cadmium dissolve
almost immediately, if the salts are pure.

The Collodion must be kept in a cool and dark place. When prepared with
Iodide of Ammonium or Potassium it becomes at length high coloured and
insensitive. The free Iodine may then be removed by a strip of pure zinc
or silver-foil.

When sensitiveness is not an object, many prefer working with an old,
coloured Collodion, finding that it gives more intensity. It has been
shown at page 97 that a peculiar change takes place in Collodion after
iodizing, by which the intensity of the image is increased.

_Directions for using Glycyrrhizine in Collodion._--The action of this
material has been described at page 114. The Collodion should be iodized
with the Iodide of Cadmium only, or with a mixture of the Iodides and
Bromides of the alkalies. The condition which calls for the employment of
Glycyrrhizine is that often found in a newly made and rather glutinous
Collodion, viz. sensitiveness of film, with good half-tones, but
insufficient intensity in the high lights. Dissolve the Glycyrrhizine
in Alcohol (not Methylated) in the proportion of 5 grains to the ounce:
this solution may perhaps keep unchanged for three or four months. To
each ounce of the Collodion add from one to four drops, and expose in the
Camera a few seconds longer than before. The effect of the Glycyrrhizine
upon the Collodion may not be fully produced immediately; if so, the fluid
must be set aside for twenty-four hours.

_Use of Nitro-glucose in Collodion._--Nitro-glucose is a substance
analogous to Pyroxyline, but more unstable. When added to Collodion
iodized with the alkaline Iodides, it slowly decomposes, liberates Iodine,
lessens the sensitiveness to a certain extent, and confers intensity. Like
Glycyrrhizine, it may be used to remedy feebleness of the image, and to
give opacity to the blacks. Prepare the Nitro-glucose by the directions
given in the Vocabulary, Part III. Dissolve twenty grains in an ounce of
pure spirit, and agitate with powdered chalk to remove free acid. Add from
five to eight drops to each ounce of Collodion. In a few days, more or
less, according to temperature, the Collodion will deepen in colour, and
will be found on trial to produce a more vigorous picture.

_Collodion for hot Climates._--In this case the Iodide of Ammonium should
be avoided, as unstable and prone to change colour. Iodide of Cadmium
may be substituted, which has been shown to remain quite colourless when
dissolved in Alcohol and Ether.

Collodion iodized with the Iodide of Potassium will usually keep for about
six weeks or two months; but no certain rule can be given, much depending
upon the condition of the Ether and the heat of the weather.

Plain Collodion may retain its properties unimpaired for five or six
months, sometimes much longer; but there is a tendency to a formation of
the acid principle (p. 85); and hence, on the addition of an alkaline
Iodide to old Collodion, the coloration is commonly very rapid. The
structure of the transparent film may also be injured by keeping plain
Collodion for too long a time.

Photographers who wish to operate with Collodion in hot climates will
find it advantageous to carry with them the prepared Pyroxyline and the
spirituous solvents, observing that the bottles are carefully _luted_,
and that a bubble of air is left in the neck of each, to allow for the
necessary expansion, which might otherwise burst the glass or force out
the stopper.


THE NITRATE BATH.

This solution may be prepared by the same formula as that given for
direct Positives at page 203, acidifying the solution with Acetic Acid in
preference to Nitric Acid.


THE DEVELOPING SOLUTION.

  Pyrogallic Acid                      1 grain.
  Acetic Acid (glacial)               10 to 20 minims,
    or Beaufoy's Acetic Acid fort.     1 fluid drachm.
  Alcohol                             10 minims.
  Distilled Water                      1 fluid ounce.

In place of Distilled Water, pure Rain-Water may be used (see Part III.,
Art. "Water").

The quantity of Acetic Acid required will vary with the strength of
the Acid and the temperature of the atmosphere. An excess enables the
manipulator to cover the plate more easily before the action begins, but
when the picture is taken in a dull light, is apt to give a bluish, inky
hue to the image. In cold weather, use less of the Acetic and twice the
quantity of Pyrogallic Acid. With Collodion prepared from Spirits nearly
anhydrous, and iodized with Iodide of Cadmium, the full quantity of Acetic
Acid will be required, as there is sometimes a little difficulty in
making the developer flow up to the edge of the film.

If the image cannot be rendered sufficiently black, two or three minims of
the Nitrate Bath solution may be added to each drachm towards the end of
the development.

If the solution be kept for some time after its first preparation, it
becomes brown and discoloured. In this state it will still develope the
image, but is less likely to give a clear and vigorous picture. A solution
of Pyrogallic Acid in Acetic Acid will keep for many weeks, and may be
diluted down when required for use.

The following is a good formula:--

  Pyrogallic acid                    12 grains.
  Beaufoy's Acetic acid               1 fluid ounce.

To one drachm add seven drachms of water.


THE FIXING LIQUID.

  Cyanide of Potassium                2 to 12 to 20 grains.
  Water                               1 fluid ounce.

  or, Hyposulphite of Soda            1/2 ounce.
  Water                               1 fluid ounce.

For remarks on the Cyanide of Potassium Fixing Bath, see the last Section,
page 207.




CHAPTER III.

MANIPULATIONS OF THE COLLODION PROCESS.


These may be classed under five heads:--Cleaning the Plates.--Coating with
Iodide of Silver.--Exposure in the Camera.--Developing the image.--Fixing
the image.--In addition to this, the present Chapter will include in
separate Sections directions for the choice and management of lenses, for
copying engravings, manuscripts, etc., and for taking stereoscopic and
microscopic photographs.


CLEANING THE GLASS PLATES.

Care should be taken in selecting glass for use in Photography. The
ordinary window-glass is inferior, having scratches upon the surface, each
of which may cause an irregular action of the developing fluid; and the
squares are seldom flat, so that they are apt to be broken in compression
during the printing process.

The patent plate answers better than any other description of glass; but
if it cannot be procured, the "flatted crown glass" may be substituted.

Before washing the glasses, each square should be roughened on the edges
by means of a file or a sheet of emery-paper; or more simply, by drawing
the edges of two plates across each other. If this precaution be omitted,
the fingers are liable to injury, and the Collodion film may contract and
separate from the sides.

In cleaning glasses, it is not sufficient, as a rule, to wash them simply
with water; other liquids are required to remove _grease_, if present. A
cream of Tripoli powder and Spirits of Wine, with a little Ammonia added,
is commonly employed. A tuft of cotton is dipped in this mixture, and the
glasses are well rubbed with it for a few minutes. They are then rinsed in
plain water and wiped dry with a cloth.

The cloths used for cleaning glasses should be kept expressly for that
purpose; they are best made of a material sold as fine "diaper," and very
free from flocculi and loosely-adhering fibres. They are not to be washed
_in soap and water_, but always in pure water or in water containing a
little Carbonate of Soda.

After wiping the glass carefully, complete the process by polishing with
an old silk handkerchief, avoiding contact with the skin of the hand.
Some object to _silk_, as tending to render the glass electrical, and so
to attract particles of dust, but in practice no inconvenience will be
experienced from this source.

Before deciding that the glass is clean, hold it in an angular position
and _breathe_ upon it. The importance of attending to this simple rule
will be at once seen by referring to the remarks made at page 39. In the
Honey preservative and Collodio-Albumen processes it is especially needful
that the glasses should be thoroughly cleaned, on account of the tendency
which the film has to become loosened or to blister during the development
and washings. Caustic Potash, sold by the druggists under the name of
"Liquor Potassæ," is very efficacious, or in place of it, a warm solution
of "washing Soda" (Carbonate of Soda). Liquor Potassæ, being a caustic and
alkaline liquid, softens the skin and dissolves it; it must therefore be
diluted with about four parts of water and applied to the glass by means
of a cylindrical roll of flannel. After wetting both sides thoroughly,
allow the glass to stand for a time until several have been treated in the
same way; then wash with water and rub dry in a cloth.

The use of an alkaline solution is usually sufficient to clean the glass,
but some plates are dotted on the surface with small white specks, not
removable by Potash. These specks may consist of hard particles of
_Carbonate of Lime_, and when such is the case they dissolve readily in
a dilute acid,--Oil of Vitriol, with about four parts of water added, or
dilute Nitric Acid.

The objection to the use of Nitric Acid is, that if allowed to come in
contact with the dress, it produces stains which cannot be removed unless
_immediately_ treated with an alkali. A drop of Ammonia should be applied
to the spot before it becomes yellow and faded.

When Positives are to be taken, it is advisable to use additional care in
preparing the glass, and especially so with pale transparent films and
neutral, Nitrate Bath.

After a glass has been once coated with Collodion, it is not necessary in
cleaning it a second time to use anything but pure water; but if the film
has been allowed to harden and become dry, possibly dilute Oil of Vitriol
or Cyanide of Potassium may be required to remove stains.

When glasses have been repeatedly used in photography they often become at
length so dull and stained, that it is better to reject them.


COATING THE PLATE WITH THE COLLODIO-IODIDE OF SILVER.

This part of the process, with that which follows, must be conducted in
a room from which chemical rays of light are excluded. It is inferred
therefore that the operator has provided himself with an apartment of that
kind.

The most simple plan of preparing the room is to nail a treble thickness
of yellow calico completely over the window, or a part of it, the
remainder being darkened. To this a single thickness of a waterproof
material made by coating linen with gutta-percha may be added as a further
security against the entrance of white light, the smallest pencil of which
admitted into the room would cause fogging.

It is often convenient to illuminate by means of a candle screened
by yellow glass. A dark orange yellow, approaching to brown, is more
impervious to chemical rays than a lighter canary-yellow. Lamps suitable
for the purpose are sold by the manufacturers of apparatus and chemicals.

Before coating the plate with Collodion, see that the fluid is perfectly
clear and transparent, and that all particles have settled to the bottom;
also that the neck of the bottle is free from hard and dry crusts, which,
if allowed to remain, would partially dissolve and produce striæ upon the
film. In taking small portraits and stereoscopic subjects, these points
are of especial importance, and every picture will be spoiled if they are
not attended to.

A useful piece of apparatus for clearing Collodion is that represented in
the following woodcut.

[Illustration]

The Collodion, having been iodized some hours previously, is allowed to
settle down and become clear in this bottle; then by gently blowing at
the point of the shorter tube, the small glass siphon is filled, and the
fluid drawn off more closely than could be done by simply pouring from one
bottle to another.

When the Collodion is properly cleared from sediment, the operator takes
a glass plate, previously cleaned, and wipes it gently with a silk
handkerchief, in order to remove any particles of dust which may have
subsequently collected. If it be a plate of moderate size, it may be
held by the corners in a horizontal position, between the forefinger and
thumb of the left hand. The Collodion is to be poured on steadily until a
circular pool is formed, extending nearly to the edges of the glass.

[Illustration]

By a slight inclination of the plate the fluid is made to flow towards the
corner marked 1, in the above diagram, until it nearly touches the thumb
by which the glass is held: from corner 1 it is passed to corner 2, held
by the forefinger; from 2 to 3, and lastly, the excess poured back into
the bottle from the corner marked No. 4. It is then to be held vertically
over the bottle for a moment, until it _nearly_ ceases to drip, and then,
by raising the thumb a little, the direction of the plate is changed,
so as to cause the diagonal lines to coalesce and produce a smooth
surface. The operation of coating a plate with Collodion must not be done
hurriedly, and nothing is required to ensure success but steadiness of
hand and a sufficiency of the fluid poured in the first instance upon the
plate.

In coating larger plates, the _pneumatic_ holder, which fixes itself by
suction, will be found the most simple and useful.

_The Proper Time for immersing the Film in the Bath._--After exposing a
layer of Collodion to the air for a short time, the greater part of the
Ether evaporates, and leaves the Pyroxyline in a state in which it is
neither wet nor dry, but receives the impression of the finger without
adhering to it. Photographers term this _setting_, and when it takes
place it is a sign that the time has come for submitting it to the action
of the Bath.

If the film be lowered into the Nitrate before it has set, the effect is
the same as that produced by adding Water to Collodion. The Pyroxyline is
precipitated in part, and consequently there are cracks, and the developer
will not always run up to the edge of the film. On the other hand, if
it be allowed to become too dry, the Iodide of Silver does not form
perfectly, and the film, on being washed and brought out to the light,
exhibits a peculiar iridescent appearance, and is paler in some parts than
in others.

No rule can be given as to the exact time which ought to elapse: it varies
with the temperature of the atmosphere, and with the proportions of Ether
and of Pyroxyline; thin Collodion containing but little Alcohol requiring
to be immersed more speedily. Twenty seconds in the common way, or ten
seconds in hot weather, will be found an average time.

When the plate is ready, rest it upon the glass dipper, Collodion side
uppermost, and lower it into the solution by a slow and steady movement:
if any pause be made, a horizontal line corresponding to the surface
of the liquid will be formed. Then place the cover upon the vertical
trough[35] and darken the room, if this has not already been done. As
the presence of white light does no injury to the plate previous to its
immersion in the Bath, it is not necessary to exclude it during the time
of coating with Collodion.

[35] Troughs made of gutta-percha, glass, or porcelain are commonly used;
the latter are the best, being quite opaque and not liable to cracks or
leaking.

When the plate has remained in the solution about twenty seconds, lift
it partially out two or three times, in order to wash away the Ether
from the surface. An immersion of one minute to a minute and a half will
usually be sufficient; or two minutes in cold weather, and with Collodion
containing but little Alcohol. Continue to move the plate until the liquid
flows off in a uniform sheet, when the decomposition may be considered
to be sufficiently perfect. The principal impediment in this part of the
process lies in the difficulty with which Ether and Water mix together,
which causes the Collodion surface on its first immersion to appear oily
and covered with streaks. By gentle motion the Ether is washed away, and a
smooth and homogeneous layer obtained.

The plate is next removed from the dipper, and held vertically in the hand
for a few seconds upon blotting paper, to drain off as much as possible of
the solution of Nitrate of Silver.[36] It is then wiped on the back with
filtering-paper, placed in a clean and dry slide, and is ready for the
Camera.

[36] This blotting-paper must be frequently changed, or stains will be
produced at the lower edge of the plate during the development.

The amateur is strongly recommended not to proceed to take pictures in the
Camera until by a little practice he has succeeded in producing a perfect
film which is uniform in every part and will bear inspection when washed
and brought out to the light.

It should, if properly prepared, present the following appearance:--Smooth
and uniform, both by reflected and transmitted light; free from wavy
lines or markings such as would be caused by a glutinous Pyroxyline, and
from opaque dots due to small particles of dust or Iodide of Silver in
suspension in the Collodion.

The evidences of a too rapid immersion in the Bath are sought for on the
side of the plate from which the Collodion was poured off. This part
remains wet longer than the other, and always suffers the most; horizontal
cracks or marks resembling vegetation are seen, each of which would cause
an irregular action of the developing fluid. On the other hand, the upper
part of the plate must be examined for the pale colour characteristic of
a film which had become too dry before immersion, since the Collodion is
thinner at that point than at any other.


EXPOSURE OF THE PLATE IN THE CAMERA.

After the plate has been rendered sensitive, it should be exposed and
developed with all convenient despatch; the intensity of the Negatives
being, with some Collodion, materially lessened by neglecting this point
(see p. 100).

Ascertain that the joints of the Camera are tight in every part--that the
sensitive plate, when placed in the slide, falls precisely in the same
plane as that occupied by the ground glass--and that the chemical and
visual foci of the Lens accurately correspond.[37]

[37] See the Second Section of this Chapter.

Supposing the case of a portrait, next proceed to arrange the sitter
as nearly as possible in a vertical position, that every part may be
equidistant from the lens. Then, an imaginary line being drawn from the
head to the knee, point the Camera slightly downwards, so that it may
stand at right angles to the line. If this point be neglected the figure
will be liable to be distorted in a manner presently to be shown (p. 228).

In order to succeed well with portraits, the sitter should be illuminated
by an even, diffused light falling horizontally. A vertical light causes a
deep shadow on the eyes and makes the hair appear grey: it must therefore
be cut off by a curtain of blue or white calico suspended over the head.
The direct rays of the sun are generally to be avoided, as causing too
great a contrast of light and shade. This is a point on which the operator
must exercise his judgment. With a feeble Collodion, a better Negative
picture may often be obtained by placing the sitter quite in the open air,
but when the Collodion and Bath are in the condition for giving great
intensity of image, the gradation of tone will be inferior unless the
light be prevented from falling too strongly upon the face and hands.

In focussing the object, cover the head and the back part of the Camera
with a black cloth, and shift the Lens gently until the greatest possible
amount of distinctness is obtained. Then insert the sensitive plate, and
having raised the door of the slide, cover all with a black cloth during
the exposure, as a security against white light finding entrance at any
part excepting through the Lens.

With regard to the proper time for the exposure, so much depends upon the
brightness of the light and the nature of the Collodion, that it must be
left almost entirely to experience. The following general rules however
may be of use:--

In a tolerably bright day in the spring or summer months, and with a
newly-mixed Collodion, allow four seconds for a Positive Portrait, and
eight seconds for a Negative. With a double-combination Lens of large
aperture and short focus, perhaps three seconds, and six seconds, or even
less, may be sufficient.

In the dull winter months, in the smoky atmosphere of large cities,
or when using an old Collodion brown from free Iodine, multiply these
numbers three or four times, which will be an approximation to the
exposure required. It is by the appearance presented under the influence
of the developer, which will immediately be described, that the operator
ascertains the proper time for exposure to light.


THE DEVELOPMENT OF THE IMAGE.

The details of developing the latent image differ so much in the case of
Positive and Negative pictures, that it is better to describe the two
separately.

The development of direct Positives.--With Sulphate of Iron as a
developer, it is most simple to develope the image by immersion. The
solution may conveniently be poured into a vertical trough, such as that
used for exciting, and the plate immersed by means of a glass dipper in
the usual way. Unless the weather be cold, the image makes its appearance
in three or four seconds, and the film is then immediately washed with
clear water. Whilst in the Bath, the plate is kept in gentle motion, and
the operator must not expect to see the image very distinctly, except
the high lights; the shadows, being faint, are partially concealed by
the unaltered Iodide, but they come out during the fixing. The action
of the Sulphate of Iron is stopped at an early period, or an excess of
development will be incurred. The Bath may be used repeatedly.

In using Pyrogallic Acid or Nitrate of Iron to develope glass Positives,
the plate may be placed upon a levelling-stand, or held in the hand, or by
the pneumatic holder, and the solution poured on quickly at one corner; by
blowing gently or inclining the hand, as the case may be, it is scattered
evenly over the film before the development commences.

If any difficulty is experienced in covering a plate evenly with a strong
developer before the action commences, it may be overcome by using a
shallow cell formed by cementing two or three thicknesses of window-glass
on a piece of patent plate to the depth of a quarter of an inch. The size
of the cell should be only slightly larger than the plate intended to be
developed, that the waste of fluid may be as little as possible.

The cell is held in the left hand, and the plate being placed in it, a
sufficient quantity of the developer is poured on at one corner. By a
slight inclination, the fluid is caused to flow in a uniform sheet over
the surface of the film, backwards and forwards. The image starts out
quickly, and the developer is then at once poured off, and the film washed
as before.

It is very important in developing Positives to use a sufficient quantity
of the solution to cover the plate easily; otherwise oily stains and marks
are formed, from the developer not combining properly with the surface of
the film. For a plate five inches by four, three or four drachms will be
required, and so in proportion for larger sizes.

The appearance of the Positive image after developing, as a guide to
the proper time of exposure.--When the plate has been developed, it is
washed, fixed, and laid upon a dark ground, such as a piece of black
velvet, for inspection.

In the case of a portrait, if the features have an unnaturally black
and gloomy appearance, the dark portions of the drapery, etc., being
invisible, the picture has been _under-exposed_.

On the other hand, in an over-exposed plate, the face is usually pale and
white, and the drapery misty and indistinct. Much however in this respect
depends upon the dress of the sitter (see p. 66), and the manner in which
the light is thrown; if the upper part of the figure is shaded too much,
the face may perhaps be the last to be seen. The operator should accustom
himself to expend pains in the preliminary focussing upon the ground
glass, and to ascertain at that time that every part of the object is
equally illuminated. For this reason, pictures taken in a room are seldom
successful; the light falls entirely upon one side, and hence the shadows
are dark and indistinct.

_The development of Negative Pictures._--This process differs in most
respects from that of Positives. In the latter case, there is a tendency
to over-develope the image; but in the former, to stop the action at too
early a period; hence it is common to find Negative Pictures which are
insufficiently developed, and too pale to print well.

In developing Negatives, many operators place the plate upon a
levelling-stand, and distribute the fluid by blowing gently upon the
surface; others prefer holding it in the hand and pouring the fluid on and
off from a glass measure. The quantity of developer required will be less
than that used for Positives, inasmuch as, if the Acetic Acid be present
in sufficient excess, it is easy to cover the plate before the action
begins. Some Collodion however, especially the glutinous kind, seems to
repel the developer and prevent it from running up to the edge of the
plate. When this is the case, or when oiliness and stains are produced,
from the Bath being old and containing Ether, Alcohol must be added to
the solution of Pyrogallic Acid.

With ordinary Negative Collodion, an addition of Nitrate of Silver to the
developer will often be required; but the Pyrogallic Acid is to be used
alone until the image has reached its maximum of intensity, which it will
do in a minute or so, according to the temperature of the developing room.
The plate may then be examined leisurely by placing it in front of, and
at some distance from, a sheet of white paper. If it is not sufficiently
black, add about four drops of the Nitrate Bath to each drachm of
developer, stir well with a glass rod, and continue the action until the
requisite amount of intensity is obtained. When there is any disposition
in the plate to _fog_ towards the end of the development, it may be
obviated by fixing with Cyanide of Potassium (not Hyposulphite), and then,
after a careful washing, intensifying with Pyrogallic Acid and Nitrate
of Silver in the usual way. The glass which contains the mixture of
Pyrogallic Acid and Nitrate of Silver must be washed out after each plate,
as the black deposit hastens the discoloration of the fresh solution (p.
179).

_Appearance of the Negative image during and after the reducing process,
as a guide to the exposure to light._--An under-exposed plate developes
slowly. By continuing the action of the Pyrogallic Acid, the high lights
_become very black_, but the shadows are invisible, nothing but the yellow
Iodide being seen on those portions of the plate. After treatment with the
Cyanide, the picture shows well as a Positive, but by transmitted light
all the minor details are invisible; the image is black and white, without
any half-tone.

An over-exposed Negative developes rapidly at first, but soon begins
to blacken slightly at every part of the plate. After the fixing is
completed, nothing can often be seen by reflected light but a uniform
grey surface of metallic Silver, without any appearance (or, at most, an
indistinct one) of an image. By transmitted light the plate may appear of
a red or brown colour, and the image is _faint_ and dull. The clear parts
of the Negative being obscured by the fogging, and the half-shadows having
acted so long as nearly to overtake the lights, there is a want of proper
_contrast;_ hence the over-exposed plate is the exact converse of the
under-exposed, where the contrast between lights and shadows is too well
marked, from the absence of intermediate tints.

A Negative which has received the proper amount of exposure,
usually possesses the following characters after the development is
completed:--The image is partially but not fully seen by reflected light.
In the case of a portrait, any dark portions of drapery show well as a
Positive, but the features of the sitter are scarcely to be discerned.
The plate has a general aspect as of fogging _about to commence_, but
not actually established. By transmitted light the figure is bright, and
appears to stand out from the glass: the dark shadows are clear, without
any misty deposit of metallic Silver; the high lights black _almost_ to
complete opacity. The _colour_ of the image however varies much with the
state of the Bath and Collodion and with the brightness of the light.

The remarks already made under the head of Positives, apply equally well
to Negatives; that is, it will be difficult to secure gradation of tone,
unless the object be _equally_ illuminated, without any strong contrast of
light and shade. Hence the direct rays of the sun are, as a rule, to be
avoided, and curtains, etc., employed when practicable.


FIXING AND VARNISHING THE IMAGE.

After the development is completed, and the plate has been carefully
washed by a stream of water, it may be brought out to the light and
treated with the Hyposulphite or Cyanide, until the unaltered Iodide is
entirely cleared off. Some use a Bath for the Cyanide; but it is doubtful
whether much saving is effected by doing so. The plate is again to be
carefully washed after the fixing; and especially if Hyposulphite of
Soda be used. Three or four minutes in running water will not be too
long, or the glass may be left in a dish of water for an hour or two. If
such precautions are neglected, crystals form on drying, and the image is
injured.

Collodion pictures should be protected by a coat of varnish, both
Negatives and Positives having been known to fade when exposed to damp air
without any covering (see p. 166). To prepare transparent varnish. Amber
may be dissolved in Chloroform according to Dr. Diamond's formula;--about
80 grains of amber-beads or pipe-stems should be digested with one ounce
of the Chloroform, and the clear portion separated by filtration. It may
be poured on the plate in the same manner as Collodion, and dries up
speedily into a hard and transparent layer. The Spirit Varnish ordinarily
sold for Negatives requires the aid of heat to prevent the gum from
chilling as it dries; the plate is first warmed gently and the varnish
poured on and off in the usual way; it is then, whilst still dripping,
held to the fire until the Spirit has evaporated. A few trials will render
the operation easy to perform. White Lac dissolved in strong Alcohol or in
Benzole has also been recommended for clear varnish.

Direct Positives are to be varnished, first with a layer of transparent
varnish, and then with black japan. Suggett's patent jet is sometimes
employed, but it has a disagreeable smell, and is apt to crack on drying.
The best black japan used by coachmakers is more elastic and less liable
to crack. Asphalt (4 oz.) dissolved in mineral Naphtha (10 oz.), with
the addition of 30 grains of Caoutchouc dissolved in half an ounce of
the same menstruum, is also said to stand well. A third formula contains
black sealing-wax dissolved in Alcohol. In either case it will be best to
apply first a layer of clear varnish to the film, and afterwards the black
varnish, which should combine with the other without dissolving it.

Positives whitened with Bichloride of Mercury are injured by varnishing;
they must therefore be backed up with black velvet, or Japan laid upon the
opposite side of the glass. Many prefer taking the picture upon coloured
glass, using only a layer of clear varnish; but in this case the Collodion
side being left uppermost, the image is necessarily reversed.


SECTION II.

_Directions for the use of Photographic Lenses._

Those who are comparatively unacquainted with the science of optics
require simple rules to guide them in the choice of a photographic lens,
and in the proper mode of using it.

Two kinds of Achromatic lenses are sold, the Portrait lens and the View
lens; the former of which is constructed to admit a large volume of light,
for the purpose of copying living objects, etc.

A convenient-sized Camera for small portraits is "the half-plate" with a
lens of about 2-1/4 inches diameter, and giving a tolerably flat field on
a surface of 5 inches by 4. Much however in this respect will depend upon
the quality of the glass and also upon its focal length; a short focus
lens taking a picture more quickly, but giving a smaller image, and a
field which is misty towards the edge. There is also a great tendency to
_distortion_ of the image in portrait lenses of large aperture and short
focus, such as those employed for operating in a dull light.

The "whole plate" portrait lens may be expected to cover 6-1/2 by 4-3/4
inches, and has a diameter of about 3-1/4 inches. It will take larger
pictures than the last, but not necessarily in a shorter time; since,
although the aperture for admitting the light is larger, the focal length
is proportionately greater and the light less condensed.

The "quarter-plate" portrait lens of 1-1/4 inch diameter is useful for
stereoscopic subjects and small portraits; which are usually more sharply
defined when taken with a small lens.

The distance at which the Camera is to be placed from the sitter in taking
a portrait, will depend upon the focal length of the lens. The effect of
bringing the Camera nearer is to add to the size of the image, but at the
same time to increase the chance of distortion; hence with every lens of
full aperture, there is a practical limit to the size of picture which can
be taken.

When it is required to obtain a large image with a small lens, a stop
with a central aperture (which may be readily made of a piece of circular
cardboard blackened with Indian ink) must be placed in front of the lens.
This will diminish the amount of light, but will render the picture more
distinct towards the edge, and bring a variety of objects at different
distances into focus at the same time. With a stop attached, the lens may
also be brought nearer to the object without distorting.

With regard to this subject of the distortion often produced by lenses,
observe particularly, that with the portrait combination of full aperture,
and especially when the powers of the glass are rather strained by its
being advanced too near to the sitter,--all objects near to the lens will
be _magnified_, and those more removed will appear diminished; hence, as
the position of the sitter is never quite vertical, the Camera must be
inclined a little _downwards_, or the hands and feet will be enlarged,
the figure in fact becoming pyramidal with the base below; whereas on the
other hand, if the inclination of the Camera be too great, the head and
forehead will be enlarged, and the figure becomes a pyramid with the base
above.

When groups are taken, arrange the objects as near as possible equidistant
from the lens, and use a stop if practicable. Long-focus lenses are the
best for this purpose, allowing the Photograph to be taken further off,
and giving a greater variety of objects in focus at the same time.

Portrait lenses may often be advantageously substituted for View lenses
in copying objects of still life which are _badly lighted_. The aperture
of the lens being large, a Negative can be obtained with an amount of
light which would not suffice if a small stop were used. On the other
hand, if the light be unusually bright, the lens of full aperture is
always the most likely, from its extent of reflecting surface, to produce
a misty and indistinct image. Hence the object should be well backed up
with some neutral colour, or, if that cannot be done, a pasteboard funnel,
projecting about a foot and a half, may be fastened in front of the
lens, in order to exclude rays of light not immediately concerned in the
formation of the image. If the lens were turned towards distant objects
brightly illuminated, and a portion of sky included, there would probably
be diffused light, and consequent fogging of the plate on the application
of the developer. This effect will also invariably follow if the sun's
rays be allowed to fall directly upon the glass.

_Directions for finding the Plane at which the Sharpest Image can be
obtained._--Non-Achromatic Lenses are understood by all to require
correction for the chemical focus; but it is usually said of the compound
glasses, that their two foci correspond. The amateur is recommended, in
order to avoid disappointment, to test the accuracy of this statement, and
also to see that his Camera is constructed with care. To do this, proceed
as follows:--

First ascertain that the prepared sensitive plate falls precisely in
the plane occupied by the ground glass. Suspend a newspaper or a small
engraving at the distance of about three feet from the Camera, and focus
the letters occupying the centre of the field; then insert the slide, with
a square of _ground glass_ substituted for the ordinary plate (the rough
surface of the glass looking inwards), and observe if the letters are
still distinct. In place of the ground glass, a transparent plate with a
square of silver-paper which has been oiled or wetted, may be used, but
the former is preferable.

If the result of this trial seems to show that the Camera is good, proceed
to test the correctness of the Lens.--

Take a Positive Photograph with the full aperture of the portrait Lens,
the central letters of the newspaper being carefully focussed as before.
Then examine at what part of the plate the greatest amount of distinctness
of outline is to be found. It will sometimes happen, that whereas the
exact centre was focussed visually, the letters on a spot midway between
the centre and edge are the sharpest in the Photograph. In that case the
chemical focus is longer than the other, and by a distance equivalent to,
but in the opposite direction of, the space which the ground glass has to
be moved, in order to define those particular letters sharply to the eye.

When the chemical focus is the shorter of the two, the letters in the
Photograph are indistinct at every portion of the plate; the experiment
must therefore be repeated, the lens being shifted an eighth of an inch
or less. Indeed it will be proper to take many Photographs at minute
variations of focal distance before the capabilities of the lens will be
fully shown.

The object of finding the point at which the sharpest image is obtained
will also be assisted by placing several small figures in different planes
and focussing those in the centre. This being done, if the more distant
figures come out distinctly in the Photograph, the chemical focus is
_longer_ than the Visual, or _vice versâ_ when the nearest ones are most
sharply defined.

_The Single Achromatic Lens._--A useful lens for landscape Photography
is one of about 3 inches diameter and 15 inches focal length, which may
be expected to cover a field of 10 inches by 8. With the lens, stops are
supplied of various diameters, the largest of which will be useful in dull
weather; the smaller when the field is required to be rendered sharp to
the very edge.

The stop is arranged at a certain distance in front of the lens, and must
not be moved. If it were brought close up to the glass, the field would
not be so flat; the effect being then the same as that of a stop placed in
front of a Portrait Lens, viz. simply to cut off the outside portion of
the glass.[38]

[38] See this subject explained in 'Photographic Journal,' vol. ii. p. 133.

In taking Photographs of architectural and other subjects with vertical
outlines, it is very important to have the Camera placed perfectly
horizontal; since, if it be inclined either upwards or downwards, the
perpendiculars will be destroyed and the object will appear of a pyramidal
form, falling inwards or outwards, as before shown. It is convenient to
rule the ground focussing glass with a number of parallel lines in both
directions, which enables the operator at once to see that the position of
the instrument is correct.


SECTION III.

_Mode of copying Engravings, Etchings, etc._

The engraving to be Photographed should be removed from its frame (the
glass causing irregular reflection) and suspended vertically and in a
reversed position, in a good diffused light. A black cloth may be placed
behind the picture with advantage if any surface likely to reflect light
be presented to the lens.

The Camera must be fixed immovably, so as not to vibrate in the least
degree when the cap of the lens is taken off. It should be pointed at
right angles to the picture, and the focus determined in the ordinary
way. Either a portrait or a single lens may be used, with a diaphragm
sufficiently small to render the image distinct up to the edge.

It is not desirable to employ too thin a Collodion, since perfect opacity
of the darkest parts of the Negative is essential. An old Collodion
containing free Iodine is better than a contractile Collodion, as giving
a more intense and clear image. Pure Collodion iodized with Iodide of
Cadmium, if found wanting in intensity, may be at once rendered fit for
use in copying engravings by adding Glycyrrhizine (p. 209), until the dark
parts of the negative become very opaque, and subsequently softening the
excessive hardness, if necessary, by dropping alcoholic solution of Iodine
into the Collodion until it reaches a straw-yellow tint. A second formula
useful in iodizing Collodion for a similar purpose is as follows.

  Iodide of Potassium                4 grains.
  Bromide of Potassium               1 grain.

This, with addition of Glycyrrhizine, will give a very black image.

Etchings, diagrams, and drawings with pencil or ink, without much
middle-tint, if on thin paper, are easily copied without the aid of the
Camera, by simply laying the sketch upon a sheet of Negative Paper,
exposing for a brief time to the light, and developing with Gallic Acid.
This yields a Negative which is employed for printing Positives in the
usual way. Full directions on this subject will be found in the Second
Section of the following Chapter.

A more simple plan, and one which will succeed when great delicacy is not
required, consists in laying the sketch upon a sheet of Positive printing
paper (a highly salted paper will be the best, as giving most intensity)
and exposing to the light until a copy is obtained. All the details are
faithfully rendered in this way, but it is sometimes difficult to obtain a
Negative sufficiently black to yield a _vigorous_ print.


SECTION IV.

_Rules for taking Stereoscopic Photographs._

Binocular pictures of a large size, for the reflecting Stereoscope, may
be taken with an ordinary View lens of about 15 inches focus. The ground
glass of the Camera having been ruled with cross lines in the manner
described at page 231, the position of some prominent object is marked
upon one of the lines with a pencil, and the first view is taken. The
stand is then moved laterally to the proper distance, and the Camera
adjusted to its second position by shifting it until the marked object
occupies the same place as before. The distance between the two positions
should be about one foot when the foreground of the picture is twenty-five
feet from the instrument, or four feet when it is at thirty or forty
yards. But, as before shown at page 71, this rule is not to be followed
implicitly, much depending upon the character of the picture and the
effect desired.

Photographs for the lenticular Stereoscope are taken with small lenses
of about 4-1/2 inches focus. For portraits, a Camera may advantageously
be fitted with two double-combination lenses, of 1-3/4 inches diameter,
exactly equal in focal length and in rapidity of action. The caps are
removed simultaneously, and the pictures impressed at the same instant.
The centres of the lenses may be separated by three inches when the Camera
is placed at about six feet from the sitter, or four inches when the
distance is increased to eight feet.

Pictures taken with a binocular Camera of this kind, require to be mounted
in a reversed position to that which they occupy on the glass: for since
the image of the Camera is _inverted_, when it is turned round and made
erect, the right-hand picture will necessarily stand on the left side, and
_vice versâ_.

Mr. Latimer Clark has devised an arrangement for taking stereoscopic
pictures with a single Camera, which is exceedingly ingenious. Its most
important feature is a contrivance for rapidly moving the Camera in a
lateral direction without disturbing the position of the image upon the
ground glass. This will be understood by a reference to the following
woodcut.

[Illustration]

"A strongly-framed Camera-stand carries a flat table, about 20 inches wide
by 16, furnished with the usual adjustments. Upon this are laid two flat
bars of wood in the direction of the object, and parallel, and about the
width of the Camera asunder. They are 18 inches in length; their front
ends carry stout pins, which descend into the table and form centres
upon which they turn. Their opposite ends also carry similar pins, but
these are directed upwards, and fit into two corresponding holes in the
tail-board of the Camera.

"Now when the Camera is placed upon these pins, and moved to and fro
laterally, the whole system exactly resembles the common parallel ruler.
The two bars form the guides, and the Camera, although capable of free
lateral motion, always maintains a parallel position. In this condition
of things it is only suited to take stereoscopic pictures of an object
at an infinite distance; but to make it move in an arc, _converging_ on
an object at any nearer distance, it is only necessary to make the two
guide-bars approximate at their nearer end so as to converge slightly
towards the object; and by a few trials some degree of convergence will
be readily found at which the image will remain as it were _fixed_ on the
focussing glass while the Camera is moved to and fro. To admit of this
adjustment, one of the pins descends through a Slot in the table and
carries a clamping-screw, by means of which it is readily fixed in any
required position.

"In order however to render the motion of the Camera smoother, it is
advisable not to place it directly upon the two guides, but to interpose
two thin slips of wood, lying across them at right angles, beneath the
front and back of the Camera respectively (and which may be fixed to the
Camera if preferred), and to dust the surfaces with powdered soap-stone or
French chalk."

In addition to this arrangement for moving the Camera laterally, the
_slide_ for holding the sensitive plates must be modified from the common
form. It is oblong in shape, and being about ten or eleven inches long,
requires some little adaptation to fit it to the end of an ordinary
Camera. The glasses are cut to about 6-3/4 inches by 3-1/4; and when
coated with Iodide of Silver, the two images are impressed side by side,
the plate being shifted laterally about 2-1/2 inches, at the same time and
in the same direction as the Camera itself.

The operation of taking a portrait is thus performed. The focus having
been adjusted for both positions, and the Camera and the slide both drawn
to the left-hand, the door is raised and the plate exposed; the Camera and
the slide are then shifted to the right-hand, and the plate in its new
position having been again exposed, the door is closed and the operation
completed.[39]

[39] See 'Photographic Journal,' vol. i. page 59.

Pictures taken with this instrument do not require to be reversed in
mounting, the left picture being purposely formed on the right-hand side
of the glass.


SECTION V.

_On the Photographic delineation of Microscopic objects._

Many specimens of Micro-photography which have been exhibited are
exceedingly elaborate and beautiful; and their production is not
difficult to one thoroughly acquainted with the use of the Microscope and
with the manipulations of the Collodion Process. It is important however
to possess a good apparatus, and to have it properly arranged.

The object-glass of the ordinary compound Microscope is the only part
actually required in Photography, but it is useful to retain the _body_
for the sake of the adjustments, and the mirrors used in the illumination.
The _eye-piece_ however, which simply magnifies the image formed by the
object-glass, is not necessary, since the same effect of enlargement may
be obtained by lengthening out the dark chamber, and throwing the image
further off.

_Arrangement of the Apparatus._--The Microscope is placed with its body
in a horizontal position, and the eye-piece being removed, a tube of
paper, properly blackened in the interior, or lined with black velvet, is
inserted into the instrument, to prevent irregular reflection of light
from the sides.

A dark chamber of about two feet in length, having at one end an aperture
for the insertion of the eye-piece end of the body, and at the other a
groove for carrying the slide containing the sensitive plate, is then
attached; care being taken to stop all crevices likely to admit diffused
light. An ordinary Camera may be employed as the dark chamber, the lens
being removed, and the body lengthened out if required by a conical tube
of gutta-percha, made to fasten into the flange of the lens in front. The
whole apparatus should be placed exactly in a straight line, that the
ground glass used in focussing may fall at right angles to the axis of the
Microscope.

The length of the chamber, measuring from the object-glass, may be from
two to three feet, according to the size of image required; but if
extended beyond this, the pencil of light transmitted by the object-glass
is diffused over too large a surface, and a faint and unsatisfactory
picture is the result. The object should be illuminated by sunlight if
it can be obtained, but a bright diffused daylight will succeed with
low-power glasses, and especially when Positives are taken. Employ the
concave mirror for reflecting the light on the object in the latter case;
but in the former the _plane mirror_ is the best, except with powers
exceeding a quarter of an inch, and of large angular aperture.

The image upon the ground glass should appear bright and distinct, and the
field of a circular form and evenly illuminated; when this is the case,
all is ready for inserting the sensitive plate.

The time of exposure must be varied according to the intensity of the
light, the sensibility of the Collodion, and the degree of magnifying
power; a few seconds to a minute will be about the extremes; but minute
directions are not required, as the operator, if a good Photographer, will
easily ascertain the proper time for exposing (see page 224).

At this point a difficulty will probably occur from the plane of the
chemical focus not corresponding, as a rule, with that of the visual
focus. This arises from the fact that the object-glasses of Microscopes
are "over-corrected" for colour, in order to compensate for a little
chromatic aberration in the eye-piece. The violet rays, in consequence
of the over-correction, are projected _beyond_ the yellow, and hence the
focus of chemical action is further from the glass than the visible image.

The allowance may be made by shifting the sensitive plate, or, what
amounts to the same thing, by removing the object-glass a little _away_
from the object with the fine adjustment screw; the latter is the most
convenient. The exact distance must be determined by careful experiment
for each glass; but it is greatest with the low powers, and decreases as
they ascend.

Mr. Shadbolt gives the following as a guide:--"An inch and a half
objective of Smith and Beck's make required to be shifted 1/50th of an
inch, or two turns of _their_ fine adjustment; a 2/3rds of an inch,
1/200th of an inch, or half a turn; and a 4/10ths of an inch, 1/1000th
of an inch, or about two divisions of the adjustment. With the 1/4th
and higher powers, the difference between the foci was so small as to be
practically unimportant."

There is also reason to think that the _kind of light_ employed has an
influence upon the separation of the foci. Mr. Delves finds that with
sunlight the difference between them is very small even with the low
powers, and inappreciable with the higher; whereas in using diffused
daylight which has undergone a previous reflection from white clouds, it
is considerable.

The object-glasses of the same maker, and particularly those of different
makers, also vary much; so that it will be necessary to test each glass
separately, and to register the allowance which is required.

Having found the chemical focus, the principal difficulty has been
overcome, and the remaining steps are the same in every respect as for
ordinary Collodion Photographs.

To those who cannot devote their time to Photography during the day,
Mr. Shadbolt's observations on the use of artificial light may be of
service. He employs _Camphine_, which gives a whiter flame than gas, or a
moderator lamp; placing the source of light in the focus of a plano-convex
lens of 2-1/2 to 3 inches diameter (the flat side towards the lamp), and
condensing the parallel rays so obtained on the object, by a second lens
of about 1-1/2-inch diameter and 3-inch focus.

This mode of illumination, being feeble in chemical rays, is best adapted
for object-glasses of low power. The exposure required to produce a
Negative impression with the one-inch glass may be from three to five
minutes. As the sensitive plate would be liable to become dry during that
time, it is recommended to coat it with some preservative solution by the
modes described in the sixth Chapter. Mr. Crookes having lately shown that
the Bromide of Silver is more sensitive than the Iodide to artificial
light, a mixture of the two salts may conveniently be used (see pp. 66 and
232).

The development may be conducted in the same manner as that for preserved
sensitive plates; fixing with Cyanide of Potassium before the development
is fully complete, if any tendency to fogging is observed (see page 224).

The Rev. W. Towler Kingsley has communicated a process by which very
beautiful Microscopic Photographs have been obtained. He illuminates (in
the absence of sunlight) with the brilliant light produced by throwing
a jet of mixed Oxygen and Hydrogen gases upon a small cone of Lime
or Magnesia. Particular stress is laid upon the object-glass of the
Microscope being a good one for the purpose; and indeed all who have given
attention to the subject are agreed upon this point--that there is a
considerable difference in the Photographic value of objectives, and this
independent of the angular aperture of the glass.




CHAPTER IV.

THE PRACTICAL DETAILS OF PHOTOGRAPHIC PRINTING.


This Chapter is divided as follows:--

  Section I.--The ordinary direct process of positive printing.
  Section II.--Positive printing by development.
  Section III.--The mode of toning Positives by Sel d'or.
  Section IV.--On printing enlarged or reduced Positives,
    transparencies, etc.


SECTION I.

_Positive Printing by the direct action of Light._

This includes--the preparation of sensitive paper,--of fixing and toning
Baths,--and the manipulatory details of the process.

_Selection of Paper for Photographic Printing._--The ordinary varieties
of paper sold in commerce are not well adapted for the production of
Positive prints. Papers are manufactured purposely which are more smooth
and uniform in texture. Many samples of even the finest paper are however
defective, and hence each sheet should be examined separately by holding
it against the light, and if spots or irregularities of texture are seen,
it should be rejected. These spots usually consist of small particles of
brass or iron, which, when the paper is rendered sensitive, decompose the
Nitrate of Silver and leave a circular mark very noticeable after fixing.

The foreign papers, French and German, are different from the English.
They are porous and sized with starch, the English being sized with
gelatinous animal matter. In all cases there is a difference in smoothness
between the two sides of the paper, which may be detected by holding each
sheet in such a manner that the light strikes it at an angle; the wrong
side is that on which dark wavy bands, of an inch to an inch and a half
in breadth, are seen, caused by the strips of felt on which the paper
was dried. With most qualities of paper no difficulty whatever will be
experienced in detecting the broad and regular bands above referred to;
but when they cannot be seen, the wrong side of the sheet may be known
by wire markings crossing each other, or if the paper be wetted at the
corner, one side may appear evidently smoother than the other.


PREPARATION OF SENSITIVE PAPER.

There are three principal varieties of sensitive paper in common use, viz.
the Albuminized, the plain, and the Ammonio-Nitrate paper.

Formula I. _Preparation of Albuminized Paper._--This includes the salting
and albuminizing, and the sensitizing with Nitrate of Silver.

_The Salting and Albuminizing._--Take of

  Chloride of Ammonium, or Pure
    Chloride of Sodium                   200 grains.
  Water                                   10 fluid ounces.
  Albumen                                 10 fluid ounces.

If distilled water cannot be procured, rain water or even common spring
water[40] will answer the purpose. To obtain the Albumen, use new-laid
eggs, and be careful that in opening the shell the yolk is not broken;
each egg will yield about one fluid ounce of Albumen.

[40] If the water contained much Sulphate of Lime, it is likely that the
sensitiveness of the paper would be impaired (?).

When the ingredients are mixed, take a bundle of quills or a fork, and
beat the whole into a perfect froth. As the froth forms, it is to be
skimmed off and placed in a flat dish to subside. The success of the
operation depends entirely upon the manner in which this part of the
process is conducted;--if the Albumen be not thoroughly beaten, flakes of
animal membrane will be left in the liquid, and will cause streaks upon
the paper. When the froth has partially subsided, transfer it to a tall
and narrow jar, and allow to stand for several hours, that the membranous
shreds may settle to the bottom. Then pour off the upper clear portion,
which is fit for use. Albuminous liquids are too glutinous to run well
through a paper filter, and are better cleared by subsidence.

A more simple plan than the above, and one equally efficacious, is to
fill a bottle to about three parts with the salted mixture of Albumen and
water, and to shake it well for ten minutes or a quarter of an hour until
it loses its glutinosity and can be poured out smoothly from the neck of
the bottle. It is then to be transferred to an open jar, and allowed to
settle as before.

The solution prepared by the above directions will contain exactly ten
grains of salt to the ounce, dissolved in an equal bulk of Albumen and
water. Some operators employ the Albumen alone without an addition of
water; but this commonly gives a highly varnished appearance, which is
thought by most to be objectionable. Much however will depend upon the
kind of paper which is employed, certain varieties taking more gloss than
others; Papier Rive, for instance, often requires the Albumen to be nearly
or quite undiluted.

The principal difficulty in Albuminizing paper, is to avoid the occurrence
of _streaky lines_, which, when the paper is rendered sensitive, _bronze_
strongly under the influence of the light. To avoid them, use the eggs
quite fresh, and lower the paper on to the liquid by one steady movement;
if a pause be made, a line will probably be formed. Some papers are not
readily wetted by the Albumen, and when such is the case, a few drops of
spirituous solution of bile, or a fragment of the prepared Ox-Gall sold
by the artists' colour-men, will be found a useful adjunct. Care must be
taken however not to add an excess, or the Albumen will be rendered too
fluid, and will sink into the paper, leaving no gloss.

In salting and albuminizing Photographic paper by the formula above given,
it is found that each quarter-sheet, measuring eleven inches by nine
inches, removes one fluid drachm and a half from the bath, equivalent
to about one grain and three-quarters of salt (including droppings). In
salting plain paper, each quarter-sheet takes up only one drachm; so that
the glutinous nature of the Albumen causes a third part more of salt to be
retained by the paper.

English papers are not good for albuminizing; they do not take the Albumen
properly, and curl up when laid upon the liquid: the process of toning the
prints is also slow and tedious. The thin negative paper of Canson, the
Papier Rive, and Papier Saxe, have succeeded with the writer better than
Canson's Positive paper, which is often recommended; they have a finer
texture, and give more smoothness of grain.

To apply the Albumen, pour a portion of the solution into a flat dish to
the depth of half an inch. Then, having previously cut the paper to the
proper size, take a sheet by the two corners, bend it into a curved form,
convexity downwards, and lay it upon the Albumen, the centre part first
touching the liquid, and the corners being lowered gradually. In this way
all bubbles of air wall be pushed forwards and excluded. One side only of
the paper is wetted: the other remains dry. Allow the sheet to rest upon
the solution for _one minute and a half_, and then raise it off, and pin
it up by two corners. If any circular spots, free from Albumen, are seen,
caused by bubbles of air, replace the sheet for the same length of time as
at first.

The paper must not be allowed to remain upon the salting Bath much longer
than the time specified, because the solution of Albumen being _alkaline_
(as is shown by the strong smell of Ammonia evolved on the addition of the
Chloride of Ammonium) tends to remove the size from the paper and to sink
in too deeply; thus losing its surface gloss.

Albuminized paper will keep a long time in a dry place. Some have
recommended to press it with a heated iron, in order to coagulate the
layer of Albumen upon the surface; but this precaution is unnecessary,
since the coagulation is perfectly effected by the Nitrate of Silver used
in the sensitizing; and it is doubtful whether a layer of _dry_ Albumen
would admit of coagulation by the simple application of a heated iron.

_To render the paper sensitive._--This operation must be conducted by the
light of a candle, or by yellow light. Take of

  Fused Nitrate of Silver                 60  grains.
  Glacial Acetic Acid                     1/3 minim.
  Distilled Water                          1  ounce.

Prepare a sufficient quantity of this solution, and lay the sheet upon it
in the same manner as before. Three minutes' contact will be sufficient
with the thin Negative paper, but if the Canson Positive paper be used,
four or five minutes must be allowed for the decomposition. The papers
are raised from the solution by a pair of bone forceps or common tweezers
tipped with sealing-wax; or a pin may be used to lift up the corner, which
is then taken by the finger and thumb and allowed _to drain a little_
before again putting in the pin, otherwise a white mark will be produced
upon the paper, from decomposition of the Nitrate of Silver. When the
sheet is hung up, a small strip of blotting-paper suspended from the
lower edge of the paper will serve to drain off the last drop of liquid.

A Bath prepared by the above formula is stronger than is really necessary.
Forty grains of Nitrate to the ounce of water is abundantly sufficient if
the sample be pure; but it must be borne in mind that the _strength_ of
the Bath diminishes _rapidly_ by use, and hence, when the prints begin to
be wanting in vigour, with pale shadows and perhaps a spotted appearance,
an addition of Nitrate of Silver must be made. Fused Nitrate of Silver
is recommended in preference to the crystallized Nitrate, on account of
the latter being occasionally contaminated with an impurity alluded to at
page 101. This when present will be likely to redden the pictures and to
interfere with the rapidity of bronzing.

The solution of Nitrate of Silver becomes after a time discoloured by the
Albumen, but may be used for sensitizing until it is nearly black. The
colour can be removed by Animal Charcoal,[41] but a better plan is to use
the "kaolin," or pure white china clay. This substance often contains
Carbonate of Lime, and effervesces with acids: it must in such a case be
purified by washing in vinegar, or the Bath will become alkaline, and
dissolve off the Albumen. It has been stated that an addition of Alcohol
to the Nitrate Bath prevents it discolouring with Albumen.

[41] Common Animal Charcoal contains Carbonate and Phosphate of Lime, the
former of which renders the Nitrate of Silver alkaline; purified Animal
Charcoal is usually acid from Hydrochloric Acid.

Sensitive albuminized paper will usually keep for several days, if
protected from the light, but afterwards turns yellow from partial
decomposition.

Formula II. _Preparation of plain paper._--Take of

  Chloride of Ammonium or Sodium         160 grains.
  Purified Gelatine                       20 grains.
  Iceland Moss[42]                         60 grains.
  Water                                   20 ounces.

[42] Iceland Moss is recommended because the writer finds that Positives
so printed stand the action of destructive tests better than prints on
plain paper, and equal to prints upon Ammonio-Nitrate paper.

Pour boiling water upon the Moss and Gelatine and stir until the latter is
dissolved, then cover the vessel and set aside until cold; add the salt,
and strain.

Use Papier Saxe or Towgood's paper,[43] floated upon the salting Bath in
the same manner as directed for Albumen at p. 243.

[43] The writer does not recommend the Positive paper of De Canson,
having noticed that prints upon that paper do not withstand the action of
sulphuretting agents so well as others (?).

Render sensitive by floating for two or three minutes upon a solution of
Nitrate of Silver, 40 grains to the ounce. Thirty grains to the ounce,
or less, will be sufficient if the sample be pure; but in that case
occasional additions of fresh Nitrate of Silver must be made, as the Bath
loses strength.

_A second Formula for plain paper._--Take of

  Chloride of Ammonium                 200 grains.
  Citrate of Soda[44]                  200   "
  Gelatine                              20   "
  Water                                 20 fluid ounces.

[44] This salt may be obtained at the operative chemists; or it may be
prepared extemporaneously by neutralizing 112 grains of pure Citric Acid,
free from Tartaric Acid, with 133 grains of the dried Bicarbonate or
"Sesquicarbonate" of Soda, used for effervescing draughts.

If Towgood's or any English paper be used, the Citric Acid, Carbonate of
Soda, and Gelatine may be omitted. With a foreign paper the Citrate tends
to give a purple tone to the Positive, when toned by Sel d'or, but the
gold toning Bath must be in active order, or the prints will be too red.
The Citric Acid also should not be in excess over the alkaline Carbonate.

Render sensitive by floating for three minutes upon a Nitrate Bath of
sixty grains to the ounce of water.

Formula III. _Ammonio-Nitrate Paper._--This is always prepared without
Albumen, which is dissolved by Ammonio-Nitrate of Silver. Take of

  Chloride of Ammonium                 100 grains.
  Citrate of Soda                      200   "
  Gelatine                              20   "
  Water                                 20 fluid ounces.

Dissolve the Gelatine by the aid of heat; add the other ingredients, and
filter. The solution cannot be kept longer than two or three weeks without
becoming mouldy. The Saxony paper, or Towgood's English paper, may be
employed; the Gelatine and Citrate being retained or omitted, according to
the taste of the operator and the mode of toning which is adopted.

Render sensitive by a solution of Ammonio-Nitrate of Silver, 60 grains to
the ounce of water, which is prepared as follows:--

Dissolve the Nitrate of Silver in one-half of the total quantity of water.
Then take a pure solution of Ammonia and drop it in carefully, stirring
meanwhile with a glass rod. A brown precipitate of Oxide of Silver first
forms, but on the addition of more Ammonia it is re-dissolved.[45] When
the liquid appears to be clearing up, add the Ammonia very cautiously, so
as not to incur an excess. In order still further to secure the absence
of free Ammonia, it is usual to direct, that when the liquid becomes
perfectly clear, a drop or two of solution of Nitrate of Silver should be
added until a _slight turbidity_ is again produced. Lastly, dilute with
water to the proper bulk. If the crystals of Nitrate of Silver employed
contain a large excess of free Nitric Acid, no precipitate will be formed
on the first addition of Ammonia. The free Nitric Acid, producing _Nitrate
of Ammonia_ with the alkali, keeps the Oxide of Silver in solution. This
cause of error however is not likely to happen frequently, since the
amount of Nitrate of Ammonia required to prevent all precipitation would
be considerable. From the same reason, viz. the presence of Nitrate of
Ammonia, it is often useless to attempt to convert an old Nitrate Bath
already used for sensitizing, into Ammonio-Nitrate.

[45] If the excess of Ammonia does not readily dissolve it, probably the
Nitrate of Silver is impure.

Ammonio-Nitrate of Silver should be kept in a dark place, being more prone
to reduction than the Nitrate of Silver.

Sensitizing paper with Ammonio-Nitrate.--It is not usual to float the
paper when, the Ammonio-Nitrate of Silver is used. If a bath of this
liquid were employed, it would not only become quickly discoloured by
the action of organic matter dissolved out of the papers, but would soon
contain abundance of free Ammonia (see the Vocabulary, Part III., art.
"Ammonio-Nitrate"); and an excess of Ammonia in the liquid produces an
injurious effect by dissolving away the sensitive Chloride of Silver.

The Ammonio-Nitrate is therefore applied with a glass rod, or by brushing,
and in neither case is any of the liquid which has once touched the paper
allowed to return into the bottle.

Brushes are manufactured purposely for applying Silver solutions, but the
hair is soon destroyed unless the brush be kept scrupulously clean. Lay
the salted sheet upon blotting-paper, and wet it thoroughly by drawing
the brush first lengthways and then across. Allow it to remain flat for
a minute or so, in order that a sufficient quantity of the solution may
be absorbed (you will see when it is evenly wet by looking along the
surface), and then pin up by the corner in the usual way. If, on drying,
white lines appear at the points last touched by the brush, it is probable
that the Ammonio-Nitrate contains free Ammonia.

The employment of a glass rod is a very simple and economical mode of
applying Silver solutions. Procure a flat piece of board somewhat smaller
than the sheet to be operated on, and having turned over the edges of the
paper, secure them with a pin. Next bring the board near to the corner
of the table, and laying the glass rod along the edge of the paper,
allow the fluid to drop into the groove so formed; then carry the rod
directly across the sheet, when an even wave of fluid will be spread over
the surface. A pipette made of glass tubing, when dipped into the bottle
and the upper end closed with the finger, will withdraw as much of the
Ammonio-Nitrate as is required; and if a scratch be made upon the tube at
a point corresponding to 30 or 40 minims, it will be found sufficient for
a quarter sheet of the Papier Saxe.

Ammonio-Nitrate paper, however prepared, cannot be kept many hours without
becoming brown and discoloured.

_Use of a solution of Oxide of Silver in Nitrate of Ammonia._--The great
objection to the use of Ammonio-Nitrate of Silver is the _decomposition_
which it sometimes experiences by keeping, metallic Silver separating
and Ammonia being set free. To obviate this liberation of Ammonia, the
Author employs Nitrate of Ammonia as the solvent for the Oxide of Silver.
The solution is prepared as follows:--Dissolve 60 grains of _Nitrate
of Silver_ in half an ounce of water, and drop in Ammonia until the
precipitated Oxide of Silver is exactly re-dissolved. Then divide this
solution of Ammonio-Nitrate of Silver into two equal parts, to one of
which add Nitric Acid cautiously, until a piece of immersed litmus-paper
is reddened by an excess of the acid; then mix the two together, fill up
to one ounce with water, and filter from the milky deposit of Chloride or
Carbonate of Silver, if any be formed.

This solution of Oxide of Silver in Nitrate of Ammonia appears to possess
all the advantages of the Ammonio-Nitrate without the inconvenience of
liberating so much free Ammonia upon the surface of the sensitive sheets.

_Hints in selecting from the above Formulæ._--Albuminized paper is the
most simple and generally useful; it is well fitted for small portraits
and stereoscopic Photographs. The Ammonio-Nitrate Process requires more
experience, but gives excellent results when black tones are required: it
may be used for larger portraits, engravings, etc.

Plain paper rendered sensitive by floating upon a Bath of Nitrate of
Silver is easier of manipulation than the Ammonio-Nitrate, and will be
found to be better adapted for toning by the Sel d'or Bath (p. 267) than
the Albuminized Paper.


PREPARATION OF THE FIXING AND TONING BATH.

Take of

  Chloride of Gold                         4 grains.
  Nitrate of Silver                       16 grains.
  Hyposulphite of Soda[46]                  4 ounces.
  Water                                    8 fluid ounces.

[46] The common kind of Hyposulphite of Soda occurring in yellow and
discoloured masses, is too impure for use in Photography, and requires
re-crystallization.

Dissolve the Hyposulphite of Soda in four ounces of the water, the
Chloride of Gold in three ounces, the Nitrate of Silver in the remaining
ounce; then pour the diluted Chloride by degrees into the Hyposulphite,
stirring with a glass rod; and afterwards the Nitrate of Silver in the
same way. This order of mixing the solutions is to be strictly observed:
if it were reversed, the Hyposulphite of Soda being added to the Chloride
of Gold, the result would be the reduction of Metallic Gold; Hyposulphite
of Gold, which is formed, being an unstable substance, and not capable
of existing in contact with unaltered Chloride of Gold. If however it be
dissolved by Hyposulphite of Soda immediately on its formation, it is
rendered more permanent, by conversion into a double salt of Soda and Gold.

In place of Nitrate of Silver, recommended in the formula, Chloride of
Silver may be used, but not Iodide of Silver, as the formation of Iodide
of Sodium would be objectionable (p. 136). For the same reason it is
better not to add any part of the Hyposulphite Bath used for fixing
Negatives, to the Positive colouring solution.

This toning Bath is not to be employed immediately after mixing,
but should be set aside until a portion of Sulphur (produced by
free Hydrochloric Acid, and Tetrathionate of Soda reacting upon the
Hyposulphite) has subsided. It will be very active at the expiration of a
few days or a week; but upon keeping for a longer time, loses much of its
efficacy by a process of spontaneous change.

The immersion of prints also lessens the quantity of Gold; and hence,
when the Bath begins to work slowly, more of the Chloride must be added,
the Sulphur being allowed to deposit as before. Filtration through
blotting-paper will not be required.

The writer finds that after a certain time, when the Bath has been long
used, and organic matters, Albumen, etc., have accumulated in it, it is
better, and more economical, to throw away what remains, and to prepare
a new solution. The addition of Chloride of Gold to an old Bath will not
always make it work as quickly as one recently mixed.


THE MANIPULATORY DETAILS OF PHOTOGRAPHIC PRINTING.

These include--the exposure to light, or printing properly so called; the
fixing and toning; and the washing, drying, and mounting of the proof.

The Exposure to Light.--For this purpose reversing frames are sold, which
admit of being opened at the back, in order to examine the progress of the
darkening by light, without producing any disturbance of position.

Simple squares of glass however succeed equally well, when a little
experience has been acquired. They may be held together by the wooden
clips sold at the American warehouses at one shilling per dozen. The lower
plate should be covered with black cloth or velvet.

Supposing the frame to be employed, the shutter at the back is removed,
and the Negative laid flat upon the glass, Collodion side uppermost. A
sheet of sensitive paper is then placed upon the Negative, sensitive side
downwards, and the whole tightly compressed by replacing and bolting down
the shutter.

This operation may be conducted in the dark room; but unless the light be
strong, such a precaution will not be required. The time of exposure to
light varies much with the density of the Negative and the power of the
actinic rays, as influenced by the season of the year and other obvious
considerations. As a general rule, the best Negatives print slowly;
whereas Negatives which have been under-exposed and under-developed print
more quickly.

In the early spring or summer, when the light is powerful, probably about
ten to fifteen minutes will be required; but from three-quarters of an
hour to an hour and a half may be allowed in the winter months, even in
the direct rays of the sun.

It is always easy to judge of the length of time which will be sufficient,
by exposing a small slip of the sensitive paper, unshielded, to the sun's
rays, and observing how long it takes to reach the coppery stage of
reduction. Whatever that time may be, nearly the same will be occupied in
the printing, if the Negative be a good one.

When the darkening of the paper appears to have proceeded to a
considerable extent, the frame is to be taken in and the picture examined.
If squares of plate glass are used to keep the Negative and sensitive
paper in contact, some difficulty may be experienced at first in returning
it precisely to its former position after the examination is complete,
but this will easily be overcome by practice. The finger and thumb should
be fixed on the lower corners or edge, and the plate raised evenly and
quickly.

If the exposure to light has been sufficiently long, the print appears
slightly darker than it is intended to remain. The toning Bath dissolves
away the lighter shades, and reduces the intensity, for which allowance is
made in the exposure to light. A little experience soon teaches what is
the proper point; but much will depend upon the state of the toning Bath;
and albuminized paper will require to be printed somewhat more deeply than
plain paper.

If, on removal from the printing-frame, a peculiar _spotted_ appearance is
seen, produced by unequal darkening of the Chloride of Silver, either the
Nitrate Bath is too weak, the sheet removed from its surface too speedily,
or the paper is of inferior quality.

On the other hand, if the general aspect of the print is a rich
chocolate-brown in the case of Albumen, a dark slate-blue with
Ammonio-Nitrate Paper, or a reddish purple with paper prepared with
Chloride and Citrate of Silver, probably the subsequent parts of the
process will proceed well.

If, in the exposure to light, the shadows of the proof become very
decidedly _coppery_ before the lights are sufficiently printed, the
Negative is in fault. Ammonio-Nitrate paper highly salted is particularly
liable to this fault of excess of reduction, and especially so if the
light be powerful; hence it is best, in the summer months, not to print
by the direct rays of the Sun. This point is important also, because the
excessive heat of the Sun's rays often cracks the glasses by unequal
expansion, and glues the Negative firmly down to the sensitive paper. An
exception however may be made in the case of Negatives of great intensity;
which are printed most successfully upon, a weakly sensitized paper
(p. 124) exposed to the full rays of the Sun; a feeble light not fully
penetrating the dark parts.

_The fixing and toning of the proof._--No injury results from postponing
this part of the process for many hours, provided the print be kept in a
dark place.

The mode often followed is to immerse the Positive in the Hyposulphite
Bath in the state in which it comes from the printing-frame; moving it
about in the liquid in order to displace air-bubbles, which, if allowed
to remain, produce spots. But the Author, for reasons given in the first
part of the Work (pp. 129 and 165), recommends that the print should
first be washed in common water until the soluble Nitrate of Silver has
been removed.[47] This is known to be the case when the liquid flows
away clear; the first milkiness being caused by the soluble Carbonates
and Chlorides in the water precipitating the Nitrate of Silver. Greater
security is thus afforded that the print will be toned in a really
permanent manner, since after removing the Nitrate of Silver from the
proof, the Bath does not work quickly unless the supply of Gold be well
maintained.

[47] This water must be free from Hyposulphite of Soda, or the print will
become discoloured.

Immediately on coming in contact with the Hyposulphite of Soda in the
fixing and toning Bath, the chocolate brown or violet tint of the Positive
disappears, and leaves the image of a red tone. Albumen proofs become
brick-red; Ammonio-Nitrate a sepia or brown-black. If the colour is
unusually _pale_ at this stage, probably the Silver Bath is too weak, or
the quantity of Chloride of Ammonium or Sodium insufficient.

After the print has been thoroughly reddened, the _toning_ action begins,
and must be continued until the desired effect is obtained. This may
happen in from ten minutes to a quarter of an hour, if the solution is in
good working order and the thermometer at 60°; but much depends upon the
temperature, and the activity of the Bath. English papers, and especially
the same prepared with Albumen, tone more slowly than foreign papers plain
salted.

The brown and purple tints are an earlier stage of coloration than the
black tones, and therefore the latter require more time. It must be borne
in mind however that prolonged immersion in the Bath is favourable to
sulphuration and yellowness; tending also to render the image unstable
and liable to fade in the half-tones. This fading may not be seen
decidedly whilst the print is in the Bath, but will show itself in the
after-processes of washing and drying.

The ultimate colour of the Print will vary much with the density of the
Negative and the character of the subject; copies of line engravings,
having but little half-tone, are easily obtained of a dark shade
resembling the original impression.

Some advise that on removal from the toning Bath the Print should be
soaked in new Hyposulphite for ten minutes, to complete the fixation;
but this precaution is not required with a Bath of the strength given in
the formula. An analysis of an old Bath which had been extensively used,
indicated only ten grains of Hyposulphite of Silver to the ounce, so that
it was far from saturated.

The occasional addition of fresh crystals of Hyposulphite of Soda to keep
up the strength of the Bath, is useful, the exact quantity added not being
material.

_The washing, drying, and mounting of the Positive Proofs._--It is
essential to wash out every trace of Hyposulphite of Soda from the Print
if it is to be preserved from fading, and to do this properly requires
considerable care.

Always wash with running water when it can be obtained, and choose a large
shallow vessel exposing a considerable surface in preference to one of
lesser diameter. A constant dribbling of water must be maintained for four
or five hours, and the prints should not lie together too closely, or the
water does not find its way between them, (see the remarks at p. 162).

When running water cannot be obtained, proceed as follows:--first wash the
Prints gently, to remove the greater part of the Hyposulphite solution.
Then transfer them to a large shallow pan, in which may be placed as many
Prints as it will conveniently hold. Leave them in for about a quarter
of an hour, with occasional movement, and then pour off the water quite
dry. This point is important, viz. to drain off the last portion of liquid
completely before adding fresh water. Repeat the process of changing at
least five or six times, or more, according to the bulk of water, number
of Prints, and degree of attention paid to them.

Lastly, proceed to remove the size from the Print by immersion in boiling
water.[48] This process will give some idea of the permanency of the
tints, since, if they become dull and red, _and do not darken on drying_,
the Print is probably toned without Gold. Ammonio-Nitrate and plain paper
Prints prepared on foreign papers by the modes described in this Work,
may be expected to stand the test of boiling water; Albumen Prints and
Positives on English paper are a little reddened, although not to an
objectionable degree.

[48] The Print must be well washed in cold water, to remove the
Hyposulphite, before using the hot water; or the half-tones will be liable
to be darkened, or changed to incipient yellowness, by sulphuration. This
point is important as regards the permanency.

The size may also be effectually removed from the Print by the common
Carbonate of Soda used in washing, although the former process is
recommended as the most secure. Dissolve about a handful of the Soda in a
pint of water, and when the milky deposit, if any occurs, has subsided,
immerse the washed Positives for twenty minutes or half an hour. The Soda
renders the paper quite porous, but produces no alteration of tint. If
the process be properly performed, ink will _run_ in attempting to write
upon the back of the finished picture. After removal from the Soda Bath
a second washing will be required, but the time of the first washing may
be proportionally shortened. Here a difficulty will occur with many kinds
of water; the Carbonate of Soda precipitating _Carbonate of Lime_, in the
form of a white powder which obscures the picture. To obviate this, use
_rain water_ until the greater part of the alkaline salt has been removed,
and do not allow a stationary layer of liquid to rest too long upon
the Print. The New River water supplied to many parts of London, being
comparatively soft, answers perfectly, and produces no white deposit, if
the proofs are moved about occasionally.

When the Prints have been thoroughly washed, blot them off between sheets
of porous paper and hang up to dry. Some press them with a hot iron,
which darkens the colour slightly, but does so in an injurious manner when
Hyposulphite of Soda is left in the paper.

Albumen proofs when dry are sufficiently bright without further treatment;
but in the case of plain paper, salted simply, the effect is improved by
laying the Print face downwards upon a square of plate-glass and rubbing
the back with an agate burnisher, sold at the artists' colour-men's. This
hardens the grain of the paper and brings out the details of the picture.
Hot-pressing has a similar effect and is often employed.

Mount the proofs with a solution of Gelatine in hot water, freshly made;
the best Scotch glue answers well. Gum water, prepared from the finest
commercial gum, and free from acidity, may also be used, but it should
be made very thick, that it may not sink into the paper, nor produce an
unpleasant "cockling up" of the cardboard, which is caused by the damp and
expanded print contracting as it dries.

Caoutchouc dissolved in mineral Naphtha to the consistence of thick glue
or gold-beaters' size, is employed by many for mounting Photographic
Prints; it may be obtained at the varnish shops, and is sold in tin
boxes. The mode of using it is as follows:--with a broad brush made of
stiff bristles, apply the cement to the back of the picture; then take a
strip of glass with a straight edge, and by drawing it across the paper,
scrape off as much as possible of the excess. The print will then be
found to adhere very readily to the cardboard, without causing expansion
or cockling; and any portion of the cement which oozes out during the
pressing may, when dry, be removed with a penknife without leaving a stain.


REMARKS UPON THE WANT OF CORRESPONDENCE BETWEEN THE FORMULA OF DIFFERENT
OPERATORS.

The formulæ for Positive printing given in the works on practical
Photography exhibit great variety; and it has been proposed to attempt to
reduce them to more uniform proportions. This cannot however easily be
done, both on account of the difference in the structure and preparation
of the various Photographic papers, and also because the mode of applying
the solutions is not always the same.

Take as an illustration the following process, which has long been
recommended for its simplicity, and which is in every respect a good
one:--Dissolve 40 grains of Chloride of Ammonium in 20 ounces of Distilled
Water, and _immerse_ about a dozen sheets of Towgood's Positive paper,
removing air-bubbles with a camels'-hair brush. When the last sheet has
been placed in the liquid, turn the batch over and take them out one by
one, so that each sheet, remaining in the liquid at least ten minutes,
may be thoroughly saturated. When dry, excite by brushing with a 40 or
60-grain solution of Ammonio-Nitrate of Silver in the usual way.

Now this formula contains less than one-fifth of the amount of salt
often employed, and if a thick foreign paper sized with starch, such as
Canson's Positive, were _floated_ upon such a salting Bath, it would be
difficult to obtain a good picture. By _immersing_ however a paper sized
with Gelatine like the one recommended, a much larger quantity of salt is
retained upon the surface, and the film is sufficiently sensitive. There
are three modes of applying solutions, viz. by brushing, floating, and
immersion. The quantity of solution left on the paper varies with each,
and consequently each requires a different formula. Immersion in a strong
salting Bath tends to give a coarse picture wanting in definition; whereas
the plan of brushing a weak salting solution, produces a paper deficient
in sensitiveness, and yielding a pale red image without proper depth of
shadow.

But independent of these differences, the chemical nature of the _size_
employed also influences the toning of the Print. For instance, in the
process above given, if the Positives, after having been fully toned in
the Gold Bath, and washed in cold water, be treated with _boiling water_,
the tint immediately changes to a dull red; but on blotting off between
sheets of bibulous paper and pressing with a hot iron, the dark tones are
restored.

This destruction of the tint by boiling water, and its restoration by _dry
heat_, is due in great part to the animal substance employed in sizing
the paper; and it will be found that prints upon a foreign paper, such as
the Saxony Positive, salted with a ten-grain solution and sensitized with
Ammonio-Nitrate, do not lose their tones in hot water and are not much
darkened by ironing.

The peculiarity of the sizing of the English Photographic papers must
therefore be borne in mind, and allowance made for the additional
sensitiveness and alteration of colour which it produces. When a formula
is given, the paper which is recommended for that particular formula
should alone be used.


SECTION II.

_Positive Printing by Development._

Negative printing processes will be found useful during the dull winter
months, and at other times when the light is feeble, or when it is
required to produce a large number of impressions from a Negative in a
short space of time. The plan of development also enables the operator to
obtain Positives of greater stability than those yielded by the direct
action of light.

Three processes may be described, the first of which gives Positives of
an agreeable colour, but the second, on Iodide of Silver, the greatest
permanency under unfavourable conditions.


NEGATIVE PRINTING PROCESSES UPON CHLORIDE OF SILVER.

Positives may be obtained by exposing paper prepared with Chloride
of Silver to the action of light until a faint image is perceptible,
and subsequently developing by Gallic Acid; but in this process it
is difficult to obtain sufficient _contrast_ of light and shade; the
impression, if sufficiently exposed and not too much developed, being
feeble, with a want of intensity in the dark parts. By associating
with the Chloride an organic salt of Silver, such as the Citrate, this
difficulty may be overcome, and the shadows be brought out with great
depth and distinctness.

The papers are salted with a mixed Chloride and Citrate as in the formula
for the Ammonio-Nitrate Process.[49] They are then rendered sensitive upon
a Bath of Nitrate of Silver _containing_ either Citric or _Acetic Acid_,
which are used in Negative processes to preserve the clearness of the
white parts under the influence of the developer.

[49] The formula at p. 246 may be modified with advantage: use double the
quantity of Gelatine, and half the amount of Citrate and Chloride.

The Bath of Aceto-Nitrate is prepared as follows:--

  Nitrate of Silver                       30 grains.
  Glacial Acetic Acid                     30 minims.
  Water                                    1 fluid ounce.

Float the papers (Papier Saxe or Papier Rive) upon the Bath for three
minutes, and suspend them to dry in a room from which actinic rays are
_perfectly_ excluded.

The exposure to light,--which is conducted in the ordinary printing frame,
the Negative and sensitive paper being laid in contact in the usual
way,--will seldom be longer than three or four minutes, even upon a dull
day. It may be regulated by the colour assumed by the projecting margin
of the paper; but it is quite possible to tell by the appearance of the
image when it has received a sufficient amount of exposure:--the whole of
the picture should be seen, excepting the _lightest shades_, and it will
be found that very few details can be brought out in the development which
were altogether invisible before the Gallic Acid was applied.

The developing solution is prepared as follows:--

  Gallic Acid                              2 grains.
  Water                                    1 fluid ounce.

In very cold weather it may be necessary to employ a saturated solution of
Gallic Acid, containing about four grains to the ounce; whereas in warm
weather the image will develope too quickly, and Acetic Acid must be added
(see the remarks at the end of the process, p. 266).

To facilitate the solution of the Gallic Acid, stand the bottle in a warm
place near the fire. A lump of Camphor floated in the liquid, or a drop
of Oil of Cloves added, will to a great extent prevent it from becoming
mouldy by keeping; but if once mould has formed, the bottle must be well
cleansed with Nitric Acid, or the decomposition of the fresh Gallic Acid
will be hastened.

Pour the solution of Gallic Acid into a flat dish, and immerse the Prints
two or three at a time, moving them about, and using a glass rod to remove
air-bubbles. The development is rapid, and will be completed in three or
four minutes. If the Print developes slowly, becomes _very dark in colour_
by continuing the action of the Gallic Acid, but shows no half-tones,
it has not been exposed sufficiently long to the light. An over-exposed
proof, on the other hand, developes with unusual rapidity, and it is
necessary to remove it speedily from the Bath in order to preserve the
clearness of the white parts; when taken out to the light, it appears pale
and red, with no depth of shadow.

The extent to which the development should be carried depends upon the
kind of Print desired. By pushing the action of the Gallic Acid, a dark
picture not much altered by the fixing Bath will be produced. But a better
result as regards colour and gradation of tone will be obtained by
removing the Print from the developing solution whilst in the light red
stage, and toning it subsequently by means of Gold; in which case it will
correspond both in appearance and properties to a Positive obtained by the
direct action of light (see the remarks at page 167).

When it is intended to follow the latter plan, the action of the developer
must be stopped at a point when the proof appears lighter than it is to
remain; since the Sel d'or Bath adds a little to the intensity, and the
image becomes somewhat more vigorous on drying.

Wash the Prints in cold water in order to extract all the Gallic Acid.
Then tone with _Sel d'or_ in the manner described in the next Section, and
fix in the usual way. The whites will with care be kept pure; or with only
a faint yellow tinge, which is not objectionable.

Upon comparing the developed Prints with others obtained by the direct
action of light upon the same sensitive paper, it is evident that the
advantage is _slightly_ on the side of the latter; but the difference is
so small that it would be overlooked in printing large subjects, for which
the Negative Process is more especially adapted. The _colour_ of both
kinds of Positives is the same, or perhaps a shade darker in the developed
proofs, which are usually of a violet-purple tone, but sometimes of a dark
chocolate-brown.

_A developing process with Serum of Milk._--The use of "whey" as a vehicle
for Chloride of Silver has something the same effect as that produced by
adding a Citrate. This may be traced to the presence of the Milk Sugar and
of a portion of uncoagulated Caseine left in the Serum.

The only difficulty in the process is to coagulate the milk in such a
way as to separate the greater part but not the whole of the Caseine.
Milk which has become sour, or to which an acid has been added, is not
considered so good for the purpose as that which has been treated with
rennet; and even when rennet is used it must be of the best quality or
its action will be imperfect. The serum must filter clear through,
blotting-paper; but it should not run very rapidly, or in all probability
the whole of the Caseine has been separated, and the fluid contains little
besides sugar. The whey which is left after cheese-making, commonly
answers the purpose, if clarified by beating it up with the white of
an egg and subsequently boiling and filtering. Globules of oil must be
separated as far as possible, or they will produce a greasiness of the
paper.[50]

[50] See the Vocabulary, Part III,, Art. "Milk," for further particulars.

Salt the prepared Serum with Chloride of Sodium or Ammonium; in quantity
about eight or ten grains to each fluid ounce, and render sensitive upon
the same Bath as that recommended for the Citrate Process.


A NEGATIVE PRINTING PROCESS UPON IODIDE OF SILVER.

Iodide of Silver is more sensitive to the reception of the invisible image
than the other compounds of that metal; and hence it is usefully employed
in printing _enlarged_ Positives from small Negatives, by means of the
Camera. The great stability of the proofs upon Iodide of Silver will also
be a recommendation of this process when unusual permanency is required.

Take of

  Iodide of Potassium                    160 grains.
  Water                                   20 fluid ounces.

The best paper to use will be either Turner's Calotype, or Whatman's or
Hollingworth's Negative; the foreign papers do not succeed with the above
formula (p. 258).

Float the paper on the iodizing Bath until it ceases to curl up and lies
flat upon the liquid: then pin up to dry in the usual way.

Render sensitive upon a Bath of Aceto-Nitrate of Silver containing 30
grains of Nitrate of Silver with 30 minims of Glacial Acetic Acid to each
ounce of water.

When the sheet is quite dry, place it in contact with the Negative in a
pressure frame, and expose _to a feeble light_. About 30 seconds will be
an average time upon a dull winter's day, on which it would be impossible
to print at all in the ordinary way. On removing the Negative nothing
whatever is seen upon the paper, the image being strictly invisible in
this process unless the exposure has been carried too far.

Develope by immersion in a saturated solution of Gallic Acid, prepared
in the manner described at page 261. The image appears slowly, and the
process may last from 15 minutes to half an hour. If the exposure has
been correctly timed, the Gallic Acid appears at length almost to cease
acting; but when the proof has been over-exposed, the development goes
on uninterruptedly, and the image becomes too dark, partaking more
of the character of a Negative than a Positive. The usual rule, that
_under_-exposed proofs develope slowly but show no half-tones, and that
the _over_-exposed develope with unusual rapidity, is also observed in the
process with Iodide of Silver.

After the picture is fully brought out, wash in cold, and subsequently
in warm water, to remove the Gallic Acid, which, if allowed to remain,
would discolour the Hyposulphite Bath. Then fix the Print in a solution
of Hyposulphite of Soda, one part to two of water, continuing the action
until the yellow colour of the Iodide disappears. The fixing Bath ought
not to produce much change in the tint. If the Positive loses its dark
colour on immersion in the Hyposulphite, and becomes pale and red,
it has been insufficiently developed. The theory of this part of the
process should be understood:--It is particularly the _second stage_ of
the development of a Photograph (see p. 144) on which the fixing Bath
produces no effect; and therefore a considerable change of colour in the
Hyposulphite indicates that too little Silver has been deposited, and the
remedy will be to push the development, adding a little Aceto-Nitrate to
the Gallic Acid if the strength of the Bath be found insufficient to yield
dark tones.

The colour of Positives developed upon Iodide of Silver is not agreeable,
and they become blue and inky when toned with gold. By fixing the proof
in Hyposulphite of Soda which has been long used and has acquired
sulphuretting properties, the tint is much improved; but the permanency of
the Print under unfavourable conditions is lessened by adopting that mode
of toning.


A NEGATIVE PRINTING PROCESS UPON BROMIDE OF SILVER.

By substituting the Bromide for the Iodide of Silver in the above process,
the proportions and details of manipulation being in other respects the
same, a more agreeable colour is obtained.

Paper prepared with Bromide of Silver is less sensitive than the Iodide,
but an exposure of one minute (in the printing frame) will usually be
sufficient even on a dull day. The image is nearly latent, but sometimes a
very faint outline of the darkest shadows can be seen. The proportion of
Bromide used is likely to influence this point; the sensitiveness being
diminished, but the image showing more of the details before development,
when the quantity of the Silver Salt is reduced to a minimum.

Either English or French papers may be used, but in the latter case the
Bromide should be dissolved in Serum of Milk (p. 262), or it will be
difficult to obtain a good surface picture. The proportion of Bromide may
be five grains to the ounce of Serum.

These proofs, even when simply fixed in plain Hyposulphite of Soda, are
superior in colour to the Positives printed by the last formula upon
Iodide of Silver; and the permanency is very great if the development be
sufficiently pushed. The use of the Serum of Milk gives an advantage in
resisting the oxidizing influences to which Positives are liable to be
exposed (p. 150).


GENERAL REMARKS ON NEGATIVE PRINTING.

Printing by development should not be attempted until the manipulation of
the ordinary process by direct exposure to light has been acquired.

Perfect cleanliness is essential. The salting or iodizing solution and
the Aceto-Nitrate Bath must be filtered clear, as the effect of small
suspended particles in producing spots is more seen when the image is
brought out by a developer.

It will be necessary to be far more careful in excluding white light
than in the ordinary process; and when Iodide of Silver is used, all the
precautions required in the case of Collodion Negatives must be taken.

Observe particularly that the dishes are kept clean, or the Gallo-Nitrate
of Silver will be rapidly discoloured (read the remarks at page 179).

Stereoscopic Negatives and small portraits are not successfully printed
by development; since it is difficult to obtain the most elaborate
definition, and there is a slight tendency to yellowness in the white
parts. Positives may be developed upon Albumen paper, but the Gallic Acid
is apt to discolour the lights.

In printing by development upon Chloride of Silver, the theory of the
subject must be particularly studied. When the weather is cold and the
light bad, the development of the image proceeds slowly, the Gallic Acid
Bath remains clear, and good half-tones are obtained; but under opposite
conditions, the developer may become turbid and the shadows be lost by
excessive deposit of Silver. This _over-development_ will be remedied by
printing the Negative in a more feeble light (near to the open window of
a room), and by adding Acetic Acid to the developer, about 5 or 10 minims
to the ounce, so as to bring out the image more slowly. The intensity of
action is thus lessened, and if the picture be not under-exposed, the
half-tones will be good.

Observe also when preparing papers with Citrate, that if too much
Carbonate of Soda be added in neutralizing the Citric Acid, Carbonate
of Silver will be deposited in the paper, the effect of which is to
remove by degrees the acidity of the Nitrate Bath, and to produce
over-development and excessive sensibility to light.

The colour of the proofs when taken from the Gallic Acid should be
_light red_; the gradation of tone not being usually so perfect when the
development is carried into the second or black stage.

It is not recommended to prepare too large a stock of the salted papers,
as they will probably be liable to mouldiness and decomposition unless
kept perfectly dry.


SECTION III.

_The Sel d'or Process for toning Positives._

This process is somewhat more troublesome than the plan of fixing and
toning in one solution, but possesses advantages which will presently be
enumerated. The description may be divided into the preparation of the
toning Bath, and the manipulatory details.


THE PREPARATION OF THE TONING BATH.

Take of

  Chloride of Gold                         1 grain.
  Pure Hyposulphite of Soda                3 grains.
  Hydrochloric Acid                        4 minims.
  Water, distilled or common               4 fluid ounces.

Dissolve the Gold and Hyposulphite of Soda each in two ounces of the
water; then mix quickly by pouring the former solution into the latter,
and add the Hydrochloric Acid. If the Chloride of Gold be neutral, the
liquid will have a red tinge, but if _acid_, then the solution may be
colourless. The commercial Chloride of Gold, containing usually much free
Hydrochloric Acid, will not require any addition of that substance. (See
the Vocabulary, Part III.)

In place of making an extemporaneous Hyposulphite of Gold by mixing the
Chloride with Hyposulphite of Soda, the Crystallized Sel d'or may be used,
adding about half a grain to the ounce of water, acidified as before; but
the objection to the employment of this salt is its expense, and also the
difficulty of obtaining it in a pure form; some samples containing less
than five per cent, of Gold.

It will be found very convenient to keep the two solutions on hand ready
for mixing, viz. the Chloride of Gold dissolved in water in the proportion
of a grain to the drachm, and the Hyposulphite of Soda, three grains to
the drachm. When required for use, measure out a fluid drachm of each,
dilute with water to two ounces, and mix.

It is possible that the three-grain solution of Hyposulphite of Soda may
by long keeping become decomposed, with precipitation of Sulphur. The
effect of this would be to produce a turbidity and deposit of Gold on
mixing the ingredients for the Bath, the Chloride of Gold being in excess
over the Hyposulphite of Soda (see p. 250).

The Bath of Sel d'or is always most active when recently mixed, but it
will keep good for some days if contact with free Nitrate of Silver be
avoided. The addition of this substance produces a red deposit in the
Bath, containing Gold, and the solution then becomes useless.


DETAILS OF MANIPULATION.

The paper may be prepared by either of the formulæ given in the first
Section of this Chapter, according to the tint desired. The pure black
tones are obtained most easily with the Ammonio-Nitrate paper, and the
purple tints, without gloss, on paper prepared with plain Chloride and
Citrate of Soda.

The printing is not carried quite to the usual intensity, as the
half-tones are very little dissolved in this process.

On being taken from the frame, the prints are washed thoroughly in common
water until it ceases to become milky; that is, until the greater part of
the Nitrate of Silver has been removed. The washing must be conducted in a
dark place, but it is not necessary to hasten it; the proofs may be thrown
into a pan of water covered with a cloth, and allowed to remain until
required for tinting.

A trace of free Nitrate of Silver usually escapes the washing; this would
cause a yellow deposit on the Print, and also in the toning Bath. It must
therefore be removed, either by adding a little _common salt_ to the water
during the last washings, or by means of a dilute solution of Ammonia.

For plain paper Prints the former plan will be found the least
troublesome; but with Albumen proofs[51] the Ammonia is required, in order
to dissolve away a portion of the Albuminate of Silver which has escaped
the action of light, before submitting the print to the gold; otherwise
the dark tones would nearly disappear in the fixing Bath, the Hyposulphite
carrying away the Gold with this superficial layer of silver salt.

[51] The amateur is recommended not to use Albuminized paper in this
process until he has become accustomed to the manipulations; the plain
paper prints being toned with more ease and certainty.

To prepare the Ammonia Bath, take of

  Liquor Ammoniæ                           1 drachm.
  Common Water                             1 pint.

The exact quantity is not material; if the liquid smells faintly of
Ammonia, it will be sufficient. Place the washed Prints in this Bath, two
or three at a time, and allow them to remain until the purple tint gives
place to a red tone. The action must be watched, because if the Ammonia
Bath be strong, the proof becomes unusually _pale and red_, and when this
is the case a little brilliancy is lost in the after-tinting.

As the Print is comparatively insensitive to light when the excess of
Nitrate has been washed away, it is not necessary to darken the room; but
a _bright light_ proceeding from an open door or window should be avoided.

After using the salt or the Ammonia, soak the Prints again for a minute
or so in common water. Then place them in the toning Bath of Gold and
acid; do not put in too many at once, and move them about occasionally, to
prevent spots of imperfect action at the point where the sheets touch each
other.

The foreign papers, plain salted, colour rapidly in two or three minutes.
English papers require five to ten minutes; Albuminized, ten minutes to
a quarter of an hour. The tendency of the Gold Bath is to give a blue
tone to the image; hence proofs which are light red after using the salt
or Ammonia, become, first red-purple, and then violet-purple in the Sel
d'or. Albumen Prints assume some shade of brown, or of purple if not too
strongly Albuminized. Ammonio-Nitrate papers highly salted, and prepared
without Citrate, become first dark purple, and then blue and inky; the
Citrate is intended to obviate this inky tint.

When the darkest tones are reached, the Bath produces no further effect,
but eventually (more especially if the solution be not shielded from light
[?]) there is a little decomposition, producing a cream-coloured deposit
upon the lights.

The toning being completed, the Prints are again washed for an instant in
water, to remove the excess of gold solution. This washing must not be
continued longer than two or three minutes, or there will be danger of
yellowness of the whites; this however ought not to happen with proper
precautions.

Lastly, the proofs are fixed in a solution of Hyposulphite of Soda, one
part to four of water; which may be used many times successively. This
Bath alters the tone very little if the deposit of Gold be well fixed
on the Print; but the writer has often observed in the case of Albumen
paper and paper prepared with Citrate (Formula II.) that if removed too
quickly from the Sel d'or, the purple tones change by immersion in the
Hyposulphite to a chocolate-brown. Ammonio-Nitrate Prints are less liable
to alter in this way.

In order that the fixing may be properly performed, the time of immersion
should not be less than ten minutes with a porous paper, plain salted; or
fifteen minutes in the case of an English or albuminized paper.

Ammonia may be used for fixing plain paper Prints; about one part of the
Liquor Ammoniæ, to four of water. Ten minutes' immersion will usually be
sufficient, and the tone is very little affected. This process is a good
one, but the pungent smell of the Ammonia is an objection, and the Bath
discolours by use. Some care too is required in order to ensure a proper
fixing of the prints (see the remarks at page 131).

For directions to wash and mount the proofs, see page 255.

It will sometimes happen in the Sel d'or process, from the toning Bath
having but little solvent action on the light shades, that the Prints,
after being washed and dried, appear too dark; this may be remedied by
laying them for a few minutes in _a very dilute solution_ of Chloride
of Gold (five or six drops of the yellow solution of the Chloride to a
few ounces of water) and washing for an additional quarter of an hour.
Or an over-printed Positive may be saved by toning it with Chloride of
Gold instead of Sel d'or. In that case, after proper removal of the free
Nitrate of Silver, a few drops of a lemon-yellow solution of Chloride of
Gold (with a fragment of Carbonate of Soda added to remove acidity, p.
132), should be poured over the Print, which is to be subsequently fixed
in the usual way.

_Advantages of toning by Sel d'or._--This process will be found especially
useful by those who print large Positives. The solutions may be mixed in
a few minutes, and, being very dilute, are economical. It is not even
necessary to employ a _Bath_ for toning, but if the Sel d'or solution be
prepared of about twice or three times the strength given in the formula,
it will be sufficient to pour a few drachms upon the surface of the
print. As the Gold solution is always used soon after mixing, a uniform
and permanent tint can be obtained; whereas the single fixing and toning
Bath of Gold and Hyposulphite loses much of its efficacy by keeping, and
_over-printing_ of the proof is required in proportion as the Bath becomes
older.


SECTION IV.

_On a mode of Printing enlarged and reduced Positives, Transparencies,
etc., from Collodion Negatives._

To explain the manner in which a Photograph may be enlarged or reduced in
the process of printing, it will be necessary to refer to the remarks made
at page 52, on the _conjugate foci_ of lenses.

If a Collodion Negative be placed at a certain distance in front of a
Camera, and (by using a tube of black cloth) the light be admitted into
the dark chamber only through the Negative, a reduced image will be formed
upon the ground glass; but if the Negative be advanced nearer, the image
will increase in size, until it becomes first equal to, and then larger
than, the original Negative; the focus becoming more and more distant from
the lens, or receding, as the Negative is brought nearer.

Again, if a Negative portrait be placed in the Camera slide, and
the instrument being carried into a dark room, a hole be cut in the
window-shutter so as to admit light through the Negative, the luminous
rays, after refraction by the lens, will form an image of the exact size
of life upon a white screen placed in the position originally occupied
by the sitter. These two planes, in fact, that of the object and of the
image, are strictly _conjugate foci_, and, as regards the result, it is
immaterial from which of the two, anterior or posterior, the rays of light
proceed.

Therefore in order to obtain a reduced or enlarged copy of a Negative, it
is necessary only to form an image of the size required, and to project
the image upon a sensitive surface either of Collodion or paper.

A good arrangement for this purpose may be made by taking an ordinary
Portrait Camera, and prolonging it in front by a deal box blackened inside
and with a double body, to' admit of being lengthened out as required; or,
more simply, by adding a framework of wood covered in with black cloth. A
groove in front carries the Negative, or receives the slide containing the
sensitive layer, as the case may be.

In _reducing_ Photographs, the Negative is placed in front of the lens, in
the position ordinarily occupied by the object; but in making an enlarged
copy, it must be fixed _behind_ the lens, or, which is equivalent, the
lens must be turned round, so that the rays of light transmitted by the
Negative enter the back glass of the combination, and pass out at the
front. This point should be attended to in order to avoid indistinctness
of image from spherical aberration.

A Portrait combination of lenses of 2-1/2 or 3-1/4 inches diameter
is the best form to use, and the actinic and luminous foci should
accurately correspond, as any difference between them would be increased
by enlarging. A stop of an inch or an inch and a half aperture placed
_between_ the lenses obviates to some extent the loss of sharp outline
usually following enlargement of the image.

The light may be admitted through the Negative by pointing the Camera
towards the sky; or direct sunlight may be used, thrown upon the Negative
by a plane reflector. A common swing looking-glass, if clear and free from
specks, does very well; it should be so placed that the centre on which it
turns is on a level with the axis of the lens.

The best Negatives for printing enlarged Positives are those which are
distinct and clear; and it is important to use a _small_ Negative, which
strains the lens less and gives a better result than one of larger size.
In printing by a 2-1/4 lens for instance, prepare the Negative upon a
plate about two inches square, and afterwards enlarge it four diameters.

Paper containing Chloride of Silver is not sufficiently sensitive to
receive the image, and the Print should be formed upon Collodion, or on
iodized paper developed by Gallic Acid (see p. 263).

The exposure required will vary not only with the intensity of the light
and the sensibility of the surface used, but also _with the degree of
reduction or enlargement of the image_.

In printing upon Collodion the resulting picture is Positive by
transmitted light; it should be backed up with white varnish, and then
becomes Positive by reflected light. The tone of the blacks is improved
by treating the plate first with Bichloride of Mercury, and then with
Ammonia, in the manner described at pages 113 and 207.

Mr. Wenham, who has written a paper on the mode of obtaining Positives
of the life size, operates in the following way:--he places the Camera,
with the slide containing the Negative, in a dark room, and reflects the
sunlight in through a hole in the shutter, so as to pass first through the
Negative and then through the lens; the image is received upon iodized
paper, and developed by Gallic Acid, in the mode described in the second
Section of this Chapter (p. 263).

_On printing Collodion transparencies for the Stereoscope._--This may
be done by using the Camera to form an image of the Negative in the
mode described in the last page; but more simply by the following
process:--Coat the glass, upon which the Print is to be formed, with
Collodio-Iodide of Silver in the usual way; then lay it upon a piece of
black cloth, Collodion side uppermost, and place two strips of paper of
about the thickness of cardboard and one-fourth of an inch broad, along
the two opposite edges, to prevent the Negative being soiled by contact
with the film. Both glasses must be _perfectly flat_, and even then it may
happen that the Negative is unavoidably wetted; if so, wash it immediately
with water, and if it be properly varnished, no harm will result.

A little ingenuity will suggest a simple framework of wood, on which the
Negative and sensitive plate are retained, separated only by the thickness
of a sheet of paper; and the use of this will be better than holding the
combination in the hand.

The printing is conducted by the light of gas, or of a camphine or
moderator lamp; diffused daylight would be too powerful.

The employment of a concave reflector, which may be purchased for a few
shillings, ensures parallelism of rays, and is a great improvement.
The lamp is placed in the focus of the mirror, which may at once
be ascertained by moving it backwards and forwards until an evenly
illuminated circle is thrown upon a white screen held in front. This in
fact is one of the disadvantages of printing by a naked flame--that the
light falls most powerfully upon the central part, and less so upon the
edges, of the Negative.

The picture must be exposed for a longer or shorter time (about ten
seconds will be an average) according to its behaviour during development
(see p, 224); this process, as well as the fixing, is conducted in the
same manner as for Collodion pictures generally.

Some adopt the plan of whitening by Corrosive Sublimate, and again
blackening by dilute Ammonia, as an improvement to the colour of the dark
shadows (see p. 113).

If this mode of printing upon Collodion be conducted with care, the
Negative being separated from the film by the smallest interval only, the
loss of distinctness in outline will scarcely be perceived.

Stereoscopic transparencies may also be printed by the dry Collodion
process described in Chapter VI., or by the Collodio-Albumen process. Mr.
Llewellyn recommends the employment of a solution of Oxymel, so dilute
that the plate becomes nearly dry, and may be laid in contact with the
Negative without fear of injury (see the footnote at page 302).




CHAPTER V.

CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS.


  Section I.--Imperfections in Collodion Photographs.
  Section II.--Imperfections in Paper Positives.


SECTION I.

_Imperfections in Negative and Positive Collodion Photographs._

The following may be mentioned:--fogging--spots-- markings, etc.


CAUSES OF FOGGING OF COLLODION PLATES.

1. _Over-exposure of the Plate._--This is likely to happen when using the
full aperture of a double combination lens for distant objects brightly
illuminated, the Collodion being highly sensitive. Also from the film
being very blue and transparent, with too little Iodide of Silver (p. 114).

2. _Diffused Light._--_a._ In the developing room. This is a frequent
cause of fogging, and especially so when the common yellow calico
is employed, which is apt to fade. Use a treble thickness, or
procure the waterproof material, in which the pores are stopped with
gutta-percha.--_b._ In the Camera. The slide may not fit accurately, or
the door does not shut close. Throw a black cloth over the Camera during
the exposure of the plate.--_c._ From direct rays of the sun or the light
of the sky falling upon the lens. With the full aperture of a double
combination Lens, a portion of sky included in the field (as for instance
to form the background of a portrait) is apt to cause fogging. The
portrait will probably be more brilliant if a funnel-shaped canvas bag, or
a curtain with an oblong aperture admitting only the rays proceeding from
the sitter, be placed in front of the Camera.

3. _Alkalinity of the Bath._--This condition, explained at page 88, may
be due to one of the following causes:--_a._ The use of Nitrate of Silver
which has been too strongly fused (p. 13).--_b._ Constant employment of a
Collodion containing free Ammonia or Carbonate of Ammonia (p. 89).--_c._
Addition of Potash, Ammonia, or Carbonate of Soda to the Nitrate Bath, in
order to remove free Nitric Acid (p. 89).--_d._ Use of rain-water or hard
water for making the Nitrate Bath (rain-water usually contains traces of
Ammonia; hard water often abounds with Carbonate of Lime).

In either case the alkalinity may easily be removed by the addition of
Acetic Acid, one drop to four ounces of the solution. The proper mode of
testing for alkalinity is described at p. 89.

4. _Decomposition of the Nitrate Bath._--_a._ By constant exposure to
light (the injurious effects of this will be mostly seen when Positives
are taken).--_b._ By organic matter: this is sometimes present in Nitrate
of Silver which has been prepared from the residues of old Baths; or
it may be introduced by floating papers for the printing process upon
the Bath, or by dissolving the crystals of Nitrate of Silver in putrid
rain-water, or in impure distilled water collected from the condensed
water of steam-boilers and contaminated with oily matter.--_c._
Decomposition of the Bath by contact with metallic iron or copper, or with
a fixing agent, or a developing agent (p. 90).

5. _Faults of the developing solution._--a. Brown and decomposed solution
of Pyrogallic Acid; this may sometimes be used with impunity, but it
tends, as a rule, to facilitate irregular reduction of Silver.--h. Impure
Acetic Acid having a smell of Garlic and which probably contains Sulphur
in organic combination.--c. Omission of the Acetic Acid in the developer:
this will produce a universal blackness.

6. _Sundry other causes of fogging._--_a._ Vapour of Ammonia or
Hydrosulphate of Ammonia, or the products of the combustion of coal-gas,
escaping into the developing room.--_b._ Development of the image by
immersion in solution of Sulphate of Iron: this is a safe plan when the
films are formed in an acid Nitrate Bath; but with pale films formed in a
chemically neutral Bath it is better to pour the fluid over the plate, and
not to use the same portion twice.--_c._ Redipping the plate in the Bath
before development: this is apt to give a foggy picture when using an old
Bath, and is not recommended.

_Systematic plan of proceeding to detect the cause of the fogging._--If
the amateur has had but little experience in the Collodion process, and is
using Collodion of moderate sensitiveness and a new Bath, the probability
is that the fogging is caused by over-exposure. Having obviated this,
proceed to test the Bath; _if it is made from pure materials, and does not
restore the blue colour of a piece of litmus-paper previously reddened
by holding it over the mouth of a glacial Acetic Acid bottle_, it may be
considered in working order.

Next prepare a sensitive plate, and after draining it for two or three
minutes in a dark place, pour on the developer: wash, fix, and bring out
to the light; if any mistiness is perceptible, either the developing room
is not sufficiently dark, or the Bath was prepared with a bad sample of
Nitrate of Silver, or with impure Alcohol, or impure water.

On the other hand, if the plate remains absolutely clear under these
circumstances, _the cause of error may be in the Camera_;--therefore
prepare another sensitive film, place it in the Camera, and proceed
exactly as if taking a picture, with the exception of not removing the
brass cap of the lens: allow to remain for two or three minutes, and then
remove and develope as usual.

If no indication of the cause of the fogging is obtained in either of
these ways, there is every reason to suppose that it is due to diffused
Light gaining entrance through the lens. This cause of error may often
be detected by looking into the Camera from the front, when an irregular
reflection will be seen upon the glass.


SPOTS UPON COLLODION PLATES.

Spots are of two kinds: spots of opacity, which appear black by
transmitted light, and white by reflected light; and spots of
transparency, the reverse of the others, being white when seen upon
Negatives, and black on Positives.

Opaque Spots are referable to an excess of development at the point where
the spot is seen; they may be caused by--

1. _The use of Collodion holding small particles in suspension._--Each
particle becomes a centre of chemical action, and produces a speck, or a
speck with a tail to it. The Collodion should be placed aside to settle
for several hours, after which the upper portion may be poured off.

2. _Turbidity of the Nitrate solution._--_a._ From flakes of Iodide of
Silver having fallen away into the solution, by use of an over-iodized
Collodion.--_b._ From a deposit formed by degrees upon the sides of the
gutta-percha trough.--_c._ From the inside of the trough being dusty at
the time of pouring in the solution.

In order to obviate these inconveniences, it is well to make at least half
as much again of the Nitrate solution as is necessary, and to keep it in a
stock-bottle, from which the upper part may be poured off when required.
The frequent filtration of Silver Baths is unadvisable, since the paper
employed may be contaminated with impurities.

3. _Dust upon the surface of the glass at the time of pouring on the
Collodion._--Perfectly clean glasses, if set aside for a few minutes,
acquire small particles of dust; each plate should therefore be gently
wiped with a silk handkerchief immediately before being used.

4. _Faults of the Slide._--Sometimes a small hole exists, which admits a
pencil of light, and produces a spot, known by its being always in the
same part of the plate; occasionally the door works too tightly so that
small particles of wood, etc., are scraped off, and projected against the
plate when it is raised. Or perhaps the operator, after the exposure is
finished, shuts down the door with a jerk, and so causes a splash in the
liquid which has drained down and accumulated in the groove below; this
cause, although not a common one, may sometimes occur.

5. _Insoluble particles in the Pyrogallic Acid._--The solution of
Pyrogallic Acid will not usually require filtering, but if specks of
Metagallic Acid are present, the developer should be passed through
blotting paper before use.

Spots of Transparency may generally be traced to some cause
_which renders the Iodide of Silver insensible to light at particular
points_, so that on the application of the developer no reduction takes
place.

1. _Concentration of the Nitrate of Silver on the surface of the film by
evaporation._--When the film becomes too dry after removal from the Bath,
the solvent power of the Nitrate increases so much that it eats away the
Iodide and produces spots.

2. _Small particles of undissolved Iodide of Potassium in the
Collodion._--These are likely to occur when Anhydrous Ether and Alcohol
are employed. They produce transparent specks at every part of the plate.
Allow the Collodion to settle, or add a drop of water, which will dissolve
the Iodide.

3. _Alcohol or Ether containing too much water_.--This causes a
reticulated appearance of the film, which is rotten and full of holes.

4. _Use of glasses improperly cleaned._--This cause is perhaps the most
frequent of all, when the film of Pyroxyline is very thin and the Bath
neutral. After glasses have been long used it is often difficult to clean
them so thoroughly that the breath lies smoothly; but the use of Potash
gives the best chance.


MARKINGS OF VARIOUS KINDS ON COLLODION PLATES.

1. _A reticulated appearance on the film after developing._--When this is
universal, it often depends upon the employment of Collodion containing
water. Or, if not due to this cause, the plate may have been immersed too
quickly in the Bath, and the soluble Pyroxyline partially precipitated.

2. _Oily spots or lines._--_a._ From raising the plate out of the Nitrate
Bath before it has been immersed sufficiently long to have become
thoroughly wetted.--_b._ Removal of the plate from the Bath before the
Ether upon the surface has been washed away.--_c._ Redipping the plate in
the Nitrate Bath after exposure to light, and pouring on the developer
_immediately_; if a few minutes be not allowed to drain off the excess of
Nitrate, the Pyrogallic Acid will not amalgamate readily with the surface
of the film.--_d._ From the Nitrate Bath being covered with an oily scum,
which is carried down by the plate. Draw a slip of blotting-paper gently
along the surface of the liquid before using it.

3. _Straight lines traversing the film horizontally._--From a check having
been made in immersing the plate in the Bath.

4. _Curved lines of over-development._--By employing the developer too
concentrated; or by not pouring it on sufficiently quickly to cover the
surface before the action begins; or by using too little Acetic Acid, and
omitting the Alcohol. The addition of Alcohol to the developer will not be
required as a rule when the Bath is newly made; but when much Ether has
accumulated in it, the developer has a tendency to run into oily lines,
unless containing Alcohol.

5. _Stains from too small a quantity of fluid having been employed to
develope the image._--In this case, the whole plate not being thoroughly
covered during the development, the action does not always proceed with
regularity.

6. _Irregular striæ._--From fragments of dried Collodion accumulating in
the neck of the bottle, and being washed on the film; to avoid this, the
finger should be passed gently round the inside of the neck before use.

7. _Markings like those represented in the woodcut._--They are caused by
using an inferior sample of Pyroxyline made from too hot acids, and are
most seen when using an old Bath.

[Illustration]

8. _Stains on the upper part of the plate, from using a dirty slide._--To
avoid these, place, if necessary, strips of blotting-paper between the
supports and the glass.

9. _Wavy marks at the lower parts of the plate._--_a._ If the Collodion
is becoming thick and glutinous from constant use, dilute it with a
little Ether containing an eighth part of Alcohol.--_b._ From reversing
the direction of the plate after its removal from the Bath, so that the
Nitrate of Silver flows back again over the surface and causes a stain on
the application of Pyrogallic Acid.--_c._ Impurities on the woodwork of
the frame ascending the film by capillary attraction. This is a frequent
source of stains.

10. _Marks from the developer not running up to the edge of the film_ (p.
212). Remedy this as far as possible by allowing the Collodion to set a
little more firmly before dipping the plate in the Bath.


IMPERFECTIONS IN COLLODION NEGATIVES.

1. _A want of Intensity._--a. From the development not having been
sufficiently pushed (p. 224).--_b._ From the Collodion film being too
blue and transparent for Negatives.--_c._ The Collodion newly made from
pure materials (p. 114).--_d._ The plate kept too long between exciting
and development (p. 100).--_e._ The Bath newly prepared from commercial
crystallized Nitrate of Silver (p. 101).--_f._ The light too feeble, as on
very dark wintry days, or in copying interiors, etc.

2. _Inferior half-tones, with great intensity of the high Lights._--_a._
From the plate being insufficiently exposed.--_b._ The Collodion of
inferior quality, either too strongly tinted with Iodine or made
from impure materials.--_c._ The Nitrate Bath old and partially
decomposed.--_d._ The light reflected too strongly from the object. When
the light is unusually bright, a feeble Collodion and a newly mixed
Nitrate Bath will be found to give better definition in the high lights
than an intense Collodion, which may produce chalky Negatives.

3. _The image pale and misty._--The plate is over-exposed (if so, the
image will probably be a reddish-brown colour by transmitted light), or
there is diffused light in the Camera or developing room. The presence of
Bromides or Chlorides in the Collodion may occasionally produce the same
effect.

4. _The high lights of the image are solarized._--A change of colour to
a light brown or red tint by transmitted light, with a dark shade by
reflected light, is favoured by over-exposure of the plate, by organic
decomposition of the Collodion, and by Acetate of Silver and other organic
bodies in the Bath.

5. _The image dissolves off on applying the Cyanide of Potassium._--The
Collodion is probably over-iodized. The same thing may also happen in
the Honey preservative process, when the plates have been long kept and
the indurated layer of syrup not properly removed before applying the
developer.

6. _The developer does not run up to the edge of the film._--This is
likely to occur when using Collodion nearly anhydrous; and particularly
so with a new Bath not containing much Alcohol. The film will be less
repellent, if a longer time be allowed before dipping in the Bath.

7. _The film does not stick to the glass._--Clean the plates very
carefully, and make the Collodion a little thinner if required. Allow
a longer time before dipping in the Bath. A very effectual plan is to
roughen the surface of the plates, about an eighth of an inch round the
edges.


IMPERFECTIONS IN COLLODION POSITIVES.

The principal difficulty in the production of Negatives is to ascertain
the right time of exposure to light and the proper point to which to carry
the development of the image. A minor amount of fogging, stains, etc., is
of less consequence, and will scarcely be noticed in the printing.

With direct Positives however the case is different. The beauty of these
pictures depends entirely upon their being clean and brilliant, without
fogging, specks, or imperfections of any kind. On the other hand, the
exposure and development of Positives is comparatively simple and easily
ascertained.

1. _The shadows dark and heavy._--The plate has not received sufficient
exposure in the Camera;--or the film being very transparent and the Silver
solution weak, Nitric Acid is present in the Bath, or the Collodion is
brown from free Iodine; in the latter case make the Collodion a little
thicker, and develope with Sulphate of Iron in preference to Pyrogallic
Acid.

2. _The shadows good, but the lights overdone._--The developing fluid may
have been kept on too long; or the object is not properly illuminated (p.
220); or the Collodion is not adapted for Positives.

3. _The high lights pale and flat, the shadows misty._--The plate is
over-exposed. Indistinctness of outline caused by over-exposure is
distinguished from that produced by fogging by holding the plate up to the
light; in the former case the image shows as a Negative.

If the Collodion is colourless, clearer shadows will probably be obtained
by dropping in Tincture of Iodine until a yellow colour is produced.

4. _The picture developes slowly; spangles of metallic Silver are
formed._--Too much Nitric Acid is present in proportion to the strength
of the Bath, to the amount of Iodide in the film, and to the quantity of
Protosalt of Iron in the developer (p. 112).

5. _Circular spots of a black colour after hacking up with the
varnish._--These are often caused by lifting the plate too quickly out of
the Bath; or by pouring on the developer at one spot, so as to wash away
the Nitrate of Silver; or by the use of glasses imperfectly cleaned.

6. _The image becomes metallic on drying._--If Sulphate of Iron is
employed, the solution is too weak, or free Nitric Acid has been added in
excess. If Pyrogallic Acid is used to develope, the proportion of Nitric
Acid is too great.

7. _A green or blue tint in certain parts of the image._--This is
caused by the deposit of Silver being too scanty, which may happen from
over-action of the light, or from the film of Pyroxyline being _very
thin_;--if the Collodion is diluted down beyond a certain point, the same
quantity of free Nitrate of Silver is not retained upon the surface of the
film. Add a few drops of the Bath to the developer before pouring it on
the plate.

8. _Vertical lines, and mistiness, on the image._--If the Bath has been
much used, add to it a third part of a simple solution of Nitrate of
Silver in water, without any Alcohol or Iodide. Also prepare the developer
with addition of Alcohol, to make it flow more readily (p. 211).


SECTION II.

_Imperfections in Paper Positives._

1. _The Print marbled and spotty._--The quality of the paper is often
inferior, which causes it to imbibe liquids unevenly at different points;
or the amount of Silver in the Nitrate Bath is insufficient. In this case
the spots are often absent at the lower and most depending part of the
sheet, where the excess of liquid drains off.

2. _The Print clean on the surface, but spotted when held up to the
light._--In this case the spots are probably due to imperfect fixation
(see p. 129).

3. _The Print becomes pale in the Hyposulphite Bath, and has a cold and
faded appearance when finished._--The Chloride of Silver in the paper may
have been in excess with regard to the free Nitrate of Silver; which is
especially likely if no bronzing could be obtained by prolonged action of
the light, or if a weak solution of Nitrate of Silver was laid on with a
brush, or by a glass rod. Prints formed on paper which has been kept too
long after sensitizing present the same appearance, the free Nitrate of
Silver having entered into combination with the organic matter.

4. _Yellowness of the light parts of the proof._--The following causes
are likely to produce yellowness:--acidity of the fixing and toning Bath
(p. 139),--its action continued for too long a time,--the first washings
of the proof not performed quickly,--the toning Bath laid aside until it
had become decomposed and nearly useless,--the paper kept for several days
after sensitizing.

A creamy yellowness is also common in Prints toned by Sel d'or, when the
Hydrochloric Acid has been omitted from the formula; the proof exposed to
light during the toning and fixing process; or too long a time allowed to
elapse between the toning and fixing. It is also more frequently met with
on albuminized paper.

5. _Intense bronzing of the deep shadows._--In this case the Negative
is in fault; remedy the evil as far as possible by printing on paper
containing but little salt.

6. _The definition of the Print imperfect, the Negative being a good
one._--Much will depend upon the quality of the paper. Towgood's Positive
gives good definition. The use of Albumen will be a great advantage.
Citrate of Soda (p. 246) will also improve the definition on plain paper.

7. _Markings of a yellow tint in the dark portions of the
Positive._--These are common on Prints toned without Gold; care should
be taken not to handle the paper too much, either before or after
sensitizing; to wash the prints in a clean vessel; and not to lay them
down whilst wet on a wooden table or in contact with anything likely to
communicate impurities.

8. _Small specks and spots of different hinds._--These, when not
corresponding to similar marks upon the Negative, are usually due to
metallic specks in the paper; or to insoluble particles floating in the
bath.

9. _Markings of the brush in Ammonio-Nitrate pictures._--In this case
there is probably an excess of Ammonia, which dissolves the Chloride of
Silver. Add a little fresh Nitrate of Silver, or use the Oxide of Silver
dissolved in Nitrate of Ammonia (p. 249).

10. _Marbled stains on the surface of the Print._--Draw a strip of
blotting-paper gently over the surface of the Nitrate Bath before
sensitizing the paper; and see that the sheet does not touch the bottom of
the dish.

11. _Streaks on Albuminized paper._--Apply the Albumen more rapidly and
evenly to the paper. If this does not succeed, add a little Ox-Gall (p.
243).

12. _Removal of the Albumen from the paper during sensitizing._--The
Nitrate Bath is probably alkaline (see page 89).




CHAPTER VI.

LANDSCAPE PHOTOGRAPHY ON PRESERVED COLLODION AND COLLODIO-ALBUMEN.


The Collodion process may be applied with success to landscape
Photography; but as the plates become dry and lose their sensitiveness
shortly after their removal from the Bath, the operator will require to
provide himself with a yellow tent or some portable vehicle in which the
operations of sensitizing and developing can be conducted. As it is a
point of great importance in the Collodion process that the plate should
receive exactly the right amount of exposure in the Camera,--a few seconds
more or less sufficing to affect the character of the picture,--many will
submit to much trouble and inconvenience in order to have the apparatus
complete upon the spot at which the view is taken.

The object of the "Collodion Preservative Processes" is to maintain the
sensitiveness of the film for a certain length of time after it has been
excited in the Bath. There is some difficulty in doing this, because if
the plate be allowed to dry spontaneously, the solution of free Nitrate of
Silver upon the surface, becoming concentrated by evaporation, eats away
the Iodide of Silver, and produces transparent spots.

Some operators have attempted to use a second plate of glass in such a way
as to enclose the sensitive film with an intervening stratum of liquid.
The difficulty however of separating the glasses again without tearing the
film, is considerable.

In the process of Messrs. Spiller and Crookes, the property possessed
by certain saline substances of remaining for a long time in a moist
condition was turned to account. Such salts are termed "deliquescent," and
many of them have so great an attraction for water that they absorb it
eagerly from the air: the solution having been formed, the water cannot
entirely be driven off except by the application of a considerable heat.

More recently, Honey has been employed by Mr. Shadbolt.[52] This
substance can scarcely be termed deliquescent, but it possesses, like
other uncrystallizable sugars, the property of remaining moist and sticky
for a long time. Honey is, according to the Author's views, superior
to inorganic deliquescent salts as a preservative agent, from its
possessing an affinity for Oxides of Silver, and thus acting chemically in
communicating organic intensity to the image.--Collodion plates when kept
long in a moist and sensitive state often give a pale and blue image, even
although the Nitrate of Silver be left upon the film; and neither Nitrate
of Magnesia nor Glycerine appears capable of supplying the deficient
element, both being nearly or quite indifferent to the Salts of Silver.

[52] A claim has lately been advanced by Mr. Maxwell Lyte to be considered
as the discoverer of the Honey Process. This gentleman appears to have
worked simultaneously with Mr. Shadbolt, and to have anticipated him in
publishing; but the object of Mr. Lyte's process was rather to increase
the sensibility of the plates than to confer upon them keeping qualities.


THE HONEY AND OXYMEL KEEPING PROCESSES.

When the weather is cool, Collodion plates may be preserved with tolerable
certainty for a few hours, by simply applying Honey to them in the state
in which they are taken from the Nitrate of Silver Bath.

The best pure Virgin Honey should be obtained by dripping it immediately
from the comb. This point is of importance, since if the sample of honey
be of inferior quality, or adulterated, the process may not succeed. The
quantity of water to be added will vary with the consistence of the honey,
from about an equal bulk to two parts: it should be sufficient to make the
preservative solution pass slowly through filtering-paper.

After the plate is removed from the Nitrate Bath, it is to be drained and
wiped on the back in the usual way. The Honey is then poured along the
edge in such a manner as to form a broad wave which forces the Nitrate of
Silver solution before it and covers the film. Next drain the plate into a
measure and pour on a second portion of Honey as before. This second dose
may be used again for the first application to the succeeding plate.

Lastly, stand the glass on blotting-paper in a dark place for about a
quarter of an hour or twenty minutes, and wipe the lower edge before
putting it into the plate box.

The exposure required will probably be about four or five times as long
as that for new and sensitive Collodion, or twice as long as the exposure
required for old and brown Collodion.

Before applying the developer, immerse the plate in a Bath of rain-water
for five minutes, moving it about occasionally to soften the honey. This
will probably be sufficient for plates which have not been kept longer
than four hours, and beyond that time the process is not considered
certain, since the Honey exercises a slow reducing action upon the Nitrate
of Silver.

The solution of Pyrogallic Acid may be used of the ordinary strength,
with a full dose of Acetic Acid. Only a faint image comes out at first,
but on pouring over the plate a fresh portion of the developer with two
or three drops of the Nitrate Bath added to each fluid drachm, it may be
intensified to any extent.

Fix with Hyposulphite of Soda, and wash in the usual way.

When the process fails, from heat of the weather or other causes, the
image will probably be feeble and red by transmitted light, and the
shadows defective and misty. This is especially likely to happen when the
Nitrate Bath is very old and contains much Acetate of Silver; or when the
same portion of Honey is used more than once, and has undergone partial
decomposition by the action of the Nitrate of Silver. The use of _pure_
Honey, free from mouldiness and fermentation, will, _in cool weather_,
almost certainly ensure success.

_A modification of the process when the plates are to be kept over four
hours._--In this case the whole, or the greater part of the Nitrate of
Silver must be removed before applying the preservative agent. Wash the
sensitive plate in water in the manner described for the Oxymel process
in the next page. Then apply the syrup as before, using it as thick as
possible. Honeyed plates, free from Nitrate of Silver, may commonly
be kept for five or six days; often much longer. Dr. Mansell, who has
employed this process with great success, speaks of _temperature_ as
a point to be attended to. In hot weather the same length of keeping
properties will not be attained.

_Use of Oxymel for preserving Collodion plates._--The principal difficulty
in the employment of Honey in Photography, is its disposition to ferment,
or to become mouldy. Fermentation occurs most readily in a dilute
solution, and will be obviated by using the syrup as thick and free from
water as possible. Mr. Llewellyn employs "Oxymel," which is a mixture of
Honey and Vinegar, as a preservative agent. This substance will keep even
in dilute solution for a long time without decomposition; and, being very
readily removed from the plates, does not interfere with the development
of the image. The preparation of Oxymel is described in the Vocabulary,
Part III.; it must be diluted with three or four parts of water, and
filtered.

Certain facts to which attention has been lately drawn by Dr. Norris and
Mr. Barnes in working with dry Collodion, may be advantageously borne in
mind when using Oxymel; the preservative solution of which is employed
in so dilute a state that the process resembles to a great extent a dry
Collodion process. The observations above referred to relate to the
quality of the Collodion best adapted for the purpose, and will be found
at page 298, to which the reader is referred.

The manipulation of the Oxymel process is very simple. Two flat
gutta-percha dishes are provided, the one containing common water and the
other diluted and filtered Oxymel. The Collodion plate, on its removal
from the Bath, is placed in the first dish, which is gently tilted up and
down, to wash away the free Nitrate of Silver. In a few seconds, when
the liquid is rendered milky, it is poured away, and fresh water being
introduced, the process is repeated _until the oily lines disappear, and
the surface of the film becomes smooth and glassy_. The plate is then,
after a slight draining, removed to the second tray, and the Oxymel waved
backwards and forwards for about half a minute, after which the glass is
lifted out and placed vertically on blotting-paper, which must be renewed
when it becomes wet and saturated.

The plates may be used any time within a fortnight from the date of their
preparation, and it is not necessary to develope immediately after the
exposure. The sensitiveness will be considerably less than that of fresh
Collodion: from two to five minutes may be allowed with a Stereoscopic
view lens having a quarter-inch diaphragm.

Before developing, the film should be gently washed for a few seconds with
common water. Solution of Pyrogallic Acid, of the ordinary strength, but
previously mixed with a portion of the Nitrate Bath solution, one or two
drops to each drachm, may then be poured on in the ordinary way. Use less
Nitrate of Silver and more Acetic Acid in hot weather. When discoloration
of the developer occurs, mix a fresh portion and proceed as before.


PRECAUTIONS TO BE OBSERVED IN KEEPING PROCESSES.

The plates must be roughened at the edges, and also upon the surface, to
make the film adhere.

It is advisable to use a tolerably thick Collodion, giving a yellow film;
the pale opalescent films being more easily affected by markings on the
glass, and not retaining so much of syrup or Nitrate of Silver upon the
surface.

The room in which the plates are prepared must be carefully guarded
from scattered pencils of white light; the films are exposed to injury
from this cause during the whole of the time occupied in applying the
preservative syrup; and hence anything short of absolute chemical darkness
will be likely to cause fogging; especially so when free Nitrate of Silver
is left upon the film.

The water used for washing away the free Nitrate of Silver before applying
the preserving liquid, need not be distilled. Common hard water containing
Carbonates and Chlorides, and producing _milkiness_ with Nitrate of
Silver, will often suffice. The water of the New River and of the River
Thames, with which many parts of London are supplied, may certainly be
used; but in the case of a very _hard_ water, containing much Sulphate of
Lime, it might perhaps be advisable to substitute clean rain-water, free
from brown organic discoloration.

The preservative Oxymel must be carefully filtered, and kept _covered_,
in order to protect it from dust. It will also be necessary occasionally,
before using it, to run it through a piece of white cambric, to stop back
suspended particles, which, if allowed to remain, would be a source of
spots. If it becomes mouldy, or discoloured by Silver, or ferments and
evolves gas, throw it away.

After the syrup is applied and the plates are drained, stow them in a
grooved box perfectly protected from light; or place them in slides,
which must be kept scrupulously clean, since any trace of impurity would
be likely to produce a stain when the plate was left a long time in the
slide. If the preserved plates are kept in a cupboard or box, see that no
volatile matter, such as Ammonia, coal-gas, etc., can find entrance.

In changing the plates after the exposure in the Camera, use a large bag
made of _several thicknesses_ of black calico, with a square of yellow
calico let in at the top; an elastic band securing it round the waist.


THE COLLODIO-ALBUMEN PROCESS.

This process, the theory of which has been briefly explained at page 181,
is more sensitive than the one last described, and has the additional
advantage of giving _dry_ plates, which do not attract dust, and are
less liable to injury. The details of manipulation are complex, but this
inconvenience is not so much felt when preparing a large number of plates.

_Cleaning the Glasses._--Success will greatly depend upon the mode in
which this part of the process is performed. The layer of Albumen which is
applied to the Collodion film tends to swell and to raise the latter in
blisters; the most effectual mode of obviating which will be to clean the
glass so that the film adheres with unusual tenacity.

The Liquor Potassæ of the Druggists, diluted with three or four parts of
water, and rubbed on the glass by a roll of flannel (page 214), is very
effectual. A mixture of Tripoli-water and Nitric Acid may however, if
desired, be substituted:--

  Tripoli                                1 drachm.
  Nitric Acid                           30 minims.
  Water                                  1 ounce.

Lay the glass flat on a cloth, and rub the surface carefully with a tuft
of cotton-wool dipped in the Tripoli; then, before the cream dries, wipe
it off with a second tuft, and polish with a third. Lastly, breathe upon
the glass, and having ascertained that it is chemically clean, apply the
Collodion.

_Coating with Collodion._--Choose a rather thin Collodion which adheres
tightly to the glass. A preparation which has been kept a long time after
iodizing will usually answer the purpose very well, and, as a rule, a
non-contractile, structureless Collodion is better than one which is
glutinous and wavy. The degree of sensibility of the Collodion is not
thought to have much influence upon the result.

_Coating the Plate._--Apply the Collodion in the usual manner, and allow
it full time to set perfectly, before dipping in the Bath, in order to
favour its adherence to the glass. With Collodion prepared from anhydrous
spirits, about half a minute may be given in cool weather.

_The Nitrate Bath._--Take of

  Fused Nitrate of Silver               40 grains.
  Glacial Acetic Acid                   30 minims.
  Alcohol                               20 minims.
  Water                                  1 fluid ounce.

Saturate with Iodide of Silver as described at page 204, and filter.
An immersion of one minute will be sufficient; after which, give the
plate an up-and-down movement, and wash it in plain water, in the manner
advised for the Oxymel preservative process, at page 292. Then stand it on
blotting-paper, to drain for a minute or two, wipe the back of the glass,
and pour on the Albumen.

This Bath may become discoloured after a time; continue to use it until it
is of a dark sherry-colour, and then treat it with "Kaolin," in the manner
and with the precautions advised at pages 91 and 245.

_The Iodized Albumen._--Procure eggs, fresh laid, or not more than two or
three days old. Separate the whites in the same way as for Albuminized
paper (p. 241), and mix by the following Formula:--

  Albumen                                9 fluid ounces.
  Water                                  3 fluid ounces.
  Liquor Ammoniæ                         2 fluid drachms.
  Iodide of Potassium                   48 grains.
  Bromide of Potassium                  12 grains.

The Iodide and Bromide should be free from Carbonate of Potash, which is
said to cause pin-holes in the Negatives. To ensure the absence of this
salt, dissolve the total quantity of both Iodide and Bromide in the three
ounces of water advised in the formula; then, previously to adding the
Ammonia and Albumen, introduce _an excessively minute particle of Iodine_,
enough barely to colour the liquid. The Iodine decomposes the Carbonate
of Potash, but it must not be used in excess, since free Iodine possesses
the property of coagulating Albumen. Iodide of Cadmium also coagulates
Albumen, so that the Iodides of Potassium and Ammonium are the best.

Having mixed the ingredients in the order above given, introduce them into
a bottle, and shake it violently until they have thoroughly amalgamated.
Then transfer to a tall narrow jar; allow to settle for twenty-four hours,
and draw off the upper clear portion for use. Particulars of this part of
the process have already been given under the head of Albuminized Paper,
to which the reader is referred (p. 241).

The ammoniacal solution of Albumen may be kept for some time in a
stoppered bottle without much decomposition. If mucous threads form in it,
filter through fine linen.

_Mode of applying the Albumen._--Cover the moist film with the Albumen
in the same way as advised for Collodion (p. 216), pouring on at once a
sufficient quantity to cause it to spread in an even and undivided sheet;
otherwise a veined appearance may be produced, which will show in the
development. Return the excess of Albumen into the bottle, and pour it
once again upon the plate: the film will remain clear and transparent, if
the whole of the Nitrate of Silver has been properly washed away from the
Collodion. Lastly, stand the plate nearly vertically on blotting-paper to
dry. This will occupy five or six hours; but the process may be hastened
by artificial heat.

After the Albumen solution has been used to coat a number of plates
successively, it becomes diluted with water; the result of which is, that
unequal intensity of image is produced at the upper and lower edge of the
film.

The iodized Albumen plates are at this stage of the process nearly or
quite insensitive to light, and may be preserved unchanged for many weeks.

_Sensitizing the Albumen film._--When the plate has become thoroughly
dry, it is again introduced into the Bath of Aceto-Nitrate of Silver,
and allowed to remain for one minute: then washed with water in the same
manner as before, but with even greater care, in order to obviate clouding
in the development. If blisters should form on drying, it will be found
useful to hasten the process by holding the plates to the fire--or a hot
iron may be placed in the centre of a covered box and the glasses reared
up round the sides. They will thus dry quickly, and there will not be time
for the Albumen to swell much by imbibition.

_Exposure in the Camera._--This may be performed at any period within a
few weeks from the date of preparation of the plates. For a landscape view
with a small Stereoscopic single lens, allow about three minutes in the
winter, or one minute and a half in the summer.

_Development of the image._--This can be deferred as long as fourteen days
after the exposure, with successful results. Pour water over the plate
until the film is thoroughly wetted; then cover it with a solution of
Pyrogallic Acid containing one grain of the acid to the ounce of water,
and twenty minims of Glacial Acetic Acid. Two drops of a neutral solution
of Nitrate of Silver made with forty grains of Nitrate to the ounce of
water must be previously added to each fluid drachm of the Pyrogallic.
The development, in the case of a landscape view taken with sunlight,
commences almost immediately, and may be completed in about ten minutes,
but the time occupied in developing will vary greatly with the length of
exposure, the quantity of Nitrate of Silver, and the nature of the subject
copied--a badly lighted interior, for instance, often taking an hour or
longer to appear in all its details. If the developer should discolour
before the proper intensity has been obtained, pour it off and mix a fresh
quantity.

_Fixing the image._--Hyposulphite of Soda (one ounce to four of water)
will be found preferable to the Cyanide of Potassium, as the latter has a
solvent effect upon the Albumen. An unusually long time will be required,
as the fixing agent must penetrate the Albumen, to reach the Collodion
beneath.

Careful washing in water for five or ten minutes removes the excess of
Hyposulphite, and the plate may then be varnished in the usual way.


THE DRY COLLODION PROCESS.

The earlier attempts to employ sensitive Collodion plates in a desiccated
condition were unsuccessful. The film of Pyroxyline shrinks on drying, and
becomes almost impervious to moisture: hence, the developing solution not
penetrating properly, density cannot easily be obtained. We are indebted
to Dr. Hill Norris, of Birmingham, for establishing the theory of the
subject upon a more correct basis. He has pointed out the importance of
distinguishing two different conditions of the Collodion surface,[53]
viz. the _contractile_, common in newly-mixed Collodion,--and the _short_
or _powdery_, in Collodion which has been iodized with the alkaline
Iodides, and kept until much Iodine has been set free. The latter is the
most suitable condition for the dry process; and the practical mode of
distinguishing between them is by sensitizing a plate and passing the
finger across it; if it can be easily pushed away in a firm and connected
skin, it will be unfit for the purpose required. In order still further
to preserve the film in a condition permeable by the developer, it is
recommended to coat it whilst moist with a solution of Gelatine.

[53] See these states of the film more fully described at page 83.

The dry Collodion process, although less sensitive, is more simple than
that on Collodio-Albumen, and possesses many of its advantages; but it is
less universally applicable, since it depends entirely for success upon
the peculiar state of the Collodion, resembling in this respect the Oxymel
process already described.

_Mode of preparing the plates._--The glasses are coated with the Collodion
in the usual way. Blistering during development being liable to happen in
this process as in the last, every care must be taken to make the films
adhere with the greatest possible tenacity, both by cleaning the glasses
with extra care (see p. 294), and also by allowing the Collodion to set
firmly before dipping in the Bath. The plate may be held from twenty to
thirty seconds previous to immersion, or even longer, provided the film,
when lifted out of the Bath, appears of uniform thickness throughout (see
page 218).

The sensitizing having been completed, wash the plates with plain water,
exactly in the same way as for Oxymel (p. 292). If Nitrate of Silver
be left, clouding will take place in the process of development. After
washing, drain for a few seconds, and immerse in the solution of Gelatine.

To prepare this Bath, take of

  Nelson's patent Gelatine             128 grains.
  Distilled water                       14 ounces.
  Alcohol                            2-1/2 ounces.

Put the Gelatine in the cold water, and allow it a quarter of an hour
to soften and swell; it will then readily dissolve on applying a gentle
heat. This may be done in a glazed saucepan or a pipkin of earthenware,
taking care not to scorch the bottom part by too strong a heat. Next
clarify the solution by adding to it, whilst barely warm, a tea-spoonful
of white of egg (previously beaten up with a silver fork), and afterwards
heating nearly to the boiling point. The Alcohol must now be added, to
facilitate the coagulation of the Albumen. When this takes place and the
liquid becomes clear, filter through a clean piece of cambric folded
three or four times. If a hot water filtering apparatus can be obtained,
the solution may be made to pass through _paper_; but as it tends to
gelatinize on cooling, the ordinary mode of filtration commonly fails.
The quantity of Alcohol in the above formula is greater than is usually
recommended, allowance having been made for a partial evaporation of the
spirit.

The filtered liquid may be poured into a flat porcelain dish, or a
vertical trough, but in either case it will be necessary to stand the
vessel in warm water, in order to prevent gelatinization.

The Collodion plate, thoroughly washed, is to be immersed in this solution
and moved up and down for two or three minutes. It is then removed,
drained on blotting-paper, and dried. The use of artificial heat in drying
will be found a great advantage; it prevents the gelatine from settling
unequally upon the plate. Those who possess an apparatus made purposely
for drying plates by hot air, will experience no difficulty, but an
ordinary deal trunk may be made to answer, with a little management. Cover
the bottom of the box with blotting-paper, and having heated one or two
"flat irons," place them in the centre: then range the glasses side by
side, with the coated surface looking inwards; in a quarter of an hour,
or from that to twenty minutes, the desiccation will be complete. If the
Collodion plates are prepared in a room containing a fire, they may be
reared up side by side at a distance of two or three feet, and in that
way may be safely dried without fear of injury, provided white light be
excluded.

When dry they can be stowed away in a box; all the precautions given at
page 293 being observed. The sensibility remains good for many days,
possibly for weeks or months in cold weather.

_Exposure in the Camera._--Allow from four to eight times the exposure
of the most sensitive moist Collodion. On a clear summer's day, a sun-lit
view may require one minute or a minute and a half, with a short focus
Stereoscopic lens, having a diaphragm of a quarter of an inch diameter.
The average time however with the same lens would be about twice as much,
viz. three minutes.

_Development of the Image._--Make a saturated solution of Gallic Acid in
water by the directions given at page 261. Then dissolve forty grains
of pure Nitrate of Silver in one ounce of distilled water. Pour into a
flat porcelain dish a sufficient quantity of the Gallic Acid solution to
flood the plate readily. Then measure it, and to each fluid ounce add
_ten minims_ of the solution of Silver, or five minims in hot weather. It
is important that no discoloration should occur on mixing these liquids
together, to obviate which, observe the following precautions:--Clean the
porcelain vessel very carefully with Nitric Acid or Cyanide before use.
Employ a pure solution of Nitrate of Silver; and mix it with the Gallic
Acid, in preference to adding the Gallic Acid to the Silver solution (read
the remarks at p. 179).

The picture may be expected to appear in five or ten minutes, and in
one hour, or from that to four hours (p. 298), the development will be
complete. It will not be necessary to keep the plates in motion, but
simply to lay them side by side in the solution of the Gallic Acid. If
in spite of all precautions the developer begins to blacken before the
intensity has reached the proper point, it must be poured off and a fresh
mixture prepared. This however will not often happen.

Lastly, when a full amount of opacity has been obtained, wash the plate
with water, and fix it in a solution of Hyposulphite of Soda, or dilute
solution of Cyanide of Potassium.

_Failures in the process._--Stains in the development may arise from
using dirty dishes, or glasses which have been left in Gallo-Nitrate
of Silver and improperly cleaned. It must be borne in mind that these
impurities are not visible to the eye, although they produce the effect of
discolouring the developer. A thorough cleansing with strong Nitric Acid
or Potash will prove a remedy.

Blisters, unless of large size, may often be disregarded, as they
disappear on drying. General cloudiness may depend upon the film having
been imperfectly washed. Irregular reduction at certain parts may be due
to the Gelatine setting before the plate has become dry, or to stains
produced by the finger applied to the upper edge of the plate.[54]

[54] Since the above was written, Mr. Maxwell Lyte has communicated to the
'Photographic Journal' (vol. iii.) a dry process in which a _modified_
Gelatine is used. The change is produced by boiling a solution of gelatine
with dilute Sulphuric Acid, which is afterwards neutralized and removed by
means of chalk. The result is to destroy the gelatinizing property of the
animal substance; the solution retains its fluidity on cooling, and the
necessity of employing artificial heat in drying the plates is avoided.




PART III.

OUTLINES or GENERAL CHEMISTRY.




OUTLINES or GENERAL CHEMISTRY.

[Illustration]




CHAPTER I.

THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS.


The limits of the present Work allow only of a simple sketch of the
subjects which it is proposed to treat in this Chapter. Our attention
therefore must be confined to an explanation of certain points which
are alluded to in the First Part of the Work, and without a proper
understanding of which it will be impossible for the reader to make
progress.

The following division may be adopted:--The more important Elementary
Bodies, with their symbols and atomic weights; the Compounds formed
by their union; the class of Salts; illustrations of the nature of
Chemical Affinity; Chemical Nomenclature; Symbolic Notation; the laws of
Combination; the Atomic Theory; the Chemistry of Organic Bodies.


THE CHEMICAL ELEMENTS, WITH THEIR SYMBOLS AND ATOMIC WEIGHTS.

The class of elementary bodies embraces all those substances which cannot,
in the present state of our knowledge, be resolved into simpler forms of
matter.

The chemical elements are divided into "metallic" and "non-metallic,"
according to the possession of certain general characters.

The following are some of the principal non-metallic elements, with the
symbols employed to designate them, and their atomic weights:[55]--

                                      Symbol.  Atomic Wt.
  Gases.  { Oxygen                      O         8
          { Hydrogen                    H         1
          { Nitrogen                    N        14
          { Chlorine                    Cl       36

  Solids. { Iodine                      I       126
          { Carbon                      C         6
          { Sulphur                     S        16
          { Phosphorus                  P        32

  Liquid.   Bromine                     Br       78
  Unknown.  Fluorine                    F        19

The metallic elements are more numerous. The following list includes only
those which are commonly known:--

                                       Symbol. Atomic Wt.

  Metals of the { Potassium              K        40
    Alkalies.   { Sodium                 Na       24

  Metals of the { Barium                 Ba       69
    Alkaline    { Calcium                Ca       20
     Earths     { Magnesium              Mg       12

  Metals        { Iron                   Fe       28
  Proper.       { Zinc                   Zn       32
                { Cadmium                Cd       56
                { Copper                 Cu       32
                { Lead                   Pb      104
                { Tin                    Sn       59
                { Arsenic                As       75
                { Antimony               Sb      129

  Nobel         { Mercury                Hg      202
  Metals.       { Silver                 Ag      108
                { Gold                   Au      197
                { Platinum               Pt       99

[55] The atomic weights, with the exception of that of Gold, are taken
from the last edition of Brande's 'Manual of Chemistry.'


ON THE BINARY COMPOUNDS OF THE ELEMENTS.

Many of the elementary bodies exhibit a strong tendency to combine with
each other, and to form compounds, which differ in properties from either
of their constituent elements. This attraction, which is termed "Chemical
Affinity," is exerted principally between bodies which are opposed to each
other in their general characters. Thus, taking for example the elements
Chlorine and Iodine--they are analogous in their reactions, and therefore
there is but little attraction between them, whereas either of the two
combines eagerly with Silver, which is an element of a different class.
So, again. Sulphur unites with the metals, but two metallic elements are
comparatively indifferent to each other.

Oxygen is by far the most important in the list of chemical elements.
It combines with all the others, with the single exception, perhaps, of
Fluorine. The attraction, or chemical affinity, however, which is exerted,
varies much, in different cases. The metals, as a class, are easily
oxidized; whilst many of the non-metallic elements, such as Chlorine,
Iodine, Bromine, etc., exhibit but little affinity for Oxygen. Nitrogen is
also a peculiarly negative element, showing little or no tendency to unite
with the others.

Classification of binary compounds containing Oxygen.--When one simple
element unites with another, the product is termed a "binary" compound.

There are three distinct classes of binary compounds of Oxygen:--Neutral
Oxides, basic Oxides, and acid Oxides.

Neutral and basic Oxides.--Take as examples--the Oxide of Hydrogen, or
Water, a neutral Oxide; the Oxide of Potassium, or Potash, a basic Oxide.

Water is termed a neutral oxide, because its affinities are low, and it is
comparatively indifferent to other bodies. Potash and Oxide of Silver are
examples of basic oxides; but there is a great difference between the two
in chemical energy, the former belonging to a superior class of bases,
viz. the alkaline.

By studying the properties of an alkali (such as Potash or Soda) which
are familiar to all, we gain a correct notion of the whole class of basic
oxides. An alkali is a substance readily soluble in water, and yielding a
solution which has a slimy feel from its solvent action upon the skin. It
immediately restores the blue colour of reddened litmus, and changes the
blue infusion of cabbage to green. Lastly, it is neutralized and loses all
its characteristic properties upon the addition of an acid.

The _weaker bases_ are, as a rule, sparingly or not at all soluble in
water, neither have they the same caustic and solvent action upon the
skin; but they restore the colour of reddened litmus, and neutralize acids
in the same manner as the more powerful bases, or alkalies.

_The_ Acid _Oxides._--This class, taking the stronger acids
as the type, may be described as follows:--very soluble in water, the
solution possessing an intensely sour taste, and a _corroding_ rather than
a solvent action upon the skin; changes the blue colour of litmus and
other vegetable substances to red, and neutralizes the alkalies and basic
oxides generally.

Observe however that these properties are possessed in very various
degrees by different acids. Prussic Acid and Carbonic Acid, for instance,
are not sour to the taste, and being feeble in their reactions, redden
litmus scarcely or not at all. All acids however, without any exception,
tend to combine with bases and to neutralize themselves; so that this may
be said to be the most characteristic property of the class.

_Chemical composition of Acid and Basic Oxides contrasted._--It is a law
commonly observed, although with many exceptions, that bases are formed
by the union of Oxygen with _metals_; and acids, by Oxygen uniting with
_non-metallic elements_. Thus, Sulphuric Acid is a compound of Sulphur and
Oxygen; Nitric Acid, of Nitrogen and Oxygen. But the alkali, Potash, is
an oxide of the _metal_ Potassium; and the oxides of Iron, Silver, Zinc,
etc. are bases, and not acids.

Again, the composition of acids and bases is different in another respect;
the former invariably contain more Oxygen in proportion to the other
element than the latter. Taking the same examples as before, the two
classes may be represented thus:--

  Acids { Oil of Vitriol,  Sulphur  1 atom, Oxygen 3 atoms.
        { Aqua-fortis,     Nitrogen 1  "    Oxygen 5  "
  Bases { Oxide of Silver, Silver   1 atom. Oxygen 1 atom.
        { Oxide of Iron,   Iron     1  "    Oxygen 1  "

The class of Hydrogen Acids.--Oxygen is so essentially the element which
forms the acidifying principle of acids, that its very name is derived
from that fact (οξυς, acid, and γενναω, to generate). Still there are
exceptions to this rule, and in some acids _Hydrogen_ appears to play the
same part; the _Hydracids_, as they are termed, are formed principally by
Hydrogen uniting with elements like Chlorine, Bromine, Iodine, Fluorine,
etc. Thus, Muriatic or Hydrochloric Acid contains Chlorine and Hydrogen;
Hydriodic Acid contains Iodine and Hydrogen.

Observe, however, that the position held by the Hydrogen in these
compounds, is different from that of the Oxygen in the "Oxyacids," as
regards the number of atoms usually present; thus--

  Aqua-fortis   = Nitrogen 1 atom, Oxygen   5 atoms,
  Muriatic Acid = Chlorine 1  "    Hydrogen 1 atom;

so that the composition of the Hydracids is analogous to the _basic_
oxides, in containing a single atom of each constituent.


THE TERNARY COMPOUNDS OF THE ELEMENTS.

As the various elementary substances unite with each other to form Binary
Compounds, so these binary compounds again unite and form _Ternary_
Compounds.

Compound bodies however do not, as a rule, unite with simple elements. In
illustration, take the action of Nitric Acid upon Silver, described at
page 12. No effect is produced upon the metal until _Oxygen_ is imparted;
then the Oxide of Silver so formed dissolves in the Nitric Acid. In other
words, it is necessary that a binary compound should be first formed,
before the solution can take place. The mutual attraction or chemical
affinity exhibited by compound bodies is, as in the case of elements, most
strongly marked when the two substances are opposed to each other in their
general properties.

Thus, _acids_ do not unite with other acids, but they combine instantly
with _alkalies_; the two mutually neutralizing each other and forming "a
salt."

_Salts_ therefore are ternary compounds produced by the union of acids and
bases; common Salt, formed by neutralizing Muriatic Acid with Soda, being
taken as the type of the whole class.

_General characters of the Salts._--An aqueous solution of Chloride of
Sodium, or common Salt, possesses those characters which are usually
termed saline; it is neither sour nor corrosive, but, on the other hand,
has a cooling agreeable taste. It produces no effect upon litmus and other
vegetable colours, and is wanting in those energetic reactions which are
characteristic of both acids and alkalies; hence, although formed by the
union of two binary compounds, it differs essentially in properties from
both.

All salts however do not correspond to this description of the properties
of Chloride of Sodium. The Carbonate of Potash, for instance, is an
acrid and alkaline salt, and the Nitrate of Iron reddens litmus-paper.
A perfectly neutral salt is formed when a strong acid unites with an
energetic base; but if, of the two constituents, one is more powerful than
the other, the properties of that one are often seen in the resulting
salt. Thus the Carbonate of Potash is _alkaline_ to test-paper, because
the Carbonic Acid is feeble in its reactions; but if _Nitric Acid_ and
_Potash_ are brought together, then a Nitrate of Potash is produced, which
is neutral in every sense of the term.

The Chloride of Sodium and salts of a similar kind are freely soluble in
water, but all salts are not so. Some dissolve only sparingly, and others
not at all. The Chloride and Iodide of Silver are examples of the latter
class; they are not bitter and caustic like the Nitrate of Silver, but are
perfectly tasteless from being insoluble in the fluids of the mouth.

It is seen therefore from these examples, and many others which might
be adduced, that the popular notion of a saline body is far from being
correct, and that, in the language of strict definition, any substance
is a salt which is produced by the union of an acid with an alkali,
independent of the properties it may possess.

Thus, _Cyanide of Potassium_ is a true salt, although highly poisonous;
Nitrate of Silver is a salt; the green Sulphate of Iron is a salt; so also
is Chalk or Carbonate of Lime, which has neither taste, colour, nor smell.

_On the "Hydracid" class of Salts._--The distinction between Oxyacids
and Hydracids has already been pointed out (p. 309), the latter having
been shown to consist of Hydrogen united with elements analogous in their
reactions to Chlorine, Iodine, Bromine, etc.

In a salt formed by an Oxygen Acid, both the basic and acid elements
appear. Thus the common Nitre, which is a Nitrate of Potash, is found
by analysis to contain Oxide of Potassium as a base, in a state of
combination with Nitric Acid. But if a salt be formed by neutralizing an
alkali with a _Hydrogen Acid_, the product in that case does not contain
all the elements. This is seen from the following example:--

    Hydrochloric Acid  + Soda
  = Chloride of Sodium + Water;

or, stated more at length,--

    (Chlorine Hydrogen) + (Oxygen Sodium)
  = (Chlorine Sodium)   + (Oxygen Hydrogen).

Observe that the Hydrogen and Oxygen, being present in the correct
proportions, unite to form Water, which is an Oxide of Hydrogen. This
water passes off when the solution is evaporated, and leaves the dry
crystals of salt. On the other hand, with the Oxyacid Salts, the
elementary Hydrogen being absent, no water is formed, and the Oxygen
remains.

It must therefore be borne in mind that salts like the Chlorides,
Bromides, Iodides, etc. contain only _two_ elements; but that in the
Oxyacid Salts, such as Sulphates, Nitrates, Acetates, _three_ are present.
Thus, Nitrate of Silver consists of Nitrogen, Oxygen, and Silver, but
Chloride of Silver contains simply Chlorine and metallic Silver united,
without Oxygen.

The Hydracid salts however, when decomposed, yield products similar to
the Oxyacid salts. For instance, if Iodide of Potassium be dissolved in
water, and dilute Sulphuric Acid added, this acid, being powerful in its
chemical affinities, tends to appropriate to itself the alkali; but it
does not remove _Potassium_ and liberate _Iodine_, but takes the _Oxide_
of Potassium and sets free _Hydriodic Acid_, In other words, as an atom of
water is produced during the _formation_ of a Hydracid Salt, so is an atom
destroyed and made to yield up its elements in the _decomposition_ of a
Hydracid Salt.

The reaction of dilute Sulphuric Acid upon Iodide of Potassium may be
stated thus:--

  Sulphuric Acid _plus_ (Iodine Potassium) _plus_ (Hydrogen Oxygen)
  _equals_ (Sulphuric Acid, Oxygen Potassium) or Sulphate of Potash,
  _and_ (Hydrogen Iodine)                     or Hydriodic Acid.


THE NATURE OF CHEMICAL AFFINITY FURTHER ILLUSTRATED.

_Illustration from the Non-metallic Elements._--If a stream of Chlorine
gas be passed into a solution containing the same salt as before
mentioned, viz. the Iodide of Potassium, the result is to liberate a
certain portion of Iodine, which dissolves in the liquid, and tinges it
of a brown colour. The element Chlorine, possessing a degree of chemical
energy superior to that of Iodine, prevails over it, and removes the
Potassium with which the Iodine was previously combined.

    Chlorine + Iodide of Potassium
  = Iodine   + Chloride of Potassium.

_The same Law illustrated by the Metals._--A strip of Iron dipped in
solution of Nitrate of Silver becomes immediately coated with metallic
Silver; but a piece of Silver-foil may be left for any length of time in
Sulphate of Iron without undergoing change: the difference depends upon
the fact, that metallic Iron has a greater attraction for Oxygen than
Silver, and hence it displaces it from its solution.

    Iron +   Nitrate of Silver
  = Silver + Nitrate of Iron.

_Illustrations amongst Binary Compounds._--If a few drops of solution
of Potash be added to solution of Nitrate of Silver, a brown deposit is
formed, which is the Oxide of Silver, sparingly soluble in water. That is
to say, as a stronger metal displaces _metallic Silver_, so does an oxide
of the same metal displace _Oxide of Silver_. Therefore bases like the
alkalies, alkaline earths, etc. cannot exist in a free state in solutions
of the salts of weaker bases,--a liquid containing Nitrate of Silver could
not also contain free Potash or Ammonia.

In the list given at page 306, the metallic elements are arranged
principally in the order of their chemical affinities; those of Potassium,
Sodium, Barium, etc. being the most marked.

As the alkalies displace the weaker bases from their combination with
acids, so the strong _acids_ displace weak acids from their combination
with bases. Thus, as

    Oxide of Potassium + Acetate of Silver
  = Oxide of Silver    + Acetate of Potash;

So

    Nitric Acid + Acetate of Silver
  = Acetic Acid + Nitrate of Silver.

In the list of acids. Sulphuric Acid is usually placed first as being the
strongest, and Carbonic Acid, which is a gaseous substance, last. The
vegetable acids, such as Acetic, Tartaric, etc., are _intermediate_, being
weaker than the mineral acids, but stronger than Carbonic, or Hydrocyanic
Acid.

_The order of decompositions affected by the insolubility or the
volatility of the products which may be formed._--It might be inferred
from remarks already made, that on mixing saline solutions, a gradual
interchange of elements would take place, until the strongest acids
were associated with the strongest bases, and _vice versâ_. There are
many causes however which interfere to prevent this; one of which is
_volatility_.---

The violent effervescence which takes place on treating a _Carbonate_ of
any kind with an acid is due to the _gaseous_ nature of Carbonic Acid and
its escape in that form, which greatly facilitates the decomposition.

_Insolubility_ is also a cause which exercises a great influence on the
result which will follow in mixing solutions. If the formation of an
insoluble substance is possible by any interchange of elements, it will
take place. A solution of Chloride of Sodium added to Nitrate of Silver
invariably produces Chloride of Silver; the _insolubility_ of Chloride of
Silver being the cause which determines its formation.

So again, Sulphate of Lead and Protonitrate of Iron are produced by mixing
Nitrate of Lead with Sulphate of Iron; but if Nitrate of _Potash_ be
substituted for Nitrate of Lead, the result is uncertain, because there
are no elements present which can, by interchanging, form an insoluble
salt; Sulphate of Potash, although _sparingly_ soluble in water, not being
_insoluble_, like the Sulphate of Lead or the Sulphate of Baryta.


ON CHEMICAL NOMENCLATURE.

The nomenclature of the chemical _elements_ is mostly independent of any
rule; but an attempt has been made to obviate this in the case of those of
later discovery. Thus the names of the newly-found _metals_ usually end
in _um_, as Potassium, Sodium, Barium, Calcium, etc.; and those elements
which possess analogous characters have corresponding terminations
assigned to them, as Chlorine, Bromine, Iodine, Fluorine, etc.

_Nomenclature of Binary Compounds._--These are often named by attaching
the termination _ide_ to the more important element of the two; as, the
Ox_ide_ of Hydrogen, or Water; the Chlor_ide_ of Silver; the Sulph_ide_
of Silver. Binary compounds of Sulphur however are sometimes termed
Sulphurets, as the _Sulphuret_ or the _Sulphide_ of Silver indifferently.

When the same body combines with Oxygen, or the corresponding element, in
more than one proportion, the prefix _proto_ is applied to that containing
the least Oxygen; _sesqui_ to that with once and a half as much as the
_proto_; _bi_ or _bin_ to that with twice as much; and _per_ to the one
containing the most Oxygen of all. As examples, take the following:--The
Protoxide of Iron; the Sesquioxide of Iron: the Protochloride of Mercury;
the Bichloride of Mercury. In these examples the Sesquioxide of Iron
is also a _Per_oxide, because no higher simple oxide is known, and the
Bichloride of Mercury is a _Per_chloride for a similar reason.

When an inferior compound is discovered, it is often termed _sub_; as the
Suboxide of Silver, the Subchloride of Silver. These bodies contain the
least known quantity of Oxygen and Chlorine respectively, and are hence
entitled to the prefix _proto_; but being of minor importance, they are
excepted from the general rule.

The combinations of metallic elements with each other are termed "alloys;"
or if containing Mercury, "amalgams."

_Nomenclature of binary Compounds possessing acid properties._--These
are named on a different principle. The termination _ic_ is applied to
one element. Thus, taking as an illustration the liquid known as "Oil of
Vitriol," it is truly an _Oxide_ of Sulphur, but as it possesses strong
acid properties it is termed Sulphur_ic_ Acid. So Nitric Acid is an Oxide
of Nitrogen; Carbonic Acid is an Oxide of Carbon, etc. When there are two
oxides of the same element, both possessing acid properties, the most
important has the termination _ic_, and the other _ous_; as Sulphuric
Acid, Sulphur_ous_ Acid; Nitric Acid, Nitr_ous_ Acid.

_Nomenclature of the Hydracids._--The Hydrogen Acids are distinguished
from Oxyacids by retaining the names of both constituents, the termination
_ic_ being annexed as usual. Thus, _Hydro_chloric Acid, or the Chloride of
Hydrogen; _Hydr_iodic Acid, or the Iodide of Hydrogen.

_Further illustrations of the nomenclature of Binary Compounds._--The
Oxides of Nitrogen, and also of Sulphur, afford an interesting
illustration of the principles of nomenclature. The former are as
follows:--

                           Nitrogen.  Oxygen.

  Protoxide of Nitrogen     1 atom.    1 atom.
  Binoxide of Nitrogen      1  "       2  "
  Nitrous Acid              1  "       3  "
  Peroxide of Nitrogen      1  "       4  "
  Nitric Acid               1  "       5  "

Observe, that two only out of the five possess acid properties, the others
being simple oxides. Nitric Acid is, strictly speaking, the "Peroxide,"
but as it belongs to the class of acids, that term naturally falls to the
compound below.

The binary compounds of Sulphur with Oxygen all possess acid properties;
they may be represented (in part) as follows:--

                           Sulphur.   Oxygen.

  Hyposulphurous Acid       2 atoms.   2 atoms.
  Sulphurous Acid           1  "       2  "
  Hyposulphuric Acid        2  "       5  "
  Sulphuric Acid            1  "       3  "

In this case the Sulphuric and Sulphurous Acids had become familiarly
known before the others, intermediate in composition, were discovered.
Hence, to avoid the confusion which would result from changing
the nomenclature, the new bodies are termed _Hypo_sulphuric and
_Hypo_sulphurous (from ὑπο, _under_).

_Nomenclature of Salts._--Salts are named according to the acid they
contain; the termination _ic_ being changed into _ate_, and _ous_ into
_ite_. Thus, Sulphuric Acid forms Sulph_ates_; Nitric Acid, Nitr_ates_;
but Sulphur_ous_ Acid forms Sulph_ites_, and Nitrous Acid, Nitr_ites_.

In naming a salt, the base is always placed _after_ the acid, the term
_oxide_ being omitted; thus. _Nitrate of Oxide of Silver_ is more shortly
known as "Nitrate of Silver," the presence of Oxygen being understood.

When there are two oxides of the same base, both of which are
_salifiable_,--in naming the salts, the term _proto_ is prefixed to the
acid of the salt formed by the lowest, and per to that of the higher
oxide; as, the _Proto_sulphate of Iron, or Sulphate of the Protoxide; the
_Per_sulphate of Iron, or Sulphate of the Peroxide.

Many salts contain more than one atom of acid to each atom of base. In
that case, the usual prefixes expressive of quantity are adopted: thus,
the _Bi_sulphate of Potash contains twice as much Sulphuric Acid as the
neutral Sulphate, etc.

On the other hand, there are salts in which the base is in excess with
regard to the acid, and which are usually known as "basic salts;" thus,
the red powder which deposits from solution of Sulphate of Iron, is a
_basic_ Persulphate of Iron, or a Sulphate of the Peroxide of Iron with
more than the normal proportion of oxide.

_Nomenclature of the Hydracid Salts._--The composition of these salts
being different from those formed by Oxygen Acids, the nomenclature varies
also. Thus, in neutralizing Hydrochloric Acid with Soda, the product
formed is not known as Hydrochlorate of Soda, but as _Chloride of Sodium_;
this salt, and others of a similar constitution, being _binary_, and not
_ternary_, compounds. The salt produced by Hydrochloric Acid and _Ammonia_
however is often called "Muriate or Hydrochlorate of Ammonia," although
more strictly it should be the _Chloride of Ammonium_.


ON SYMBOLIC NOTATION.

The list of symbols employed to represent the various elementary bodies is
given at page 306.--Commonly the initial letter of the Latin name is used,
a second or smaller letter being added when two elements correspond in
their initials: thus C stands for Carbon, Cl for Chlorine, Cd for Cadmium,
and Cu for Copper.

The chemical symbol however does not simply represent a particular
element; it denotes also a definite weight, or equivalent proportion, of
that element. This will be explained more fully in the succeeding pages,
when speaking of the Laws of Combination.

_Formulæ of Compounds._--In the _nomenclature_ of compounds it is usual
to place the Oxygen or analogous element _first_ in the case of binary
compounds, and the acid before the base in the ternary compounds, or
salts; but in representing them _symbolically_ this order is reversed:
thus, Oxide of Silver is written AgO, and never as OAg; Nitrate of Silver
as AgO NO{5}, not NO{5}AgO.

The juxtaposition of symbols expresses combination; thus, FeO is a
compound of one proportion of Iron with one of Oxygen, or the "Protoxide
of Iron," If more than one equivalent be present, small figures are
placed below the symbols: thus, Fe{2}O{3} represents two equivalents of
Iron united with three of Oxygen, or the "Peroxide of Iron;" SO{3}, one
equivalent of Sulphur with three of Oxygen, or Sulphuric Acid.

Larger figures placed before and in the same line with the symbols, affect
the _whole compound_ which the symbols express: thus, 2 SO{3} means two
equivalents of Sulphuric Acid; 3 NO{5}, three equivalents of Nitric Acid.
The interposition of a comma prevents the influence of the large figure
from extending further. Thus, the double Hyposulphite of Soda and Silver
is represented as follows:--

  2 NaO S{2}O{2}, AgO S{2}O{2},

or _two_ equivalents of Hyposulphite of Soda with one of Hyposulphite of
Silver; the large figure referring only to the first half of the formula.
Sometimes brackets, etc. are employed, in order to render a complicated
formula more plain. For example, the formula for the double Hyposulphite
of Gold and Soda, or "Sel d'or," may be written thus;--

  3 (NaO S{2}O{2}) AuO S{2}O{2} + 4 HO.

In this formula, the _plus sign_ (+) denotes that the four atoms of water
which follow, are less intimately united with the framework of the salt
than the other constituents.

The use of a plus sign is commonly adopted in representing salts which
contain water of crystallization. Thus, the formula for the crystallized
Protosulphate of Iron is written as follows:--

  FeO SO{3} + 7 HO.

These atoms of water are driven off by the application of heat, leaving a
white substance, which is the Anhydrous salt, and would be written simply
as FeO SO{3}.

The _plus_ sign however is often employed in token of simple _addition_,
no combination of any kind being intended. Thus the decomposition which
follows on mixing Chloride of Sodium with Nitrate of Silver may be
written as follows:--

  NaCl + AgO NO{5} = AgCl + NaO NO{5};

that is,--

    Chloride of Sodium _added to_ Nitrate of Silver.
  = Chloride of Silver _and_ Nitrate of Soda.


ON EQUIVALENT PROPORTIONS.

When elementary or compound bodies enter into chemical union with each
other, they do not combine in indefinite proportions, as in the case of a
mixture of two liquids, or the solution of a saline body in water. On the
other hand, a certain definite weight of the one unites with an equally
definite weight of the other; and if an excess of either be present, it
remains free and uncombined.

Thus, if we take a _single grain_ of the element Hydrogen--to convert that
grain into Water there will be required exactly 8 grains of Oxygen; and
if a larger quantity than this were added, as for instance _ten grains_,
then two grains would be over and above. So, to form Hydrochloric Acid, 1
grain of Hydrogen takes 36 grains of Chlorine:--for the _Hydriodic Acid_,
1 grain of Hydrogen unites with 126 grains of Iodine.

Again, if separate portions of metallic Silver, of 108 grains each, are
weighed out,--in order to convert them into Oxide, Chloride, and Iodide of
Silver respectively, there would be required

  Oxygen                                 8 grains.
  Chlorine                              36  "
  Iodine                               126  "

Therefore it appears that 8 grains of Oxygen are _equivalent_ to 36 grains
of Chlorine and to 126 grains of Iodine, seeing that these quantities all
play the same part in combining; and so it is with regard to the other
elements,--to every one of them a figure can be assigned which represents
the number of parts by weight in which that element unites with others.
These figures are the "equivalents" or "combining proportions," and they
are denoted by the _symbol_ of the element. A symbol does not stand as
a simple representative of an element, but as a representative of _one
equivalent_ of an element. Thus "O" indicates 8 parts by weight of Oxygen;
"Cl" one equivalent, or 36 parts by weight, of Chlorine; and so with the
rest.

Observe however that these figures, termed "equivalents," do not refer
to the _actual number_ of parts by weight, but only to the _ratio_ which
exists between them: if Oxygen is 8, then Chlorine is 36; but if we term
Oxygen 100, as some have proposed, then Chlorine would be 442·65.

In the scale of equivalents now usually adopted, Hydrogen, as being the
lowest of all, is taken as unity, and the others are related to it.

_Equivalents of Compounds._--The law of equivalent proportions applies
to compounds as well as to simple bodies, the combining proportion of a
compound being always the sum of the equivalents of its constituents. Thus
Sulphur is 16, and Oxygen 8, therefore Sulphuric Acid, or SO{3}, equals
40. The equivalent of Nitrogen is 14, that of Nitric Acid, or NO{5}, is 54.

The same rule applies with regard to salts. Take for instance the Nitrate
of Silver: it contains

                                        Equivalent.
  Nitrogen                                   14
  6 Oxygen                                   48
  Silver                                    108
                                            ---
  Total of equivalents, or equivalent }     170
    of the Nitrate of Silver          }

_Practical application of the Laws of Combination ._--The utility of being
acquainted with the law of combining proportions is obvious when their
nature is understood. As bodies both unite with and replace each other
in equivalents, a simple calculation shows at once how much of each
element or compound will be required in a given reaction. Thus, supposing
it be desired to convert 100 grains of Nitrate of Silver into _Chloride_
of Silver, the weight of Chloride of Sodium which will be necessary is
deduced thus:--one equivalent, or 170 parts, of Nitrate of Silver, is
decomposed by an equivalent, or 60 parts, of Chloride of Sodium. Therefore

  as 170 : 60 :: 100 : 35·2;

that is, 35·2 grains of Salt will precipitate, in the state of Chloride,
the whole of the Silver contained in 100 grains of Nitrate.

So again, in order to form the Iodide of Silver, the proportions in which
the two salts should be mixed is thus shown. The equivalent of Iodide of
Potassium is 166, and that of Nitrate of Silver is 170. These numbers so
nearly correspond, that it is common to direct that equal weights of the
two salts should be taken.

One more illustration will suffice. Supposing it be required to form 20
grains of Iodide of Silver--how much Iodide of Potassium and Nitrate of
Silver must be used? One equivalent, or 166 parts, of Iodide of Potassium,
will yield an equivalent, or 234 parts, of Iodide of Silver; therefore

  as 234 : 166 :: 20 : 14·2.

Hence, if 14·2 grains of the Iodide of Potassium be dissolved in water,
and an equivalent quantity, viz. 14·5 grains, of the Nitrate of Silver
added, the yellow precipitate, when washed and dried, will weigh precisely
20 grains.


ON THE ATOMIC THEORY.

The atomic theory, originally proposed by Dalton, so much facilitates the
comprehension of chemical reactions generally, that it may be useful to
give a short sketch of it.

It is supposed that all matter is made up of an infinite number of minute
atoms, which are elementary, and do not admit of further division. Each
of these atoms possesses an actual weight, although inappreciable by our
present methods of investigation. Simple atoms, by uniting with each
other, form _compound atoms_; and when these compounds are broken up, the
elementary constituent atoms are not destroyed, but separate from each
other, in possession of all their original properties.

In representing the simple atomic structure of bodies, _circles_ may be
used, as in the following diagram.

[Illustration: Fig. 1. Fig. 2. Fig. 3.]

Fig. 1 is a compound atom of Sulphuric Acid, consisting of an atom of
Sulphur united intimately with three of Oxygen; fig. 2 is an atom of
Peroxide of Nitrogen, NO{4}; and fig. 3, an atom of Nitric Acid, composed
of Nitrogen 1 atom. Oxygen 5 atoms, or in symbols NO{5}.

_The term "atomic weight" substituted for equivalent proportion._--If
we suppose that the simple atoms of different kinds of matter _differ
in weight_, and that this difference is expressed by their equivalent
numbers, the whole laws of combination follow by the simplest reasoning.
It is easy to understand that an atom of one element, or compound, would
displace, or be substituted for, a single atom of another; therefore,
taking as the illustration the decomposition of Iodide of Potassium by
Chlorine,--the weight of the latter element required to liberate 126
grains of Iodine is 36 grains, because _the weights of the atoms of those
two elementary bodies are as 36 to 126_. So again, in the reaction
between Chloride of Sodium and Nitrate of Silver, a compound atom of the
former, represented by the weight 60, reacts upon a compound atom of the
latter, which equals 170.

Therefore in place of the term "equivalent" or "combining proportion," it
is more usual to employ that of "atomic weight." Thus the atomic weight
of Oxygen is 8, represented by the symbol O; that of Sulphur is 16; hence
the atomic weight of the compound atom of Sulphuric Acid, or SO{3}, is
necessarily equal to the combined weights of the four simple atoms; _id
est_, 16 + 24 = 40.


ON THE CHEMISTRY OF ORGANIC SUBSTANCES.

By "organic" substances are meant those which have possessed _life_, with
definite organs and tissues, in contra-distinction to the various forms of
dead inorganic matter, in which no structural organization of that kind is
found.

The term organic however is also applied to substances which are obtained
by chemical processes from the vegetable and animal kingdoms, although
they cannot themselves be said to be living bodies; thus Acetic Acid,
procured by the distillation of woody fibre, and Alcohol, by fermentation
from sugar, are strictly organic substances.

The class of organic bodies embraces a great variety of products; which,
like inorganic Oxides, may be divided into neutral, acid, and basic.

The organic _acids_ are numerous, including Acetic Acid, Tartaric, Citric,
and a variety of others.

The _neutral substances_ cannot easily be assimilated to any class of
inorganic compounds; as examples, take Starch, Sugar, Lignine, etc.

The _bases_ are also a large class. They are mostly rare substances, not
familiarly known: Morphia, obtained from Opium; Quinia, from Quinine;
Nicotine, from Tobacco, are illustrations.

_Composition of organic and inorganic bodies contrasted._--There are more
than fifty elementary substances found in the inorganic kingdom, but
only _four_, commonly speaking, in the organic: these four are Carbon,
Hydrogen, Nitrogen, and Oxygen.

Some organic bodies,--oil of turpentine, naphtha, etc., contain only
Carbon and Hydrogen; many others, such as sugar, gum, alcohol, fats,
vegetable acids--Carbon, Hydrogen, and Oxygen. The _Nitrogenous bodies_,
so called, containing Nitrogen in addition to the other elements, are
principally substances derived from animal and vegetable tissues, such as
Albumen, Caseine, Gelatine, etc.; Sulphur and Phosphorus are also present
in many of the Nitrogenous bodies, but only to a small extent.

Organic substances, although simple as regards the _number_ of elements
involved in their formation, are often highly complex in the arrangement
of the atoms; this may be illustrated by the following formulæ:--

  Starch                      C{24}H{20}O{20}
  Lignine                     C{24}H{20}O{20}
  Cane Sugar                  C{24}H{22}O{22}
  Grape Sugar                 C{24}H{28}O{28}

Inorganic bodies, as already shown, unite _in pairs_,--two elements join
to form a binary compound; two binary compounds produce a salt; two salts
associated together form a double salt. With organic bodies however the
arrangement is different,--the elementary atoms are all grouped equally
in one compound atom, which is highly complex in structure, and cannot be
split up into binary products.

Observe also, as characteristic of Organic Chemistry, the apparent
similarity in composition between bodies which differ widely in
properties. As examples take _Lignine_, or cotton fibre, and Starch,--each
of which contains the three elements united as C{24}H{20}O{20}.

_Mode of distinguishing between Organic and Inorganic matter._--A simple
means of doing this is as follows:-- place the suspected substance upon
a piece of Platinum-foil, and heat it to redness with a spirit-lamp: if
it first _blackens_, and then burns completely away, it is probably of
organic origin. This test depends upon the fact, that the constituent
elements of organic bodies are all either themselves volatile, or capable
of forming volatile combinations with Oxygen. Inorganic substances, on the
other hand, are often unaffected by heat, or, if volatile, are dissipated
without previous charring.

The action of heat upon organic matter may further be illustrated by the
combustion of coal or wood in an ordinary furnace;--first, an escape
of Carbon and Hydrogen, united in the form of volatile gaseous matter,
takes place, leaving behind a black cinder, which consists of Carbon and
inorganic matter combined; afterwards this Carbon burns away into Carbonic
Acid, and a grey ash is left which is composed of inorganic salts, and is
indestructible by heat.




CHAPTER II.

VOCABULARY OF PHOTOGRAPHIC CHEMICALS.


ACETIC ACID.

Symbol, C{4}H{3}O{3} + HO. Atomic weight, 60.

Acetic Acid is a product of the _oxidation_ of Alcohol. Spirituous
liquids, when perfectly pure, are not affected by exposure to air; but if
a portion of yeast, or Nitrogenous organic matter of any kind, be added,
it soon acts as a _ferment_, and causes the spirit to unite with oxygen
derived from the atmosphere, and so to become sour from formation of
Acetic Acid, or "vinegar."

Acetic Acid is also produced on a large scale by heating _wood_ in close
vessels: a substance distils over which is Acetic Acid contaminated with
empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much
used in commerce.

The most concentrated Acetic Acid may be obtained by neutralizing common
vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda
so formed; this Acetate of Soda is then distilled with Sulphuric Acid,
which removes the Soda and liberates Acetic Acid: the Acetic Acid being
volatile, distils over, and may be condensed.

_Properties of Acetic Acid._--The strongest acid contains only a single
atom of water; it is sold under the name of "Glacial Acetic Acid," so
called from its property of solidifying at a moderately low temperature.
At about 50° the crystals melt, and form a limpid liquid of pungent odour
and a density nearly corresponding to that of water; the specific gravity
of Acetic Acid however is no test of its real strength, which can only be
estimated by analysis.

The commercial _Glacial_ Acetic Acid is often diluted with water, which
may be suspected if it does not solidify during the cold winter months.
Sulphurous and Hydrochloric Acids are also common impurities. They are
injurious in Photographic Processes, from their property of precipitating
Nitrate of Silver. To detect them proceed as follows:--dissolve a small
crystal of Nitrate of Silver in a few drops of water, and add to it about
half a drachm of the Glacial Acid; the mixture should remain quite clear
even when exposed to the light. Hydrochloric and Sulphurous Acid produce
a white deposit of Chloride or Sulphite of Silver; and if _Aldehyde_ or
volatile tarry matter be present in the Acetic Acid, the mixture with
Nitrate of Silver, although clear at first, becomes discoloured by the
action of light.

Glacial Acetic Acid sometimes has a smell of garlic. In this state it
probably contains an organic Sulphur Acid, and is unfit for use.

Many employ a cheaper form of Acetic Acid, sold by druggists as
"Beaufoy's" acid; it should be of the strength of the Acetic Acid fortiss.
of the London Pharmacopœia, containing 30 per cent, real acid. It will be
advisable to test it for Sulphuric Acid (see Sulphuric Acid), and other
impurities, before use.


ACETATE OF SILVER. _See_ Silver, Acetate of.


ALBUMEN.

Albumen is an organic principle found both in the animal and vegetable
kingdom. Its properties are best studied in the _white of egg_, which is
a very pure form of Albumen.

Albumen is capable of existing in two states; in one of which it is
soluble, in the other insoluble, in water. The aqueous solution of the
soluble variety gives a slightly alkaline reaction to test-paper; it is
somewhat thick and glutinous, but becomes more fluid on the addition of a
small quantity of an alkali, such as Potash or Ammonia.

Soluble Albumen may be converted into the _insoluble_ form in the
following ways:--

1. _By the application of heat._--A moderately strong solution of
Albumen becomes opalescent and coagulates on being heated to about 150°
Fahrenheit, but a temperature of 212° is required if the liquid is very
dilute. A layer of _dried_ Albumen cannot easily be coagulated by the mere
application of heat.

2. _By addition of strong acids._--Nitric Acid coagulates Albumen
perfectly without the aid of heat. Acetic Acid however acts differently,
appearing to enter into combination with the Albumen, and forming a
compound soluble in warm water acidified by Acetic Acid.

3. _By the action of metallic salts._--Many of the salts of the metals
coagulate Albumen completely. Nitrate of Silver does so; also the
Bichloride of Mercury. Ammoniacal Oxide of Silver however does not
coagulate Albumen.

The white precipitate formed on mixing Albumen with Nitrate of Silver
is a chemical compound of the animal matter with Protoxide of Silver.
This substance, which has been termed Albuminate of Silver, is soluble
in Ammonia and Hyposulphite of Soda; but after exposure to light, or
heating in a current of Hydrogen gas, it assumes a brick-red colour, being
probably reduced to the condition of an organic compound of a _Suboxide_
of Silver. It is then almost insoluble in Ammonia, but enough dissolves
to tinge the liquid wine-red. The _red coloration_ of solution of Nitrate
of Silver employed in sensitizing the Albuminized photographic paper is
probably produced by the same compound, although, often referred to the
presence of Sulphuret of Silver.

Albumen also combines with Lime and Baryta. When Chloride of Barium is
used with Albumen, a white precipitate of this kind usually forms.

_Chemical composition of Albumen._--Albumen belongs to the _Nitrogenous_
class of organic substances (see page 325). It also contains small
quantities of Sulphur and Phosphorus.


ALCOHOL.

Symbol, C{4}H{6}O{2}. Atomic weight, 46.

Alcohol is obtained by the careful distillation of any spirituous or
fermented liquor. If wine or beer be placed in a retort, and heat applied,
the Alcohol, being more volatile than water, rises first, and is condensed
in an appropriate receiver; a portion of the vapour of water however
passes over with the Alcohol, and dilutes it to a certain extent, forming
what is termed "Spirits of Wine." Much of this water may be removed by
redistillation from Carbonate of Potash, in the manner described at
page 196 of this work; but in order to render the Alcohol thoroughly
_anhydrous_, it is necessary to employ _Quicklime_, which possesses a
still greater attraction for water. An equal weight of this powdered lime
is mixed with strong Alcohol of ·823, and the two are distilled together.

_Properties of Alcohol._--Pure anhydrous Alcohol is a limpid liquid, of an
agreeable odour and pungent taste; sp. gr. at 60°, ·794. It absorbs vapour
of water, and becomes diluted by exposure to damp air; boils at 173° Fahr.
It has never been frozen.

Alcohol distilled from Carbonate of Potash has a specific gravity of ·815
to ·823, and contains 90 to 93 per cent, of real spirit.

The specific gravity of ordinary rectified Spirits of Wine is usually
about ·840, and it contains 80 to 83 per cent, of absolute Alcohol.


AMMONIA.

Symbol, NH{3} or NH{4}O. Atomic weight, 17.

The liquid known by this name is an aqueous solution of the volatile gas
Ammonia. Ammoniacal gas contains one atom of Nitrogen combined with three
of Hydrogen: these two elementary bodies exhibit no affinity for each
other, but they can be made to unite under certain circumstances, and the
result is Ammonia.

_Properties of Ammonia._--Ammoniacal gas is soluble in water to a large
extent; the solution possessing those properties which are termed alkaline
(see page 308). Ammonia however differs from the other alkalies in one
important particular--it is volatile: hence the original colour of
turmeric-paper affected by Ammonia is restored on the application of heat.
Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is
converted into Carbonate of Ammonia; it should therefore be preserved in
stoppered bottles. Besides Carbonate, commercial Ammonia often contains
Chloride of Ammonium, recognized by the white precipitate given by Nitrate
of Silver after acidifying with pure Nitric Acid.

The strength of commercial Ammonia varies greatly; that sold for
pharmaceutical purposes under the name of Liquor Ammoniæ, contains about
10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes
with the proportion of Ammonia present, the Liquor Ammoniæ being usually
about ·936.

Ammonia, although forming a large class of salts, appears at first sight
to contrast strongly in composition with the alkalies proper, such as
Potash and Soda. Mineral bases generally are _protoxides of metals_, as
already shown at page 308, but Ammonia consists simply of Nitrogen and
Hydrogen united without Oxygen. The following remarks may perhaps tend
somewhat to elucidate the difficulty:--

_Theory of Ammonium._--This theory supposes the existence of a substance
possessing the properties of a _metal_, but differing from metallic
bodies generally in being _compound_ in structure: the formula assigned
to it is NH{4}, one atom of Nitrogen united with four of Hydrogen. This
hypothetical metal is termed "Ammonium;" and Ammonia, associated with an
atom of water, may be viewed as its _Oxide_, for NH{3} + HO plainly equals
NH{4}O. Thus, as Potash is the Oxide of _Potassium_, so Ammonia is the
Oxide of _Ammonium_.

The composition of the _salts_ of Ammonia is on this view assimilated to
those of the alkalies proper. Thus, Sulphate of Ammonia is a Sulphate of
the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride
of Ammonium, etc.


AMMONIO-NITRATE OF SILVER. _See_ Silver, Ammonio-Nitrate of.


AQUA-REGIA. _See_ Nitro-Hydrochloric Acid.


BARYTA, NITRATE OF. _See_ Nitrate of Baryta.


BICHLORIDE OF MERCURY. _See_ Mercury, Bichloride of.


BROMINE.

Symbol, Br. Atomic weight, 78.

This elementary substance is obtained from the uncrystallizable residuum
of sea-water, termed _bittern_. It exists in the water in very minute
proportion, combined with Magnesium in the form of a soluble Bromide of
Magnesium.

_Properties._--Bromine is a deep reddish-brown liquid of a disagreeable
odour, and fuming strongly at common temperatures; sparingly soluble
in water (1 part in 23, Löwig), but more abundantly so in Alcohol, and
especially in Ether. It is very heavy, having a specific gravity of 3·0.

Bromine is closely analogous to Chlorine and Iodine in its chemical
properties. It stands on the list intermediately between the two; its
affinities being stronger than those of Iodine, but weaker than Chlorine
(see Chlorine).

It forms a large class of salts, of which the Bromides of Potassium,
Cadmium, and Silver are the most familiar to Photographers.


BROMIDE OF POTASSIUM.

Symbol, KBr. Atomic weight, 118.

Bromide of Potassium is prepared by adding Bromine to Caustic Potash,
and heating the product, which is a mixture of Bromide of Potassium and
Bromate of Potash, to redness, in order to drive off the Oxygen from the
latter salt. It crystallizes in anhydrous cubes, like the Chloride and
Iodide of Potassium; it is easily soluble in water, but more sparingly so
in Alcohol; it yields red fumes of Bromine when acted upon by Sulphuric
Acid.


BROMIDE OF SILVER. _See_ Silver, Bromide of.


CARBONATE OF SODA.

Symbol, NaO CO{2} + 10 Aq.

This salt was formerly obtained from the ashes of seaweeds, but is
now more economically manufactured on a large scale from common salt.
The Chloride of Sodium is first converted into Sulphate of Soda, and
afterwards the Sulphate into Carbonate of Soda.

_Properties._--The perfect crystals contain ten atoms of water, which
are driven off by the application of heat, leaving a white powder--the
anhydrous Carbonate. _Common Washing Soda_ is a neutral Carbonate,
contaminated to a certain extent with Chloride of Sodium and Sulphate of
Soda. The Carbonate used for effervescing draughts is either a Bicarbonate
with 1 atom of water, or a Sesquicarbonate, containing about 40 per cent,
of real alkali; it is therefore nearly double as strong as the washing
Carbonate, which contains about 22 per cent, of Soda. Carbonate of Soda is
soluble in twice its weight of water at 60°, the solution being strongly
alkaline.


CARBONATE OF POTASH. See Potash, Carbonate of.


CASEINE. _See_ Milk.


CHARCOAL, ANIMAL.

Animal Charcoal is obtained by heating animal substances, such as bones,
dried blood, horns, etc., to redness, in close vessels, until all
volatile empyreumatic matters have been driven off, and a residue of
Carbon remains. When prepared from bones it contains a large quantity of
inorganic matter in the shape of Carbonate and Phosphate of Lime, the
former of which produces _alkalinity_ in reacting upon Nitrate of Silver
(see p. 89). Animal Charcoal is freed from these earthy salts by repeated
digestion in Hydrochloric Acid; but unless very carefully washed it is apt
to retain an acid reaction, and so to liberate free Nitric Acid when added
to solution of Nitrate of Silver.

_Properties._--Animal Charcoal, when pure, consists, solely of Carbon, and
burns away in the air without leaving any residue: it is remarkable for
its property of decolorizing solutions; the organic colouring substance
being separated, but not actually _destroyed_, as it is by _Chlorine_
employed as a bleaching agent. This power of absorbing colouring matter is
not possessed in an equal degree by all varieties of Charcoal, but is in
great measure peculiar to those derived from the animal kingdom.


CHINA CLAY, OR KAOLIN.

This is prepared, by careful levigation, from mouldering granite and
other disintegrated felspathic rocks. It consists of the _Silicate of
Alumina_,--that is, of Silicic Acid or _Flint_, which is an Oxide of
Silicon, united with the base Alumina (Oxide of Aluminum). Kaolin is
perfectly insoluble in water and acids, and produces no decomposition
in solution of Nitrate of Silver. It is employed by Photographers to
decolorize solutions of Nitrate of Silver which have become brown from the
action of Albumen or other organic matters.

Commercial Kaolin may contain chalk, in which state it produces
alkalinity in solution of Nitrate of Silver. The impurity, detected by
its effervescence with acids, is removed by washing the Kaolin in diluted
vinegar and subsequently in water.


CHLORINE.

Symbol, Cl. Atomic weight, 36.

Chlorine is a chemical element found abundantly in nature, combined with
metallic Sodium in the form of Chloride of Sodium, or Sea-salt.

_Preparation._--By distilling common Salt with Sulphuric Acid, Sulphate of
Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine
combined with Hydrogen; by the action of nascent Oxygen (see Oxygen), the
Hydrogen may be removed in the form of water, and the Chlorine left alone.

_Properties._--Chlorine is a greenish-yellow gas, of a pungent and
suffocating odour; soluble to a considerable extent in water, the solution
possessing the odour and colour of the gas. It is nearly 2-1/2 times as
heavy as a corresponding bulk of atmospheric air.

_Chemical properties._--Chlorine belongs to a small natural group of
elements which contains also Bromine, Iodine, and Fluorine. They are
characterized by having a strong affinity for Hydrogen, and also for
the metals; but are comparatively indifferent to Oxygen. Many metallic
substances actually undergo _combustion_ when projected into an atmosphere
of Chlorine, the union between the two taking place with extreme violence.
The characteristic bleaching properties of Chlorine gas are explained in
the same manner:--Hydrogen is removed from the organic substance, and in
that way the structure is broken up and the colour destroyed.

Chlorine is more powerful in its affinities than either Bromine or
Iodine. The salts formed by these three elements are closely analogous
in composition and often in properties. Those of the Alkalies, Alkaline
Earths, and many of the Metals, are soluble in water; but the Silver salts
are insoluble; the Lead salts sparingly so.

The combinations of Chlorine, Bromine, Iodine, and Fluorine, with
Hydrogen, are acids, and neutralize Alkalies in the usual, manner, with
formation of Alkaline Chloride and water (see page 311).

The test by which the presence of Chlorine is detected, either free or in
combination with bases, is _Nitrate of Silver_; it gives a white curdy
precipitate of Chloride of Silver, insoluble in Nitric Acid, but soluble
in Ammonia. The solution of Nitrate of Silver employed as the test must
not contain Iodide of Silver, as this compound is precipitated by dilution.


CHLORIDE OF AMMONIUM.

Symbol, NH{4}Cl. Atomic weight, 54.

This salt, also known as Muriate or Hydrochlorate of Ammonia, occurs in
commerce in the form of colourless and translucent masses, which are
procured by _sublimation_, the dry salt being volatile when strongly
heated. It dissolves in an equal weight of boiling, or in three parts of
cold water. It contains more Chlorine in proportion to the weight used
than Chloride of Sodium, the atomic weights of the two being as 54 to 60.


CHLORIDE OF BARIUM.

Symbol, BaCl + 2 HO. Atomic weight, 123.

Barium is a metallic element very closely allied to Calcium, the
elementary basis of Lime. The Chloride of Barium is commonly employed as
a test for Sulphuric Acid, with which it forms an insoluble precipitate
of Sulphate of Baryta. It is also said to affect the colour of the
Photographic image when used in preparing Positive paper, which may
possibly be due to a chemical combination of Baryta with Albumen; but
it must be remembered that this Chloride, from its high atomic weight,
contains less Chlorine than the alkaline Chlorides (see page 124).

_Properties of Chloride of Barium._--Chloride of Barium occurs in the
form of white crystals, soluble in about two parts of water, at common
temperature. These crystals contain two atoms of water of crystallization,
which are expelled at 212°, leaving the anhydrous Chloride.


CHLORIDE OF GOLD. See Gold, Chloride of.


CHLORIDE OF SODIUM.

Symbol, NaCl. Atomic weight, 60.

Common Salt exists abundantly in nature, both in the form of solid
rock-salt and dissolved in the waters of the ocean.

Properties of the pure Salt.--Fusible without decomposition at low
redness, but sublimes at higher temperatures; the melted salt concretes
into a hard white mass on cooling. Nearly insoluble in absolute alcohol,
but dissolves in minute quantity in rectified spirit. Soluble in three
parts of water, both hot and cold. Crystallizes in cubes, which are
anhydrous.

_Impurities of Common Salt._--Table Salt often contains large quantities
of the Chlorides of Magnesium and Calcium, which, being deliquescent,
produce a dampness by absorption of atmospheric moisture: Sulphate of
Soda is also commonly present. The salt may be purified by repeated
re-crystallization, but it is more simple to prepare the pure compound
_directly_, by neutralizing Hydrochloric Acid with Carbonate of Soda.


CHLORIDE OF SILVER. _See_ Silver, Chloride of.


CITRIC ACID.

This acid is found abundantly in lemon-juice and in lime-juice. It occurs
in commerce in the form of large crystals, which are soluble in less than
their own weight of water at 60°.

Commercial Citric Acid is sometimes mixed with Tartaric Acid. The
adulteration may be discovered by making a concentrated solution of the
acid and adding _Acetate of Potash_; crystals of Bitartrate of Potash will
separate if Tartaric Acid be present.

Citric Acid is tribasic. It forms with Silver a white insoluble salt,
containing 3 atoms of Oxide of Silver to 1 atom of Citric Acid. When the
Citrate of Silver is heated in a current of Hydrogen gas, a part of the
acid is liberated and the salt is reduced to a Citrate of Suboxide of
Silver; which is of a red colour. The action of white light in reddening
Citrate of Silver is shown by the Author to be of a similar nature.


CYANIDE OF POTASSIUM.

Symbol, KC{2}N, or KCy. Atomic weight, 66.

This salt is a compound of Cyanogen gas with the metal Potassium.
Cyanogen is not an elementary body, like Chlorine or Iodine, but consists
of Carbon and Nitrogen united in a peculiar manner. Although a compound
substance, it reacts in the manner of an element, and is therefore
(like Ammonium, previously described) an exception to the usual laws of
chemistry. Many other bodies of a similar character are known.

Properties of Cyanide of Potassium.--These have been sufficiently
described at page 44, to which the reader is referred.


ETHER.

Symbol, C{4}H{5}O. Atomic weight, 37.

Ether is obtained by distilling a mixture of Sulphuric Acid and Alcohol.
If the formula of Alcohol (C{4}H{6}O{2}) be compared with that of Ether,
it will be seen to differ from it in the possession of an additional atom
of Hydrogen and of Oxygen: in the reaction the Sulphuric Acid removes
these elements in the form of water, and by so doing converts one atom
of Alcohol into an atom of Ether. The term Sulphuric applied to the
commercial Ether has reference only to the manner of its formation.

Properties of Ether.--The properties of Ether have been described to some
extent at pages 85 and 195. The following particulars however may be
added. It is neither acid nor alkaline to test-paper. Specific gravity, at
60°, about ·720. Boils at 98° Fahrenheit. The vapour is exceedingly dense,
and may be seen passing off from the liquid and falling to the ground:
hence the danger of pouring Ether from one bottle to another if a flame be
near at hand.

Ether does not mix with water in all proportions; if the two are shaken
together, after a short time the former rises and floats upon the surface.
In this way a mixture of Ether and Alcohol may be purified to some extent,
as in the common process of washing Ether. The water employed however
always retains a certain portion of Ether (about a tenth part of its
bulk), and acquires a strong ethereal odour; washed Ether also contains
water in small quantity.

Bromine and Iodine are both soluble in Ether, and gradually react upon and
decompose it.

The strong alkalies, such as Potash and Soda, also decompose Ether
slightly after a time, but not immediately. Exposed to air and light.
Ether is oxidized and acquires a peculiar odour (page 85).

Ether dissolves fatty and resinous substances readily, but inorganic salts
are mostly insoluble in this fluid. Hence it is that Iodide of Potassium
and other substances dissolved in Alcohol are precipitated to a certain
extent by the addition of Ether.


FLUORIDE OF POTASSIUM.

Symbol, KF. Atomic weight, 59.

_Preparation._--Fluoride of Potassium is formed by saturating Hydrofluoric
Acid with Potash, and evaporating to dryness in a platinum vessel.
Hydrofluoric Acid contains Fluorine combined with Hydrogen; it is a
powerfully acid and corrosive liquid, formed by decomposing Fluor Spar,
which is a Fluoride of Calcium, with strong Sulphuric Acid; the action
which takes place being precisely analogous to that involved in the
preparation of Hydrochloric Acid.

_Properties._--A deliquescent salt, occurring in small and imperfect
crystals. Very soluble in water: the solution acting upon glass in the
same manner as Hydrofluoric Acid.


FORMIC ACID.

Symbol, C{2}HO{3}. Atomic weight, 37.

This substance was originally discovered in the _red ant_ (_Formica
rufa_), but it is prepared on a large scale by distilling Starch with
Binoxide of Manganese and Sulphuric Acid.

_Properties._--The strength of commercial Formic Acid is uncertain, but
it is always more or less dilute. The strongest acid, as obtained by
distilling Formiate of Soda with Sulphuric Acid, is a fuming liquid with a
pungent odour, and containing only one atom of water. It inflames the skin
in the same manner as the sting of the ant.

Formic Acid reduces the Oxides of Gold, Silver, and Mercury to the
metallic state, and is itself oxidized into Carbonic Acid. The alkaline
formiates also possess the same properties.


GALLIC ACID.

Symbol, C{7}H{3}O{5} + H{3}O. Atomic weight, 94.

The chemistry of Gallic Acid is sufficiently described at page 27, to
which the reader is referred.


GELATINE.

Symbol, C{13}H{10}O{5}N{2}. Atomic weight, 156.

This is an organic substance somewhat analogous to Albumen, but differing
from it in properties. It is obtained by subjecting bones, hoofs, horns,
calves' feet, etc., to the action of boiling water. The jelly formed
on cooling is termed size, or, when dried and cut into slices, _glue_.
Gelatine, as it is sold in the shops, is a pure form of Glue. _Isinglass_
is gelatine prepared, chiefly in Russia, from the air-bladders of certain
species of sturgeon.

_Properties of Gelatine._--Gelatine softens and swells up in cold water,
but does not _dissolve_ until heated: the hot solution, on cooling, forms
a tremulous jelly. One ounce of cold water will retain about three grains
of Isinglass without gelatinizing; but much depends upon the temperature,
a few degrees greatly affecting the result.

When long boiled in water, and especially in presence of an acid, such
as the Sulphuric, Gelatine undergoes a peculiar modification, and the
Solution loses either partially or entirely its property of solidifying to
a jelly.


GLYCERINE.

Fatty bodies are resolved by treatment with an alkali into an Acid--which
combines with the alkali, forming a _soap_,--and Glycerine, remaining in
solution.

Pure Glycerine, as obtained by Price's patent process of distillation,
is a viscid liquid of sp. gr. about 1·23; miscible in all proportions
with water and Alcohol. It is peculiarly a neutral substance, exhibiting
no tendency to combine with acids or bases. It has little or no action
upon Nitrate of Silver in the dark, and reduces it very slowly even when
exposed to light.


GLYCYRRHIZINE.

Glycyrrhizine, obtained from the fresh root of Liquorice, is a substance
intermediate in properties between a sugar and a resin. Sparingly soluble
in water but very soluble in Alcohol. It precipitates strong solution of
Nitrate of Silver white, but the deposit becomes reddened by exposure
to light. Its preparation is described in the larger works on organic
chemistry.


GOLD, CHLORIDE OF.

Symbol, AuCl{3}. Atomic weight, 303.

This salt is formed by dissolving pure metallic Gold in Nitro-hydrochloric
Acid, and evaporating at a gentle heat. The solution affords deliquescent
crystals of a deep orange colour.

Chloride of Gold, in a state fit for Photographic use, may easily be
obtained by the following process:--Place a half-sovereign in any
convenient vessel, and pour on it half a drachm of Nitric Acid mixed with
two and a half drachms of Hydrochloric Acid and three drachms of water;
digest by a gentle heat, but do not _boil_ the acid, or much of the
Chlorine will be driven off in the form of gas. At the expiration of a few
hours add fresh Aqua-Regia in quantity the same as at first, which will
probably complete the solution, but if not, repeat the process a third
time.

Lastly, neutralize the liquid by adding Carbonate of Soda until all
effervescence ceases, and a green precipitate forms; this is _Carbonate
of Copper_, which must be allowed several hours to separate thoroughly.
The Chloride of Gold is thus freed from Copper and Silver, with which the
metallic Gold is alloyed in the standard coin of the realm. The solution
so prepared will be _alkaline_, and consequently prone to a reduction
of metallic Gold: a slight extra quantity of Hydrochloric acid should
therefore be added, sufficient to redden a piece of immersed litmus-paper.

The weight of a half-sovereign is about 61 grains, of which 56 grains are
pure Gold. This is equivalent to 86 grains of Chloride of Gold, which will
be the quantity contained in the solution.

The following process for preparing Chloride of Gold is more perfect than
the last:--Dissolve the Gold coin in Aqua-Regia as before; then boil with
excess of Hydrochloric Acid, to destroy the Nitric Acid,--dilute largely
with distilled water, and add a filtered aqueous solution of common
Sulphate of Iron (6 parts to 1 of Gold); collect the precipitated Gold,
which is now free from copper; redissolve in Aqua-Regia, and evaporate to
dryness on a water bath.

Avoid using _Ammonia_ to neutralize Chloride of Gold, as it would occasion
a deposit of "Fulminating Gold," the properties of which are described in
the next page.

_Properties of Chloride of Gold._--As sold in commerce it usually contains
excess of Hydrochloric Acid, and is then of a bright yellow colour; but
when neutral and somewhat concentrated, it is dark red (_Leo ruber_ of the
alchemists). It gives no precipitate with Carbonate of Soda unless heat
be applied; the free Hydrochloric Acid present forms, with the alkali.
Chloride of Sodium, which unites with the Chloride of Gold, and produces
a double salt, Chloride of Gold and Sodium, soluble in water.

Chloride of Gold is decomposed with precipitation of metallic Gold by
Charcoal, Sulphurous Acid, and many of the vegetable acids; also by
Protosulphate and Protonitrate of Iron. It tinges the cuticle of an
indelible purple tint. It is soluble in Alcohol and in Ether.


GOLD, FULMINATING.

This is a yellowish-brown substance, precipitated on adding Ammonia to a
strong solution of Chloride of Gold.

It may be dried carefully at 212°, but explodes violently on being heated
suddenly to about 290°. Friction also causes it to explode when dry; but
the moist powder may be rubbed or handled without danger. It is decomposed
by Sulphuretted Hydrogen.

Fulminating Gold is probably an Aurate of Ammonia, containing 2 atoms of
Ammonia to 1 atom of Peroxide of Gold.


GOLD, HYPOSULPHITE OF.

Symbol, AuO S{2}O{2}. Atomic weight, 253.

Hyposulphite of Gold is produced by the reaction of Chloride of Gold upon
Hyposulphite of Soda (see page 133).

The salt sold in commerce as Sel d'or is a double Hyposulphite of Gold and
Soda, containing one atom of the former salt to three of the latter, with
four atoms of water of crystallization. It is formed by adding one part of
Chloride of Gold, in solution, to three parts of Hyposulphite of Soda, and
precipitating the resulting salt by Alcohol: the Chloride of Gold must be
added to the Hyposulphite of Soda, and not the Soda salt to the Gold (see
page 250).

Properties.--Hyposulphite of Gold is unstable and cannot exist in an
isolated state, quickly passing into Sulphur, Sulphuric Acid, and metallic
Gold. When combined with excess of Hyposulphite of Soda in the form of Sel
d'or, it is more permanent.

Sel d'or occurs crystallized in fine needles, which are very soluble in
water. The commercial article is often impure, containing little else than
Hyposulphite of Soda, with a trace of Gold. It may be analyzed by adding
a few drops of strong Nitric Acid (free from Chlorine), diluting with
water, and afterwards collecting and igniting the yellow powder, which is
metallic Gold.


GRAPE SUGAR.

Symbol, C{24}H{28}O{28}. Atomic weight, 396.

This modification of Sugar, often termed _Granular Sugar_, or _Glucose_,
exists abundantly in the juice of grapes and in many other varieties of
fruit. It forms the saccharine concretion found in honey, raisins, dried
figs, etc. It may be produced artificially by the action of fermenting
principles and of dilute mineral acids, upon Starch.

_Properties._--Grape Sugar crystallizes slowly and with difficulty from
a concentrated aqueous solution, in small hemispherical nodules, which
are hard, and feel gritty between the teeth. It is much less sweet to the
taste than Cane Sugar, and not so soluble in water (1 part dissolves in
1-1/2 of cold water).

Grape Sugar tends to absorb Oxygen, and hence it possesses the property of
decomposing the salts of the noble metals, and reducing them by degrees
to the metallic state, even without the aid of light. _Cane_ Sugar does
not possess these properties to an equal extent, and hence it is readily
distinguished from the other variety. The product of the action of Grape
Sugar upon Nitrate of Silver appears to be a very low form of Oxide of
Silver combined with organic matter.


HONEY.

This substance contains two distinct kinds of Sugar, Grape Sugar, and an
uncrystallizable substance analogous to, or identical with, the Treacle
found associated with common Sugar in the cane-juice. The agreeable
taste of Honey probably depends upon the latter, but its reducing power
on metallic oxides is due to the former. Pure Grape Sugar can readily
be obtained from inspissated Honey, by treating it with Alcohol, which
dissolves out the syrup, but leaves the crystalline portion.

Much of the commercial article is adulterated, and, for Photographic use,
the Virgin Honey should be obtained direct from the comb.


HYDROCHLORIC ACID.

Symbol, HCl. Atomic weight, 37.

Hydrochloric Acid is a volatile gas, which may be liberated from most of
the salts termed Chlorides by the action of Sulphuric Acid. The acid, by
its superior affinities, removes the base; thus,--

  NaCl + HO SO{3} = NaO SO{3} + HCl.

Properties.--Abundantly soluble in water, forming the liquid Hydrochloric
or Muriatic Acid of commerce. The most concentrated solution of
Hydrochloric Acid has a sp. gr. 1·2, and contains about 40 per cent, of
gas; that commonly sold is somewhat weaker, sp. gr. 1·14 = 28 per cent,
real acid.

Pure Hydrochloric Acid is colourless, and fumes in the air. The yellow
colour of the commercial acid depends upon the presence of traces of
Perchloride of Iron, or of organic matter; commercial Muriatic Acid also
often contains a portion of free Chlorine and of Sulphuric Acid.


HYDRIODIC ACID.

Symbol, HI. Atomic weight, 127.

This is a gaseous compound of Hydrogen and Iodine, corresponding in
composition to the Hydrochloric Acid. It cannot however, from its
instability, be obtained in the same manner, since, on distilling
an Iodide with Sulphuric Acid, the Hydriodic Acid first formed is
subsequently decomposed into Iodine and Hydrogen. An aqueous solution of
Hydriodic Acid is easily prepared by adding Iodine to water containing
Sulphuretted Hydrogen gas; a decomposition takes place, and Sulphur is set
free: thus, HS + I = HI + s.

Properties.--Hydriodic Acid is very soluble in water, yielding a strongly
acid liquid. The solution, colourless at first, soon becomes brown from
decomposition, and liberation of free Iodine. It may be restored to its
original condition by adding solution of Sulphuretted Hydrogen.


HYDROSULPHURIC ACID.

Symbol, HS. Atomic weight, 17.

This substance, also known as Sulphuretted Hydrogen, is a gaseous compound
of Sulphur and Hydrogen, analogous in composition to the Hydrochloric and
Hydriodic Acid. It is usually prepared by the action of dilute Sulphuric
Acid upon Sulphuret of Iron, as described at page 373; the decomposition
being similar to that involved in the preparation of the Hydrogen acids
generally:--

  FeS + HO SO{3} = FeO SO{3} + HS.

Properties.--Cold water absorbs three times its bulk of Hydrosulphuric
Acid, and acquires the peculiar putrid odour and poisonous qualities
of the gas. The solution is faintly acid to test-paper, and becomes
opalescent on keeping, from gradual separation of Sulphur. It is
decomposed by Nitric Acid, and also by Chlorine and Iodine. It
precipitates Silver from its solutions in the form of black Sulphuret
of Silver; also Copper, Mercury, Lead, etc.; but Iron and other metals
of that class are not affected, if the liquid contains free acid.
Hydrosulphuric Acid is constantly employed in the chemical laboratory for
these and other purposes.


HYDROSULPHATE OF AMMONIA.

Symbol, NH{4}S HS. Atomic weight, 51.

The liquid known by this name, and formed on passing Sulphuretted Hydrogen
gas into Ammonia, is a double Sulphuret of Hydrogen and Ammonium. In the
preparation, the passage of the gas is to be continued until the solution
gives no precipitate with Sulphate of Magnesia, and smells strongly of
Hydrosulphuric Acid.

_Properties._--Colourless at first, but afterwards changes to yellow,
from liberation and subsequent solution of Sulphur. Becomes milky on the
addition of any acid. Precipitates, in the form of Sulphuret, all the
metals which are affected by Sulphuretted Hydrogen, and, in addition,
those of the class to which Iron, Zinc, and Manganese belong.

Hydrosulphate of Ammonia is employed in Photography to darken the Negative
image, and also in the preparation of Iodide of Ammonium, the separation
of Silver from Hyposulphite solutions, etc.


HYPOSULPHITE OF SODA.

Symbol, NaO S{2}O{2} + 5 HO. Atomic weight, 125.

The chemistry of Hyposulphurous Acid and the Hyposulphite of Soda has been
sufficiently described at pages 43, 129, and 137 of the present Work. The
crystallized salt includes five atoms of water of crystallization.


HYPOSULPHITE OF GOLD. _See_ Gold, Hyposulphite of.


HYPOSULPHITE OF SILVER. _See_ Silver, Hyposulphite of.


ICELAND MOSS.

_Cetraria Islandica._--A species of Lichen found in Iceland and the
mountainous parts of Europe; when boiled in water, it first swells up, and
then yields a substance which gelatinizes on cooling.

It contains Lichen Starch, a bitter principle soluble in Alcohol, termed
"Cetrarine," and common Starch; traces of Gallic Acid and Bitartrate of
Potash are also present.


IODINE.

Symbol, I. Atomic weight, 126.

Iodine is chiefly prepared at Glasgow, from _kelp_, which is the fused
ash obtained on burning seaweeds. The waters of the ocean contain minute
quantities of the Iodides of Sodium and Magnesium, which are separated and
stored up by the growing tissues of the marine plant.

In the preparation, the mother-liquor of kelp is evaporated to dryness
and distilled with Sulphuric Acid; the Hydriodic Acid first liberated is
decomposed by the high temperature, and fumes of Iodine condense in the
form of opaque crystals.

_Properties._--Iodine has a bluish-black colour and metallic lustre; it
stains the skin yellow, and has a pungent smell, like diluted Chlorine.
It is extremely volatile when moist, boils at 350°, and produces dense
violet-coloured fumes, which condense in brilliant plates. Specific
gravity 4·946. Iodine is very sparingly soluble in water, 1 part requiring
7000 parts for perfect solution; even this minute quantity however
tinges the liquid of a brown colour. Alcohol and Ether dissolve it more
abundantly, forming dark-brown solutions. Iodine also dissolves freely in
solutions of the alkaline Iodides, such as the Iodide of Potassium, of
Sodium, and of Ammonium.

_Chemical Properties._--Iodine belongs to the Chlorine group of elements,
characterized by forming acids with Hydrogen, and combining extensively
with the metals (see Chlorine). They are however comparatively indifferent
to Oxygen, and also to each other. The Iodides of the alkalies and
alkaline earths are soluble in water; also those of Iron, Zinc, Cadmium,
etc. The Iodides of Lead, Silver, and Mercury are nearly or quite
insoluble.

Iodine possesses the property of forming a compound of a deep blue colour
with Starch. In using this as a test, it is necessary first to liberate
the Iodine (if in combination) by means of Chlorine, or Nitric Acids
saturated with Peroxide of Nitrogen. The presence of Alcohol or Ether
interferes to a certain extent with the result.


IODIDE OF AMMONIUM.

Symbol, NH{4}I. Atomic weight, 144.

The preparation and properties of this salt are described at page 198, to
which the reader is referred.


IODIDE OF CADMIUM.

Symbol, CdI. Atomic weight, 182.

See page 199, for the preparation and properties of this salt.


IODIDE OF IRON.

Symbol, FeI. Atomic weight, 154.

Iodide of Iron is prepared by digesting an excess of Iron filings with
solution of Iodine in Alcohol. It is very soluble in water and Alcohol,
but the solution rapidly absorbs Oxygen and deposits Peroxide of Iron;
hence the importance of preserving it in contact with metallic Iron, with
which the separated Iodine may recombine. By very careful evaporation,
hydrated crystals of Proto-iodide may be obtained, but the composition of
the solid salt usually sold under that name cannot be depended on.

The _Periodide_ of Iron, corresponding to the _Perchloride_, has not been
examined, and it is doubtful if any such compound exists.


IODIDE OF POTASSIUM.

Symbol, KI. Atomic weight, 166.

This salt is usually formed by dissolving Iodine in solution of Potash
until it begins to acquire a brown colour; a mixture of Iodide of
Potassium and _Iodate of Potash_ (KO IO{5}) is thus formed; but by
evaporation and heating to redness, the latter salt parts with its Oxygen,
and is converted into Iodide of Potassium.

_Properties._--It forms cubic and prismatic crystals, which should be
hard, and _very slightly or not at all deliquescent_. Soluble in less than
an equal weight of water at 6O°; it is also soluble in Alcohol, but not
in Ether. The proportion of Iodide of Potassium contained in a saturated
alcoholic solution, varies with the strength of the spirit:--with common
Spirits of Wine, sp. gr. ·836, it would be about 8 grains to the drachm;
with Alcohol rectified from Carbonate of Potash, sp. gr. ·823, 4 or 5
grains; with absolute Alcohol, 1 to 2 grains. The solution of Iodide
of Potassium is instantly coloured brown by free Chlorine; also very
rapidly by Peroxide of Nitrogen (page 86); ordinary acids however act less
quickly, Hydriodic Acid being first formed, and subsequently decomposing
spontaneously.

The impurities of commercial Iodide of Potassium, with the means to be
adopted for their removal, are fully given at page 197.


IODIDE OF SILVER. _See_ Silver, Iodide of.


IODOFORM.

The composition of this substance is analogous to that of Chloroform,
Iodine being substituted for Chlorine. It is obtained on boiling together
Iodine, Carbonate of Potash, and Alcohol.

Iodoform occurs in yellow nacrous crystals, which have a saffron-like
odour. It is insoluble in water, but soluble in spirit.


IRON, PROTOSULPHATE OF.

Symbol, FeO SO{3} + 7 HO. Atomic weight, 139.

The properties of this salt, and of the two salifiable Oxides of Iron, are
described at page 29. It dissolves in rather more than an equal weight of
cold water, or in less of boiling water.

Aqueous solution of Sulphate of Iron absorbs the Binoxide of Nitrogen,
acquiring a deep olive-brown colour: as this gaseous Binoxide is itself a
reducing agent, the liquid so formed has been proposed as a more energetic
developer than the Sulphate of Iron alone (?).


IRON, PROTONITRATE OF.

Symbol, FeO NO{3} + 7 HO. Atomic weight, 153.

This salt, by careful evaporation _in vacuo_ over Sulphuric Acid, forms
transparent crystals, of a light green colour, and containing 7 atoms
of water, like the Protosulphate. It is exceedingly unstable, and soon
becomes red from decomposition, unless preserved from contact with air.
The preparation of solution of Protonitrate of Iron for developing
Collodion Positives, is given at page 206.


IRON, PERCHLORIDE OF.

Symbol, Fe{2}Cl{3}. Atomic weight, 164.

There are two Chlorides of Iron, corresponding in composition to the
Protoxide and the Sesquioxide respectively. The Protochloride is very
soluble in water, forming a green solution, which precipitates a dirty
white Protoxide on the addition of an alkali. The Perchloride, on the
other hand, is dark brown, and gives a foxy-red precipitate with alkalies.

_Properties._--Perchloride of Iron may be obtained in the solid form by
heating Iron wire in excess of Chlorine; it condenses in the shape of
brilliant and iridescent brown crystals, which are volatile, and dissolve
in water, the solution being acid to test-paper. It is also soluble in
Alcohol, forming the Tinctura Ferri Sesquichloridi of the Pharmacopœia.
Commercial Perchloride of Iron ordinarily contains an excess of
Hydrochloric Acid.


LITMUS.

Litmus is a vegetable substance prepared from various _lichens_, which are
principally collected on rocks adjoining the sea. The colouring matter is
extracted by a peculiar process, and afterwards made up into a paste with
chalk, plaster of Paris, etc.

Litmus occurs in commerce in the form of small cubes of a fine violet
colour. In using it for the preparation of test-papers, it is digested
in hot water, and sheets of porous paper are soaked in the blue liquid
so formed. The red papers are prepared at first in the same manner, but
afterwards placed in water which has been rendered faintly acid with
Sulphuric or Hydrochloric Acid.


MERCURY, BICHLORIDE OF.

Symbol, HgCl{2}. Atomic weight, 274.

This salt, also called Corrosive Sublimate, and sometimes _Chloride of
Mercury_ (the atomic weight of Mercury being halved), may be formed by
heating Mercury in excess of Chlorine, or more economically, by subliming
a mixture of Persulphate of Mercury and Chloride of Sodium.

_Properties._--A very corrosive and poisonous salt, usually sold in
semi-transparent, crystalline masses, or in the state of powder. Soluble
in 16 parts of cold, and in 3 of hot water; more abundantly so in Alcohol,
and also in Ether. The solubility in water may be increased by the
addition of free Hydrochloric Acid, or of Chloride of Ammonium.

The Protochloride of Mercury is an insoluble white powder, commonly known
under the name of _Calomel_.


METHYLIC ALCOHOL.

This liquid, known also by the names of _wood naphtha_ and _pyroxylic
spirit_, is one of the volatile products of the destructive distillation
of wood. It is very volatile and limpid, with a pungent odour.

By a recent excise regulation, ordinary Spirit mixed with ten per cent, of
wood naphtha is sold free of duty, under the name of "Methylated Spirit."


MILK.

The Milk of herbivorous animals contains three principal
constituents--Fatty matter, Caseine, and Sugar; in addition to these,
small quantities of the Chloride of Potassium, and of Phosphates of Lime
and Magnesia, are present.

The fatty matter is contained in small cells, and forms the greater part
of the cream which rises to the surface of the milk on standing; hence
shimmed milk is to be preferred for Photographic use.

The second constituent, Caseine, is an organic principle somewhat
analogous to Albumen in composition and properties. Its aqueous solution
however does not, like Albumen, _coagulate_ on boiling, unless _an acid_
be present, which probably removes a small portion of alkali with which
the Caseine was previously combined. The substance termed "rennet," which
is the dried stomach of the calf, possesses the property of coagulating
Caseine, but the exact mode of its action is unknown. Sherry-wine is also
commonly employed to curdle Milk; but brandy and other spirituous liquids,
when free from acid and astringent matter, have no effect.

In all these cases a portion of the Caseine usually remains in a soluble
form in the _whey_; but when the Milk is coagulated by the addition of
acids, the quantity so left is very small, and hence the use of the
rennet is to be preferred, since the presence of Caseine facilitates the
reduction of the sensitive Silver salts.

Caseine combines with Oxide of Silver in the same manner as Albumen,
forming a white coagulum, which becomes _brick-red_ on exposure to light.

Sugar of Milk, the third principal constituent, differs from both
cane and grape sugar; it may be obtained by evaporating _whey_ until
crystallization begins to take place. It is hard and gritty, and only
slightly sweet; slowly soluble, without forming a syrup, in about two and
a half parts of boiling, and six of cold water. It does not ferment and
form Alcohol on the addition of yeast, like grape sugar, but by the action
of _decomposing animal matter_ is converted into Lactic Acid.

When skimmed Milk is exposed to the air for some hours, it gradually
becomes _sour_, from Lactic Acid formed in this way; and if then heated to
ebullition, the Caseine coagulates very perfectly.


NITRIC ACID.

Symbol, NO{5}. Atomic weight, 54.

Nitric Acid, or _Aqua-fortis_, is prepared by adding Sulphuric Acid to
Nitrate of Potash, and distilling the mixture in a retort. Sulphate
of Potash and free Nitric Acid are formed, the latter of which, being
volatile, distils over in combination with one atom of water previously
united with the Sulphuric Acid.

_Properties._--Anhydrous Nitric Acid is a solid substance, white and
crystalline, but it cannot be prepared except by an expensive and
complicated process.

The concentrated _liquid_ Nitric Acid contains 1 atom of water, and has
a sp. gr. of about 1·5; if perfectly pure, it is colourless, but usually
it has a slight yellow tint, from partial decomposition into Peroxide of
Nitrogen: it fumes strongly in the air.

The strength of commercial Nitric Acid is subject to much variation. An
acid of sp. gr. 1·42, containing about 4 atoms of water, is commonly
met with. If the specific gravity is much lower than this (less than
1·36), it will scarcely be adapted for the preparation of Pyroxyline.
The yellow _Nitrous Acid_, so called, is a strong Nitric Acid partially
saturated with the brown vapours of Peroxide of Nitrogen; it has a high
specific gravity, but this is somewhat deceptive, being caused in part by
the presence of the Peroxide. On mixing with Sulphuric Acid, the colour
disappears, a compound being formed which has been termed a _Sulphate of
Nitrous Acid_.

In the Appendix a Table is given which exhibits the quantity of real
anhydrous Nitric Acid contained in samples of different densities.

_Chemical Properties._--Nitric Acid is a powerful oxidizing agent (see
page 13); it dissolves all the common metals, with the exception of Gold
and Platinum. Animal substances, such as the cuticle, nails, etc., are
tinged of a permanent yellow colour, and deeply corroded by a prolonged
application. Nitric Acid forms a numerous class of salts, _all of which
are soluble in water_. Hence its presence cannot be determined by any
precipitating reagent, in the same manner as that of Hydrochloric and
Sulphuric Acid.

_Impurities of Commercial Nitric Acid._--These are principally Chlorine
and Sulphuric Acid; also Peroxide of Nitrogen, which tinges the acid
yellow, as already described. Chlorine is detected by diluting the acid
with an equal bulk of distilled water, and adding a few drops of Nitrate
of Silver,--a _milkiness_, which, is Chloride of Silver in suspension,
indicates the presence of Chlorine. In testing for Sulphuric Acid, dilute
the Nitric Acid as before, and drop in _a single drop_ of solution of
Chloride of Barium; if Sulphuric Acid be present, an insoluble precipitate
of Sulphate of Baryta will be formed.


NITROUS ACID. _See_ Silver, Nitrite of.


NITRATE OF POTASH.

Symbol, KO NO{5}. Atomic weight, 102.

This salt, also termed _Nitre_, or _Saltpetre_, is an abundant natural
product, found effloresced upon the soil in certain parts of the East
Indies. It is also produced artificially in what are called Nitre-beds.

The properties of Nitrate of Potash are described as far as necessary at
page 190.


NITRATE OF BARYTA.

Symbol, BaO NO{5}. Atomic weight, 131.

Nitrate of Baryta forms octahedral crystals, which are anhydrous. It is
considerably less soluble than the Chloride of Barium, requiring 12 parts
of cold and 4 of boiling water for solution. It may be substituted for the
Nitrate of Lead in the preparation of Protonitrate of Iron.


NITRATE OF LEAD.

Symbol, PbO NO{5}. Atomic weight, 166.

Nitrate of Lead is obtained by dissolving the metal, or the Oxide of Lead,
in _excess_ of Nitric Acid, diluted with 2 parts of water. It crystallizes
on evaporation in white anhydrous tetrahedra and octahedra, which are
hard, and decrepitate on being heated; they are soluble in 8 parts of
water at 60°.

Nitrate of Lead forms with Sulphuric Acid, or soluble Sulphates, a white
precipitate, which is the insoluble Sulphate of Lead. The _Iodide_ of Lead
is also very sparingly soluble in water.


NITRATE OF SILVER, _See_ Silver, Nitrate or.


NITRO-GLUCOSE.

When 3 fluid ounces of cold Nitro-Sulphuric Acid, consisting of 2 ounces
of Oil of Vitriol and 1 ounce of highly concentrated Nitric Acid, are
mixed with 1 ounce of finely powdered Cane Sugar, there is formed at first
a thin, transparent, pasty mass. If it is stirred with a glass rod for a
few minutes without interruption, the paste coagulates as it were, and
separates from the liquid as a thick tenacious mass, aggregating into
lumps, which can easily be removed from the acid mixture.

This substance has a very acid and intensely bitter taste. Kneaded in
warm water until the latter no longer reddens litmus-paper, it acquires a
silver colour and a beautiful silky lustre. It may be used in Photography
to confer intensity upon newly mixed Collodion; but is inferior to
Glycyrrhizine employed for the same purpose.


NITRO-HYDROCHLORIC ACID.

Symbol, NO{4} + Cl.

This liquid is the Aqua-Regia of the old alchemists. It is produced by
mixing Nitric and Hydrochloric Acids: the Oxygen contained in the former
combines with the Hydrogen of the latter, forming water and liberating
Chlorine, thus:--

  NO{5} + HCl = NO{4} + HO + Cl.

The presence of free Chlorine confers on the mixture the power of
dissolving Gold and Platinum, which neither of the two acids possesses
separately. In preparing Aqua-Regia it is usual to mix one part, by
measure, of Nitric Acid with four of Hydrochloric Acid, and to dilute
with an equal bulk of water. The application of a gentle heat assists the
solution of the metal; but if the temperature rises to the boiling point,
a violent effervescence and escape of Chlorine takes place.


NITRO-SULPHURIC ACID.

For the chemistry of this acid liquid, see page 77.


OXYGEN.

Symbol, O. Atomic weight, 8.

Oxygen gas may be obtained by heating Nitrate of Potash to redness, but in
this case it is contaminated with a portion of Nitrogen. The salt termed
Chlorate of Potash (the composition of which is closely analogous to that
of the Nitrate, Chlorine being substituted for Nitrogen) yields abundance
of pure Oxygen gas on the application of heat, leaving behind Chloride of
Potassium.

_Chemical Properties._--Oxygen combines eagerly with many of the chemical
elements, forming Oxides. This chemical affinity however is not well
seen when the elementary body is exposed to the action of _Oxygen in the
gaseous form_. It is the _nascent_ Oxygen which acts most powerfully as
an oxidizer. By nascent Oxygen is meant Oxygen on the point of separation
from other elementary atoms with which it was previously associated; it
may then be considered to be in the liquid form, and hence it comes more
perfectly into contact with the particles of the body to be oxidized.

Illustrations of the superior chemical energy of nascent Oxygen are
numerous, but none perhaps are more striking than the mild and gradual
oxidizing influence exerted by atmospheric air, as compared with the
violent action of Nitric Acid and bodies of that class which contain
Oxygen loosely combined.


OXYMEL.

This syrup of Honey and Vinegar is prepared as follows. Take of

  Honey                                     1 pound.
  Acid, Acetic, fortiss. (Beaufoy's Acid)  11 drachms.
  Water                                    13 drachms.

Stand the pot containing the Honey in boiling water until a scum rises
to the surface, which is to be removed two or three times. Then add the
Acetic Acid and water, and skim once more if required. Allow to cool, and
it will be fit for use.


POTASH.

Symbol, KO + HO. Atomic weight, 57.

Potash is obtained by separating the Carbonic Acid from Carbonate of
Potash by means of Caustic Lime. Lime is a more feeble base than Potash,
but the Carbonate of Lime, being _insoluble_ in water, is at once formed
on adding Milk of Lime to a solution of Carbonate of Potash (see page 314).

_Properties._--Usually met with in the form of solid lumps, or in
cylindrical sticks, which are formed by melting the Potash and running it
into a mould. It always contains one atom of water, which cannot be driven
off by the application of heat.

Potash is soluble almost to any extent in water, much heat being evolved.
The solution is powerfully alkaline (p. 308), and acts rapidly upon the
skin; it dissolves fatty and resinous bodies, converting them into soaps.
Solution of Potash absorbs Carbonic Acid quickly from the air, and should
therefore be preserved in stoppered bottles; the glass stoppers must be
wiped occasionally, in order to prevent them from becoming immovably
fixed by the solvent action of the Potash upon the Silica of the glass.

The Liquor Potassæ of the London Pharmacopœia has a sp. gr. of 1·063, and
contains about 5 per cent, of real Potash. It is usually contaminated
with _Carbonate_ of Potash, which causes it to effervesce on the addition
of acids; also, to a less extent, with Sulphate of Potash, Chloride of
Potassium, Silica, etc.


POTASH, CARBONATE OF.

Symbol, KO CO{2}. Atomic weight, 70.

The impure Carbonate of Potash, termed _Pearlash_, is obtained from the
ashes of wood and vegetable matter, in the same manner as Carbonate of
Soda is prepared from the ashes of seaweeds. Salts of Potash and of Soda
appear essential to vegetation, and are absorbed and approximated by
the living tissues of the plant. They exist in the vegetable structure,
combined with organic acids in the form of salts, like the Oxalate,
Tartrate, etc., which, when burned are converted into Carbonates.

_Properties._--The Pearlash of commerce contains large and variable
quantities of Chloride of Potassium, Sulphate of Potash, etc. A purer
Carbonate is sold, which is free from Sulphates, and with only a trace of
Chlorides. Carbonate of Potash is a strongly alkaline salt, deliquescent,
and soluble in twice its weight of cold water; insoluble in Alcohol, and
employed to deprive it of water (see page 196).


PYROGALLIC ACID.

Symbol, C{8}H{4}O{4} (Stenhouse). Atomic weight, 84.

The chemistry of Pyrogallic Acid has been described at page 28.


SEL D'OR. _See_ Gold, Hyposulphite of.


SILVER.

Symbol, Ag. Atomic weight, 108.

This metal, the _Luna_ or _Diana_ of the alchemists, is found native in
Peru and Mexico; it occurs also in the form of Sulphuret of Silver.

When pure it has a sp. gr. of 10·5, and is very malleable and ductile;
melts at a bright red heat. Silver does not oxidize in the air, but
when exposed to an impure atmosphere containing traces of Sulphuretted
Hydrogen, it is slowly tarnished from formation of Sulphuret of Silver. It
dissolves in Sulphuric Acid, but the best solvent is Nitric Acid.

The standard coin of the realm is an alloy of Silver and Copper,
containing about one-eleventh of the latter metal.

To prepare pure Nitrate of Silver from it, dissolve in Nitric Acid and
evaporate until crystals are obtained. Then wash the crystals with a
little dilute Nitric Acid, redissolve them in water, and crystallize by
evaporation a second time. Lastly, fuse the product at a moderate heat, in
order to expel the last traces of Nitric and Nitrous Acids.


SILVER, AMMONIO-NITRATE OF.

Crystallized Nitrate of Silver absorbs Ammoniacal gas rapidly, with
production of heat sufficient to fuse the resulting compound, which
is white, and consists of 100 parts of the Nitrate + 29·5 of Ammonia.
The compound however which Photographers employ under the name of
Ammonio-Nitrate of Silver may be viewed more simply as a solution of the
Oxide of Silver in Ammonia, without reference to the Nitrate of Ammonia
necessarily produced in the reaction.

Very strong Ammonia, in acting upon Oxide of Silver, converts it
into a black powder, termed _Fulminating Silver_, which possesses the
most dangerous explosive properties. Its composition is uncertain. In
preparing Ammonio-Nitrate of Silver by the common process, the Oxide
first precipitated occasionally leaves a little black powder behind, on
re-solution; this does not appear however, according to the observations
of the Author, to be Fulminating Silver.

In sensitizing salted paper by the Ammonio-Nitrate of Silver, _free
Ammonia_ is necessarily formed. Thus--

    Chloride of Ammonium + Oxide of Silver in Ammonia
  = Chloride of Silver   + Ammonia + Water.


SILVER, OXIDE OF.

Symbol, AgO. Atomic weight, 116.

This compound has already been described in Part I., page 17.


SILVER, CHLORIDE OF.

Symbol, AgCl. Atomic weight, 144.

The preparation and properties of Chloride of Silver are given in Part I.
page 14.


SILVER, BROMIDE OF.

Symbol, AgBr. Atomic weight, 186.

See Part I. page 17.


SILVER, CITRATE OF. _See_ Citric Acid.


SILVER, IODIDE OF.

Symbol, AgI. Atomic weight, 234.

See Part I. page 16.


SILVER, FLUORIDE OF.

Symbol, AgF. Atomic weight, 127.

This compound differs from those last described in being soluble in
water. The dry salt fuses on being heated, and is reduced by a higher
temperature, or by exposure to light.


SILVER, SULPHURET OF.

Symbol, AgS. Atomic weight, 124.

This compound is formed by the action of Sulphur upon metallic Silver,
or of Sulphuretted Hydrogen or Hydrosulphate of Ammonia upon the Silver
salts; the decomposition of Hyposulphite of Silver also furnishes the
black Sulphuret.

Sulphuret of Silver is insoluble in water, and nearly so in those
substances which dissolve the Chloride, Bromide, and Iodide, such as
Ammonia, Hyposulphites, Cyanides, etc.; but it dissolves in Nitric Acid,
being converted into soluble Sulphate and Nitrate of Silver. (For a
further account of the properties of the Sulphuret of Silver, see page
146.)


SILVER, NITRATE OF.

Symbol, AgO NO{5}. Atomic weight, 170.

The preparation and properties of this salt have been explained at pages
12 and 362.


SILVER, NITRITE OF.

Symbol, AgO NO{3}. Atomic weight, 154.

Nitrite of Silver is a compound of Nitrous Acid, or NO{3}, with Oxide of
Silver. It is formed by heating Nitrate of Silver, so as to drive off a
portion of its Oxygen, or more conveniently, by mixing Nitrate of Silver
and Nitrite of Potash in equal parts, fusing strongly, and dissolving in a
small quantity of boiling water: on cooling, the Nitrite crystallizes out,
and may be purified by pressing in blotting-paper. Mr. Hadow describes
an economical method of preparing Nitrite of Silver in quantity, viz. by
heating 1 part of Starch in 8 of Nitric Acid of 1·25 specific gravity,
and conducting the evolved gases into a solution of pure Carbonate of
Soda until effervescence has ceased. The Nitrite of Soda thus formed is
afterwards added to Nitrate of Silver in the usual way.

_Properties._--Nitrite of Silver is soluble in 120 parts of cold water;
easily soluble in boiling water, and crystallizes, on cooling, in long
slender needles. It has a certain degree of affinity for Oxygen, and tends
to pass into the condition of Nitrate of Silver; but it is probable that
its Photographic properties depend more upon a decomposition of the salt
and liberation of Nitrous Acid.

_Properties of Nitrous Acid._--This substance possesses very feeble
acid properties, its salts being decomposed even by Acetic Acid. It is
an unstable body, and splits up, in contact with water, into Binoxide
of Nitrogen and Nitric Acid. The Peroxide of Nitrogen, NO{4}, is also
decomposed by water, and yields the same products.


SILVER, ACETATE OF.

Symbol, AgO (C{4}H{3}O{3}). Atomic weight, 167.

This is a difficultly soluble salt, deposited in lamellar crystals when
an Acetate is added to a strong solution of Nitrate of Silver. If _Acetic
Acid_ be used in place of an Acetate, the Acetate of Silver does not fall
so readily, since the Nitric Acid which would then be liberated impedes
the decomposition. Its properties have been sufficiently described at page
89.


SILVER, HYPOSULPHITE OF.

Symbol, AgO S{2}O{2}. Atomic weight, 164.

This salt is fully described in Part I. page 129. For the properties of
the soluble double salt of Hyposulphite of Silver and Hyposulphite of
Soda, see page 43.


SUGAR OF MILK. _See_ Milk.


SULPHURETTED HYDROGEN. _See_ Hydrosulphuric Acid.


SULPHURIC ACID.

Symbol, SO{3}. Atomic weight, 40.

Sulphuric Acid may be formed by oxidizing Sulphur with boiling Nitric
Acid; but this plan would be too expensive to be adopted on a large
scale. The commercial process for the manufacture of Sulphuric Acid is
exceedingly ingenious and beautiful, but it involves reactions which are
too complicated to admit of a superficial explanation. The Sulphur is
first burnt into gaseous Sulphurous Acid (SO{2}), and then by the agency
of Binoxide of Nitrogen gas, an additional atom of Oxygen is imparted from
the atmosphere, so as to convert the SO{2} into SO{3}, or Sulphuric Acid.

_Properties._--Anhydrous Sulphuric Acid is a white crystalline solid. The
strongest liquid acid always contains one atom of water, which is closely
associated with it, and cannot be driven off by the application of heat.

This _mono-hydrated_ Sulphuric Acid, represented by the formula HO SO{3},
is a dense fluid, having a specific gravity of about 1·845; boils at
620°, and distils without decomposition. It is not volatile at common
temperatures, and therefore does not _fume_ in the same manner as Nitric
or Hydrochloric Acid. The concentrated acid maybe cooled down even to
zero without solidifying; but a weaker compound, containing twice the
quantity of water, and termed _glacial_ Sulphuric Acid, crystallizes at
40° Fahr. Sulphuric Acid is intensely acid and caustic, but it does not
destroy the skin or dissolve metals so readily as Nitric Acid. It has an
energetic attraction for water, and when the two are mixed, condensation
ensues, and much heat is evolved; four parts of acid and one of water
produce a temperature equal to that of boiling water. Mixed with aqueous
Nitric Acid, it forms the compound know a as Nitro-Sulphuric Acid.

Sulphuric Acid possesses intense chemical powers, and displaces the
greater number of ordinary acids from their salts. It _chars_ organic
substances, by removing the elements of water, and converts Alcohol into
Ether in a similar manner. The _strength_ of a given sample of Sulphuric
Acid may be calculated, nearly, from its specific gravity, and a Table is
given by Dr. Ure for that purpose. (See Appendix.)

_Impurities of Commercial Sulphuric Acid._--The liquid acid sold as Oil
of Vitriol is tolerably constant in composition, and seems to be as well
adapted for Photographic use as the _pure_ Sulphuric Acid, which is far
more expensive. The specific gravity should be about 1·836 at 60°. If
a drop, evaporated upon Platinum-foil, gives a fixed residue, probably
Bisulphate of Potash is present. A milkiness, on dilution, indicates
Sulphate of Lead (see page 186).

_Test for Sulphuric Acid._--If the presence of Sulphuric Acid, or a
soluble Sulphate, be suspected in any liquid, it is tested for by adding a
few drops of dilute solution of Chloride of Barium, or Nitrate of Baryta.
A white precipitate, _insoluble in Nitric Acid_, indicates Sulphuric Acid.
If the liquid to be tested is very acid, from Nitric or Hydrochloric Acid,
it must be largely diluted before testing, or a crystalline precipitate
will form, caused by the sparing solubility of the Chloride of Barium
itself in acid solutions.


SULPHUROUS ACID.

Symbol, SO{2}. Atomic weight, 32.

This is a gaseous compound, formed by burning Sulphur in atmospheric air
or Oxygen gas: also by heating Oil of Vitriol in contact with metallic
Copper, or with Charcoal.

When an acid of any kind is added to Hyposulphite of Soda, Sulphurous Acid
is formed as a product of the decomposition of Hyposulphurous Acid, but it
afterwards disappears from the liquid by a secondary reaction, resulting
in the production of Trithionate and Tetrathionate of Soda.

_Properties._--Sulphurous Acid possesses a peculiar and suffocating odour,
familiar to all in the fumes of burning Sulphur. It is a feeble acid, and
escapes with effervescence, like Carbonic Acid, when its salts are treated
with Oil of Vitriol. It is soluble in water.


TETRATHIONIC ACID.

Symbol, S{4}O{5}. Atomic weight, 104.

The chemistry of the Polythionic Acids and their salts will be found
described in the First Part of this Work, page 157.


WATER.

Symbol, HO. Atomic weight, 9.

Water is an Oxide of Hydrogen, containing single atoms of each of the
gases.

_Distilled water_ is water which has been vaporized and again condensed;
by this means it is freed from earthy and saline impurities, which, not
being volatile, are left in the body of the retort. _Pure_ distilled water
leaves no residue on evaporation, and should remain perfectly clear on the
addition of Nitrate of Silver, _even when exposed to the light_; it should
also be neutral to test-paper.

The condensed water of steam-boilers sold as distilled water is apt to be
contaminated with oily and empyreumatic matter, which discolours Nitrate
of Silver, and is therefore injurious.

_Rain-water_, having undergone a natural process of distillation, is
free from inorganic salts, but it usually contains a minute portion of
_Ammonia_, which gives it an alkaline reaction to test-paper. It is very
good for Photographic purposes if collected in clean vessels, but when
taken from a common rain-water tank should always be examined, and if much
organic matter be present, tingeing it of a brown colour and imparting an
unpleasant smell, it must be rejected.

_Spring_ or _River_ water, commonly known as "hard water," usually
contains Sulphate of Lime, and Carbonate of Lime dissolved in Carbonic
Acid; also Chloride of Sodium in greater or less quantity. On boiling
the water, the Carbonic Acid gas is evolved, and the greater part of
the Carbonate of Lime (if any is present) deposits, forming an earthy
incrustation on the boiler.

In testing water for Sulphates and Chlorides, acidify a portion with a few
drops of _pure_ Nitric Acid, free from Chlorine (if this is not at hand,
use pure Acetic Acid); then divide it into two parts, and add to the first
a _dilute_ solution of Chloride of Barium, and to the second, Nitrate of
Silver,--a milkiness indicates the presence of Sulphates in the first case
or of Chlorides in the second. The _Photographic Nitrate Bath_ cannot be
used as a test, since the Iodide of Silver it contains is precipitated
on dilution, giving a milkiness which might be mistaken for Chloride of
Silver.

Common hard water can often be used for making a Nitrate Bath when nothing
better is at hand. The Chlorides it contains are precipitated by the
Nitrate of Silver, leaving soluble _Nitrates_ in solution, which are not
injurious. The Carbonate of Lime, if any is present, neutralizes free
Nitric Acid, rendering the Bath alkaline in the same manner as Carbonate
of Soda. (See page 89.) Sulphate of Lime, usually present in well water,
is said to exercise a retarding action upon the sensitive Silver Salts,
but on this point the writer is unable to give certain information.

Hard water is not often sufficiently pure for the developing fluids. The
Chloride of Sodium it contains decomposes the Nitrate of Silver upon
the film, and the image cannot be brought out perfectly. The _New River
water_, however, supplied to many parts of London, is almost free from
Chlorides, and answers very well. In other cases a few drops of Nitrate of
Silver solution may be added, to separate the Chlorine, taking care not to
use a large excess.




APPENDIX.


QUANTITATIVE TESTING OF SOLUTIONS OF NITRATE OF SILVER.

The amount of Nitrate of Silver contained in solutions of that salt may be
estimated with sufficient delicacy for ordinary Photographic operations by
the following simple process.

Take the _pure_ crystallized Chloride of Sodium, and either dry it
strongly or fuse it at a moderate heat, in order to drive off any water
which may be retained between the interstices of the crystals; then
dissolve in distilled water, in the proportion of 8-1/2 grains to 6 fluid
ounces.

In this way, a standard solution of salt is formed, each drachm of
which (containing slightly more than one-sixth of a grain of salt) will
precipitate exactly half a grain of Nitrate of Silver.

In order to use it, measure out accurately one drachm of the Bath in a
minim measure and place it in a two-ounce stoppered phial, taking care to
rinse out the measure with a drachm of distilled water, which is to be
added to the former; then pour in the salt solution, in the proportion
of a drachm for every 4 grains of Nitrate _known to be present_ in an
ounce of the Bath which is to be tested; shake the contents of the
bottle briskly, until the white curds have perfectly separated, and the
supernatant liquid is clear and colourless; then add fresh portions of the
standard solution, by 30 minims at a time, with constant shaking. When the
last addition causes no _milkiness_, read off the total number of drachms
employed (the last half-drachm being subtracted), and multiply that
number by 4 for the weight in grains of the Nitrate of Silver present in
an ounce of the Bath.

In this manner the strength of the Bath is indicated within two grains to
the ounce, or even to a single grain if the last additions of standard
salt-solution be made in portions of 15, instead of 30 minims.

Supposing the Bath to be tested is thought to contain about 35 grains of
Nitrate to the ounce, it will be convenient to begin by adding to the
measured drachm, 7 _drachms_ of the standard solution; afterwards, as
the milkiness and precipitation become less marked, the process must be
carried on more cautiously, and the bottle shaken violently for several
minutes, in order to obtain a clear solution. A few drops of Nitric Acid
added to the Nitrate of Silver facilitate the deposition of the Chloride;
but care must be taken that the sample of Nitric Acid employed is pure and
free from Chlorine, the presence of which would cause an error.


RECOVERY OF SILVER FROM WASTE SOLUTIONS,--FROM THE BLACK DEPOSIT OF
HYPO-BATHS, ETC.

The manner of separating metallic Silver from waste solutions varies
according to the presence or absence of alkaline Hyposulphites and
Cyanides.

a. _Separation of metallic Silver from old Nitrate Baths._--The Silver
contained in solutions of the Nitrate, Acetate, etc. may easily be
precipitated by suspending a strip of sheet Copper in the liquid; the
action is completed in two or three days, the whole of the Nitric Acid and
Oxygen passing to the Copper, and forming a blue solution of the Nitrate
of Copper. The metallic Silver however, separated in this manner, always
contains a portion of Copper, and gives a blue solution when dissolved in
Nitric Acid.

A better process is to commence by precipitating the Silver entirely in
the form of _Chloride of Silver_, by adding common Salt until no further
milkiness can be produced. If the liquid is well stirred, the Chloride of
Silver sinks to the bottom, and may be washed by repeatedly filling the
vessel with common water, and pouring off the upper clear portion when
the clots have again settled down. The Chloride of Silver thus formed may
afterwards be reduced to metallic Silver by a process which will presently
be described (p. 374).

b. _Separation of Silver from solutions containing alkaline Hyposulphites,
Cyanides, or Iodides._--In this case the Silver cannot be precipitated by
adding Chloride of Sodium, since the Chloride of Silver is soluble in such
liquids. It is necessary therefore to use the Sulphuretted Hydrogen, or
the Hydrosulphate of Ammonia, and to separate the Silver in the form of
Sulphuret.

Sulphuretted Hydrogen gas is readily prepared, by fitting a cork and
flexible tubing to the neck of a pint bottle, and having introduced
Sulphuret of Iron (sold by operative chemists for the purpose), about as
much as will stand in the palm of the hand, pouring upon it 1-1/2 fluid
ounce of Oil of Vitriol diluted with 10 ounces of water. The gas is
generated gradually without the application of heat, and must be allowed
to bubble up through the liquid from which the Silver is to be separated.
The smell of Sulphuretted Hydrogen being offensive, and highly poisonous
if inhaled in a concentrated form, the operation must be carried on in the
open air, or in a place where the fumes may escape without doing injury.

When the liquid begins to acquire a strong and persistent odour of
Sulphuretted Hydrogen, the precipitation of Sulphuret is completed. The
black mass must then be collected upon a filter, and washed by pouring
water over it, until the liquid which runs through gives little or no
precipitate with a drop of Nitrate of Silver.

The Silver may also be separated in the form of Sulphuret from old
Hypo-Baths, by adding Oil of Vitriol in quantity sufficient to decompose
the Hyposulphite of Soda; and burning off the free Sulphur from the brown
deposit.

Conversion of Sulphuret of Silver into metallic Silver.--The black
Sulphuret of Silver may be reduced to the state of metal by roasting and
subsequent fusion with Carbonate of Soda; but it is more convenient, in
operating on a small scale, to proceed in the following manner:--first
convert the Sulphuret into Nitrate of Silver, by boiling with Nitric Acid
diluted with two parts of water; when all evolution of red fumes has
ceased, the liquid may be diluted, allowed to cool, and filtered from
the insoluble portion, which consists principally of Sulphur, but also
contains a mixture of Chloride and Sulphuret of Silver, unless the Nitric
Acid employed was free from Chlorine; this precipitate may be heated, in
order to volatilize the Sulphur, and then digested with Hyposulphite of
Soda, or added to the Hypo-Bath.

The solution of Nitrate of Silver obtained by dissolving Sulphuret of
Silver, is always strongly acid with Nitric Acid, and also contains
_Sulphate_ of Silver. It may be crystallized by evaporation; but unless
the quantity of material operated on is large, it will be better to
precipitate the Silver in the form of Chloride, by adding common Salt, as
already recommended.


REDUCTION OF CHLORIDE OF SILVER TO THE METALLIC STATE.

The Chloride of Silver is first to be carefully washed, by filling up
the vessel which contains it, many times with water, and pouring off the
liquid, or drawing it off close with a siphon. It may then be dried at a
gentle heat, and fused with twice its weight of dry Carbonate of Potash,
or better still, with a mixture of the Carbonates of Potash and Soda.

The process for reducing Chloride of Silver in the moist way, by metallic
Zinc and Sulphuric Acid, is more economical and less troublesome than
that just given; it is conducted as follows:--The Chloride, after having
been well washed as before, is placed in a large flat dish, and a bar
of metallic Zinc laid in contact with it. A small quantity of Oil of
Vitriol, diluted with four parts of water, is then added, until a slight
effervescence of Hydrogen gas is seen to take place. The vessel is set
aside for two or three days, and is not to be disturbed, either by
stirring or by moving the bar. The reduction begins with the Chloride
immediately in contact with the Zinc, and radiates in all directions. When
the whole mass has become of a grey colour, the bar is to be carefully
removed and the adhering Silver washed off with a stream of water; the
Zinc usually presents a honeycombed appearance, with irregularities upon
the surface, which however are not metallic Silver;--they consist only of
Zinc or of Oxide of Zinc.

In order to ensure the purity of the Silver, a fresh addition of Sulphuric
Acid must be made, after the Zinc bar has been removed, and the digestion
continued for several hours, in order to dissolve any fragments of
metallic Zinc which may have been inadvertently detached. The grey powder
must be repeatedly washed, first with Sulphuric Acid and water (this is
necessary to dissolve a portion of an insoluble Salt of Zinc, probably
an oxychloride) and then with water alone, until the liquid runs away
_neutral_, and gives no precipitate with Carbonate of Soda; it may
then be fused into a button, to burn off organic matter if present, and
subsequently converted into Nitrate of Silver by boiling with Nitric Acid
diluted with two parts of water.

In reducing Chloride of Silver precipitated from old Nitrate Baths
_containing Iodide of Silver_, the grey metallic powder is sometimes
contaminated with unreduced Iodide of Silver, which dissolves in
the solution of Nitrate of Silver formed on treating the mass with
Nitric Acid. To avoid this, wash the purified Silver with solution of
Hyposulphite of Soda, and then again with water.


MODE OF TAKING THE SPECIFIC GRAVITY OF LIQUIDS.

Instruments are sold, termed "Hydrometers," which indicate specific
gravity by the extent to which a glass bulb containing air, and properly
balanced, rises or sinks, in the liquid; but a more exact process, and one
equally simple, is by the use of the specific gravity bottle.

These bottles are made to contain exactly 1000 grains of distilled water,
and with each is sold _a brass weight_, which counterbalances it when
filled with pure water.

In taking the specific gravity of a liquid, fill the bottle quite full
and insert the stopper, which being pierced through by a fine capillary
tube allows the excess to escape. Then, having wiped the bottle quite dry,
place it in the scale-pan, and ascertain the number of grains required to
produce equilibrium; this number added to, or subtracted from, _unity_
(the assumed specific gravity of water), will give the density of the
liquid.

Thus, to take examples, supposing the bottle filled with _rectified
Ether_ to require 250 grains to enable it to counterbalance the brass
weight,--then 1· _minus_ ·250, or ·750, is the specific gravity; but in
the case of _Oil of Vitriol_ the bottle, when full, will be heavier than
the counterpoise by perhaps 836 grains; therefore 1· _plus_ ·836, _id est_
1·836, is the density of the sample examined.

Sometimes the bottle is made to hold only 500 grains of distilled water,
in place of 1000; in this case the number of grains to be added or
subtracted must be multiplied by 2.

In taking specific gravities, observe that the temperature be within a few
degrees of 60° Fahrenheit (if higher or lower, immerse the bottle in warm
or cold water); and wash out the bottle thoroughly with water each time
after use.


ON FILTRATION AND WASHING PRECIPITATES.

In preparing filters, cut the paper into squares of a sufficient size,
and fold each square neatly upon itself, first into a half-square, and
then again, at right angles, into a quarter-square;--round off the corners
with a pair of scissors, and open out the filter into a conical form, when
it will be found to drop exactly into the funnel, and to be uniformly
supported throughout.

Before pouring in the liquid, always moisten the filter with distilled
water, in order to expand the fibres; if this precaution be neglected,
the pores are apt to become choked in filtering liquids which contain
finely divided matter in suspension. The solution to be filtered may be
poured gently down a glass rod, held in the left hand (_a silver spoon_
may be used, in case of necessity, for Nitrate Baths, and all liquids not
containing Nitric or Hydrochloric Acid), and directed against the side of
the funnel, near to the upper part. If it does not immediately run clear,
it will usually do so on returning it into the filter and allowing it to
pass through a second time.

_Mode of Washing Precipitates._--Collect the precipitate upon a filter and
drain off as much of the mother-liquor as possible; then pour in distilled
water by small portions at a time, allowing each to percolate through the
deposit before adding a fresh quantity. When the water passes through
perfectly pure, the washing is complete; in testing it, a single drop
may be laid upon a strip of glass and allowed to evaporate spontaneously
in a warm place, or the proper chemical reagents may be applied, and the
washing continued until no impurity can be detected. Thus, for example,
in washing the Sulphuret of Silver precipitated from a Hypo-Bath by means
of Hydrosulphate of Ammonia, the process will be completed when the water
which runs through causes no deposit with a drop of Nitrate of Silver
solution.


ON THE USE OF TEST-PAPERS.

The nature of the colouring matter which is employed in the preparation of
litmus-paper has already been described at page 353.

In testing for the alkalies and basic oxides generally, the blue
litmus-paper which has been reddened by an acid may be used, or, in place
of it, the _turmeric_-paper. Turmeric is a yellow vegetable substance
which possesses the property of becoming brown when treated with an
alkali; it is however less sensitive than the reddened litmus, and is
scarcely affected by the weaker bases, such as Oxide of Silver.

In using test-papers, observe the following precautions:--they should be
kept in a dark place, and protected from the action of the air, or they
soon become purple from Carbonic Acid, always present in the atmosphere
in small quantity. By immersion in water containing about one drop of
Liquor Potassæ or Ammoniæ, or a grain of Carbonate of Soda to four
ounces, the blue colour is restored. As the quantities which are tested
for in Photography are often infinitesimally small, it is essential that
the litmus-paper should be in good condition; and test-papers prepared
with porous paper will be found to show the colour better than those
upon glazed or strongly-sized paper. The mode of employing the paper is
as follows:--Place a small strip in the liquid to be examined: if it
becomes at once _bright red_, a strong acid is present; but if it changes
_slowly to a wine-red_ tint, a weak acid, such as Acetic or Carbonic, is
indicated. In the case of the Photographic Nitrate Bath faintly acidified
with Acetic Acid, a purple colour only may be expected, and a decided red
colour would suggest the presence of Nitric Acid. In the Hypo fixing and
toning Bath which has acquired acidity, the litmus-paper will perhaps
redden in about three or four minutes.

Blue litmus-papers may be changed to the red papers used for alkalies by
soaking in water acidified with Sulphuric Acid, one drop to half a pint;
or by holding for an instant near the mouth of a bottle containing Glacial
Acetic Acid. In examining a Nitrate Bath for alkalinity by means of the
reddened litmus-paper, at least five or ten minutes should be allowed for
the action, since the change of colour from red to blue takes place very
slowly.


REMOVAL OF SILVER STAINS FROM THE HANDS, LINEN, ETC.

The black stains upon the hands caused by Nitrate of Silver, may readily
be removed by moistening them and rubbing with a lump of Cyanide of
Potassium. As this salt however is highly poisonous, many may prefer the
following plan:--Wet the spot with a saturated solution of Iodide of
Potassium, and afterwards with Nitric Acid (the strong Nitric Acid acts
upon the skin and turns it yellow, it must therefore be diluted with two
parts of water before use); then wash with solution of Hyposulphite of
Soda.

Stains upon white linen may be easily removed by brushing them with a
solution of Iodine in Iodide of Potassium, and afterwards washing with
water and soaking in Hyposulphite of Soda, or Cyanide of Potassium, until
the yellow Iodide of Silver is dissolved out; the Bichloride of Mercury
(neutral solution) also answers well in many cases, changing the dark spot
to white (p. 151).


A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID IN DILUTE SULPHURIC ACID OF
DIFFERENT SPECIFIC GRAVITIES. (URE.)

  +---------+------------++---------+------------++---------+------------+
  |         |Real Acid   ||         |Real Acid   ||         |Real Acid   |
  |Specific |  in 100    ||Specific |  in 100    ||Specific |  in 100    |
  |Gravity. |parts of the||Gravity. |parts of the||Gravity. |parts of the|
  |         |  Liquid.   ||         |  Liquid.   ||         |  Liquid.   |
  +---------+------------++---------+------------++---------+------------+
  |  1·8485 |   81·54    ||  1·8115 |   73·39    ||  1·7120 |   65·23    |
  |  1·8475 |   80·72    ||  1·8043 |   72·57    ||  1·6993 |   64·42    |
  |  1·8460 |   79·90    ||  1·7962 |   71·75    ||  1·6870 |   63·60    |
  |  1·8439 |   79·09    ||  1·7870 |   70·94    ||  1·6750 |   62·78    |
  |  1·8410 |   78·28    ||  1·7774 |   70·12    ||  1·6630 |   61·97    |
  |  1·8376 |   77·46    ||  1·7673 |   69·31    ||  1·6520 |   61·15    |
  |  1·8336 |   76·65    ||  1·7570 |   68·49    ||  1·6415 |   60·34    |
  |  1·8290 |   75·83    ||  1·7465 |   67·68    ||  1·6321 |   59·52    |
  |  1·8233 |   75·02    ||  1·7360 |   66·86    ||  1·6204 |   58·71    |
  |  1·8179 |   74·20    ||  1·7245 |   66·05    ||  1·6090 |   57·89    |
  +---------+------------++---------+------------++---------+------------+

A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID IN THE LIQUID NITRIC ACID
OF DIFFERENT SPECIFIC GRAVITIES. (URE.)

  +---------+------------++---------+------------++---------+------------+
  |         |Real Acid   ||         |Real Acid   ||         |Real Acid   |
  |Specific |  in 100    ||Specific |  in 100    ||Specific |  in 100    |
  |Gravity. |parts of the||Gravity. |parts of the||Gravity. |parts of the|
  |         |  Liquid.   ||         |  Liquid.   ||         |  Liquid.   |
  +---------+------------++---------+------------++---------+------------+
  |  1·5000 |   79·700   ||  1·4640 |   69·339   ||  1·4147 |   58·978   |
  |  1·4980 |   78·903   ||  1·4600 |   68·542   ||  1·4107 |   58·181   |
  |  1·4960 |   78·106   ||  1·4570 |   67·745   ||  1·4065 |   57·384   |
  |  1·4940 |   77·309   ||  1·4530 |   66·948   ||  1·4023 |   56·587   |
  |  1·4910 |   76·512   ||  1·4500 |   66·155   ||  1·3978 |   55·790   |
  |  1·4880 |   75·715   ||  1·4460 |   65·354   ||  1·3945 |   54·993   |
  |  1·4850 |   74·918   ||  1·4424 |   64·557   ||  1·3882 |   54·196   |
  |  1·4820 |   74·121   ||  1·4385 |   63·760   ||  1·3833 |   53·399   |
  |  1·4790 |   73·324   ||  1·4346 |   62·963   ||  1·3783 |   52·602   |
  |  1·4760 |   72·527   ||  1·4306 |   62·166   ||  1·3732 |   51·805   |
  |  1·4730 |   71·730   ||  1·4269 |   61·369   ||  1·3681 |   51·068   |
  |  1·4700 |   70·933   ||  1·4228 |   60·572   ||  1·3630 |   50·211   |
  |  1·4670 |   70·136   ||  1·4189 |   59·775   ||  1·3579 |   49·414   |
  +---------+------------++---------+------------++---------+------------+


WEIGHTS AND MEASURES.


_Troy, or Apothecaries' Weight._

  1 Pound = 12 Ounces. 1 Ounce = 8 Drachms. 1 Drachm
  = 3 Scruples. 1 Scruple = 20 Grains. (1 Ounce Troy = 480
  Grains, or 1 Ounce Avoirdupois _plus_ 42·5 grains.)


_Avoirdupois Weight._

  1 Pound = 16 Ounces. 1 Ounce = 16 Drachms. 1 Drachm
  = 27·343 grains. (1 Ounce Avoirdupois = 437·5 grains.) (1 Pound
  Avoirdupois = 7000 Grains, or 1 Pound Troy _plus_ 2-1/2 Troy Ounces
  _plus_ 40 grains.)


_Imperial Measure._

  1 Gallon = 8 Pints. 1 Pint = 20 Ounces. 1 Ounce = 8
  Drachms. 1 Drachm = 60 Minims. (A Wine Pint of water
  measures 16 Ounces, and _weighs_ a Pound.)

  An Imperial Gallon of water _weighs_ 10 Pounds Avoirdupois, or
  70,000 Grains. An Imperial Pint of water _weighs_ 1-1/4 Pound Avoirdupois.
  A fluid Ounce of water _weighs_ 1 Ounce Avoirdupois, or
  437·5 Grains. A Drachm of water _weighs_ 54·7 Grains.


_French Measures of Weight._

  1 Kilogramme = 1000 Grammes = something less than 2-1/4 Pounds
  Avoirdupois.

  1 Gramme = 10 Decigrammes--100 Centigrammes = 1000
  Milligrammes = 15·433 English Grains.

  A Gramme of water _measures_ 17 English Minims, nearly.
  1000 Grammes of water _measure_ 35-1/4 English fluid Ounces.


_French Measures of Volume._

  1 Litre = 13 Decilitres = 100 Centilitres = 1000 Millilitres =
  35-1/4 English fluid Ounces.

  1 Litre = 1 Cubic Decimetre = 1000 Cubic Centimetres.

  1 Cubic Centimetre = 17 English Minims.

  A Litre of water _weighs_ a Kilogramme, or something less than
  2-1/4 Pounds Avoirdupois. A Cubic Centimetre of water _weighs_ a
  Gramme.




INDEX.[56]

[56] The preparation and properties of the Chemicals used in Photography
will be found in the Alphabetical List commencing at page 327.


  Aberration, chromatic, 54;
    spherical, 56.
  Accelerating agents, their mode of action in Collodion explained, 95.
  Acetate of Silver, its preparation and formula, 365;
    its formation in Nitrate Bath explained, 89;
    ensures   absence of free Nitric Acid, 116;
    tends slightly to favour fogging and spots, 104;
    contra-indicated for glass Positives, 111.
  Acetic Acid, properties and mode of testing purity of, 327;
    useful in preventing fogging, 104;
    in rendering the development slow and even, 99;
    does not coagulate Albumen, 329;
    a good commercial form of acid, 212;
  Acetic Acid essential in Calotype, waxed paper, and Albumen
      processes, 177;
    also in printing paper Positives by development, 260.
  Aceto-Nitrate of Silver, term explained, 177.
  Achromatic Lenses, their construction explained, 55;
    the visual and chemical foci often coincident in, 60.
  Acids, nature of, 308.
  Actinism, explained, 61;
    importance of distinguishing Actinic from visual rays, 62;
    mode of finding Actinic focus, 229.
  Affinity, chemical, 312.
  Albumen, its chemistry, 328;
    forms a compound with oxide of Silver, 20;
    used in Positive printing to produce a fine surface layer, 122;
    to increase sensitiveness, 125;
    affects the colour of the prints, 127;
    protects the image from oxidation, 150;
    putrifies when exposed to moisture, 155;
    discolours the Nitrate Bath, 245.
  Albumen negative process, its invention, 10;
    theory of, 180;
    Collodio-Albumen process of M. Taupenot, 294.
  Albuminized paper, formula for, 241;
    slow in fixing, 131;
    not well adapted for toning by Sel d'or, 269;
    good for stereoscopic subjects and small portraits, 249;
    cannot be sensitized with Ammonio-Nitrate of Silver, 246.
  Alcohol, its chemistry, 330;
    sometimes too dilute for making Collodion, 84;
    mode of rectifying, 196;
    must not contain impurities, 96;
    effects of adding to Collodion, 84, 96;
    to developer, 205.
  Alkalies, nature of, 308.
  Alkalinity of Nitrate Bath, explained, 88;
    the evils it produces, 104;
    how to test for it, 377;
    how to remove it, 277.
  Amber varnish, 226.
  Ammonia, preparation and properties, 331;
    its use in fixing, 42;
    Mr. Shadbolt's formula for, 271;
    its action upon Chloride of Gold, 343;
    effect of concentrated Ammonia upon Oxide of Silver, 362.
  Ammonio-Nitrate of Silver, its chemistry, 262;
    used in Positive printing to increase sensitiveness, 125;
    to give black tones, 127;
    cannot be used with Albumen, 246;
    increases permanency of print, 169;
    old Nitrate Baths not easily convertible into Ammonio-Nitrate, 248;
    mode of preparing, 247;
    best applied to the paper by brush or rod, 248;
    Oxide of Silver in Nitrate of Ammonia, a useful substitute for it, 249.
  Ammonio-Nitrate paper, formula for, 246;
    a more simple formula, but less sensitive than the last, 258.
  Atomic theory explained, 322.

  Bath for fixing and toning Positives. _See_ Fixing and toning Bath.
  Bichloride of Mercury, whitening action on glass Positives
      explained, 113;
    solution for, 207;
    used to intensify Negatives, 118;
    bleaches paper prints, 151;
    should not be added to paste used in mounting prints, 164;
    removes Silver stains, 377.
  Binocular vision, phenomena of, explained, 66.
  Blackening Negatives, 37, 117.
  Black tones, mode of obtaining, in paper Positives, 168, 246.
  Bromide of Silver, its preparation and properties, 17;
    its superior sensibility to coloured light, 63;
    less acted on by white light than Chloride, 19;
    less sensitive to invisible image than Iodide, 25;
    employment in Collodion, 101;
    found useful in Photographing by artificial light, 66;
    diagram of chemical spectrum on, 64.
  Bromo-Iodide of Silver, 173.
  Brushes, mode of applying Silver solutions by, 248.

  Calotype process, theory of, 176.
  Camera, its first invention, 7;
    theory of its construction, 54;
    mode of testing accuracy of, 229;
    cause of the image being inverted, 53;
    the term "flatness of field" explained, 54;
    best position of the Camera for portraits, 220;
    for architectural subjects, 231;
    a funnel-shaped tube placed in front of the lens, 229;
    stereoscopic Camera, 234;
    microscope Camera, 236.
  Causes of failure in Collodion process, 276.
  Chemical affinity, illustrations of, 312.
  Chemical elements, 306.
  Chemical focus, directions for finding, 229;
    shorter than visual in non achromatic lenses, 60;
    longer than visual in microscopic objectives, 237;
    varies slightly with the nature of the light, 238.
  Chemical spectrum, 61.
  Chemicals, Photographic, Vocabulary of, 327.
  Chloride of Silver, its preparation and properties, 14;
    more sensitive to white light than Bromide or Iodide, 19;
    less sensitive to invisible image, 24;
    its blackening by light explained, 20, 141;
    accelerated by excess of Nitrate, 19;
    by organic matter, 20, 142;
    experiments illustrating darkening of papers prepared with, 21;
    simple explanation of the mode of preparing sensitive papers with, 22;
    agents which dissolve it, 42;
    mode of reducing it to metallic state. 374.
  Chloride of Gold, its preparation and properties, 342;
    action of Ammonia upon it, 343;
    use of an alkaline solution of, for toning, 132, 271;
    compounds formed on adding it to Hyposulphite of Soda, 133;
    mode of preparing the fixing and toning Bath with, 250;
    the Sel d'or Bath with, 267.
  Chromatic aberration, 54.
  Citric Acid, forms a red compound with Suboxide of Silver, 21, 338;
    used in printing to give purple tones, 128;
    formula for preparing paper with, 246.
  Cleaning glass plates, theory of, 39;
    details of, 213.
  Collodion, its discovery, 10;
    chemistry of Pyroxyline, 75;
    physical effect of Ether and Alcohol in, 83;
    of water in, 85;
    glutinosity of, 83;
    coloration of iodized, explained, 85;
    sensitiveness and intensity affected by the change, 97, 99;
    details of manufacture of Collodion, 185;
    Positive Collodion, theory of, 108;
    formula for, 201;
    Negative Collodion, theory of, 113;
    formula for, 208;
    Collodion for copying engravings, 231;
    for keeping processes, 298;
    for hot climates, 210;
    for working by artificial light, 238;
    to remove the brown colour from Collodion, 86.
  Collodion film, the proper time for immersing it in the Bath, 219;
    a thin film often good for direct Positives, 109;
    a thicker film for Negatives, 113;
    cause of the film falling away from the glass, 83, 293;
    spots and markings on, 281;
    conditions which affect its sensitiveness to light, 92;
    causes influencing its behaviour with the developer, 98;
    mode of preserving sensitiveness of film, 289.
  Collodio-Albumen process, theory of, 181;
    practical details of, 294.
  Colours, their nature explained, 47;
    their chemical action on sensitive film, 64;
    their photographic action assisted by reflection of white light, 66.
  Combination, laws of, 307.
  Conjugate foci, explained, 52, 272.
  Crookes, Mr., remarks upon chemical spectrum, 63;
    upon waxed paper process, 180;
    preservative process for Collodion films, 289.
  Curvature of luminous image formed by lens, explained, 53.
  Cyanide of Potassium, its fixing action explained, 44;
    preparation of solution of, 207;
    used to remove stains, 377.
  Daguerreotype, its invention, 8;
    theory of the process, 171.
  Development of invisible image, explanation of, 34-40;
    second, or intensifying stage explained, 37;
    details of developing glass Positives and Negatives, 221-223;
    development of paper Positives, 259;
    conditions which increase or diminish rapidity of development, 98;
    irregularities of development, 103.
  Developers, their preparation and properties, 26;
    comparative strength of, 98;
    theory of, for Positives, 111;
    for Negatives, 117;
    formulæ for Positive developers, 205;
    for Negative, 211.
  Diagrams, mode of copying, 232.
  Diaphragms for lenses. _See_ Stops.
  Double decomposition, illustrated, 14;
    explained, 314.
  Dry Collodion process, 298.

  Elementary bodies, table of, 306;
    combination of, 307.
  Engravings, mode of copying, 231;
    often yield dark-coloured prints, 255.
  Equivalent proportions, 320.
  Ether, properties of, 339;
    purification of, for Photography, 195;
    must be kept in a dark place, 196;
    should not be distilled from residues of old Collodion, 96.
  Experiments, illustrating action of Light upon Chloride of Silver, 21;
    illustrating formation and development of invisible images, 25;
    illustrating photographic action of coloured light, 62.
  Exposure in the Camera, rules for Positives, 221;
    for Negatives, 225;
    for preserved Collodion plates, 292;
    for microscopic photographs, 238;
    effects of under and over-exposing, 35;
    exposure required in Calotype process, 177;
    in waxed paper, 180;
    in Albumen negative process, 181;
    in dry Collodion process, 301;
    in Taupenot's process, 297.

  Fading of Positives, explained at length, 160;
    Author's researches on, 153.
  Film, sensitive. _See_ Collodion film.
  Filters, mode of cutting, 376.
  Fixing, theory of, 41;
    of paper prints explained, 128;
    solution for fixing glass Positives and Negatives, 212;
    manipulatory details of fixing, 225;
    fixing paper Positives with Ammonia, 271.
  Fixing and toning Bath, its preparation, 250;
    conditions which favour or retard its action, 135;
    certain states of the Bath injurious to the proofs, 136;
    importance of keeping it in an active condition, 168;
    must not be employed immediately after mixing, 251;
    must not be allowed to become acid by constant use, 168;
    theory of the gradual change of properties it undergoes, 156.
  Foci, actinic and luminous, 60;
    actinic, mode of finding, 229;
    variation between them in microscopic objectives, 237.
  Focussing the object, 220.
  Fogging, theory of, 103;
    mode of detecting causes of, 276.
  Formulæ for solutions required in Collodion process, 201;
    for papers used in Positive printing, 241;
    want of correspondence between, 257.

  Gallic Acid, its preparation and properties, 27;
    used in paper processes, 178;
    becomes mouldy by keeping, 261.
    formula for developing paper Positives with, 261.
  Gallo-Nitrate of Silver, 177;
    discolours rapidly when developing dishes are not clean, 179.
  Gelatine, its properties, 341;
    forms a compound with an Oxide of Silver, 21;
    employed in dry Collodion process, 299;
    modified form of, 302;
    affects the colour in printing processes, 128;
    used in Positive printing to form an even surface layer, 126;
    as a cement to mount Photographs, 257.
  Glass plates, rules for cleaning, 39;
    details of cleaning, 213;
    mode of coating with Collodion, 215;
    with Albumen, 180.
  Glutinous Collodion, explained, 83.
  Glycyrrhizine, its nature, 342;
    its action in Collodion, 114;
    formula for solution of, 209.
  Gold, Chloride of. _See_ Chloride of Gold.
  Gold salts, their use in Photographic printing explained, 131;
    in the Daguerreotype process, 175.
  Gradation of tone, in Collodion Photographs, affected by the density of
     the film, 109, 113;
    by use of Glycyrrhizine, 115.
  Gradation of tone, in paper Positives, conditions affecting it in prints
      obtained by direct exposure, 123;
    in Positives printed by development, 266.

  Hadow, Mr., researches on Collodion, 77;
    formula for making Pyroxyline, 187.
  Heliography, invented by M. Niépce, 7.
  Historical sketch of Photography, 6.
  Honey keeping process, 289.
  Hunt, Mr., introduces Protosalts of Iron in developing, 111.
  Hypo Bath. _See_ Fixing and Toning Bath.
  Hyposulphite of Silver, its peculiar changes in colour, 129;
    the sweet compound which it forms with Hyposulphite of Soda, 44.
  Hyposulphite of Soda, preparation and properties, 43;
    theory of its fixing action, 43;
    blackens Nitrate of Silver, 129;
    causes a milkiness with acids, 137;
    its decomposition by constant use in fixing, 138;
    the salts it forms with Chloride of Gold, 133;
    its conversion into sulphuretting Tetrathionate by Iodine and
      Perchloride of Iron, 139;
    test for presence of, 169.

  Iceland moss, its use in Positive printing, 128;
    formula for preparing paper with, 245.
  Imperfections in Collodion Negatives, 282;
    in Positives, 284;
    in paper Positives, 285.
  Intensity, explanation of term, 92;
    mode of increasing in Negatives, 99,   114;
    effect of Acetate of Silver upon, 116;
    of Nitrite of Silver upon, 102;
    mode of diminishing, in glass Positives, 109, 110;
    conditions affecting intensity in paper Positives, 123;
    in developed paper Positives, 266.
  Invisible images, theory of formation of, 34;
    development of, 36;
    experiments illustrating, 25.
  Iodate, how formed in Collodion film, 94;
    produces insensitiveness, 198.
  Iodide of Ammonium, preparation of, 198;
    not fitted for iodizing Collodion required to be kept long, 210.
  Iodide of Iron, an accelerator to Collodion, 116.
  Iodide of Potassium and Silver, properties of, 42;
    mode of iodizing Calotype papers by, 177.
  Iodide of Potassium, tests of purity of, 197;
    extent of solubility in Alcohol, 351;
    dissolves Iodide of Silver, 42.
  Iodide of Silver, its preparation and properties, 16;
    unaffected by direct action of light, 19;
    highly sensitive to invisible image, 24;
    hypothesis of formation of latent image on, 34;
    possibility of its reduction by Pyrogallic Acid shown, 33;
    excess of Nitrate of Silver essential to its blackening by
      developer, 36;
    diagrams of chemical spectrum on, 61, 64;
    fixing agents for, 42;
    its solubility in the Nitrate Bath, 86;
    retards the action of Hypo fixing and toning Bath, 136;
    superior permanency of developed prints on, 167;
    details of Negative printing process on, 263.
  Iodine, in Collodion, diminishes sensitiveness, 94;
    forms Nitric Acid and Iodate in the Bath, 94;
    often useful in Positive Collodion, 110;
    in Negative Collodion, if fogging occurs, 105;
    mode of removing from Collodion, 86.
  Iodized Collodion. See Collodion.

  Kaolin, properties of, 335;
    used to decolorize Nitrate Baths, 91;
    importance of purifying it before use, 245.

  Landscape Photography, with preserved Collodion plates, 288.
  Latent image. See Invisible image.
  Laws of substitution explained, 78.
  Le Grey, M,, his toning process with Chloride of Gold, 132;
    his waxed paper Negative process, 178.
  Lenses, various forms of, 51;
    foci of, 52;
    formation of images by, 53;
    use of stops, 58;
    portrait, 59;
    chemical foci of, 60;
    chromatic aberration of, 54;
    spherical aberration of, 56;
    simple directions for using lenses, 227;
    for finding chemical focus, 229.
  Light; its action upon Silver Salts, 19;
    experiments illustrating, 21;
    formation of invisible images by, 24;
    its alternating action upon Daguerreotype plate, 39;
    its compound nature, 46;
    photographic action of coloured light, 60;
    refraction of light, 49.
  Llewellyn, Mr., his Oxymel process, 291.

  Manipulations of Collodion process, 213;
    of Photographic printing, 251;
    of toning by Sel d'or, 267.
  Manuscripts, mode of copying, 231.
  Markings on Collodion Pictures, 281.
  Measures and Weights, 379.
  Microscopic Photography, 235.
  Moser, M. Ludwig, his researches on the development of invisible
      images, 37.
  Mounting Positive Prints, substances which should be avoided in,
      155, 164;
    details of, 257.

  Negative processes for printing Positives, 259, 263.
  Negatives, definition of, 106;
    Collodion Negatives, theory of production of, 113;
    Calotype, 176;
    waxed paper, 178;
    Albumen, 180;
    Collodio-Albumen (Taupenot), 181;
    mode of developing Collodion Negatives, 37, 117, 223;
    of converting Positives into, 117;
    formula for solutions for Negatives, 208;
    the Collodion best adapted for Negatives, 114;
    spots and markings upon Negatives, 282;
    decomposition of Pyroxyline a cause of fading of, 166.
  Nitrate of Silver, preparation and properties of, 12;
    preparation of from standard coin of realm, 362;
    often contains free Nitric Acid, 13;
    when very strongly fused, contains Nitrite
      (_see_ Nitrite of Silver), 14;
    not acted on by light, 18;
    its reduction by Pyrogallic Acid explained, 31;
    the melted Nitrate more certain in its action, 13, 101;
    its presence essential in developing the image, 36, 98;
    increases sensitiveness of Collodion plate, 92;
    dissolves Iodide of Silver, 86;
    discoloured by Albumen, 245, 329;
    forms a compound with Honey, 289;
    with various other organic bodies, 21;
    very little acted on by Glycerine, 342;
    mode of recovering the Silver from, 372.
  Nitrate Bath, mode of preparing for glass Positives, 110, 203;
    for Collodion Negatives, 116, 211;
    its power of dissolving Iodide of Silver, its occasional acidity and
      alkalinity explained, 86;
    the mode in which Acetate of Silver may be formed in it, 89;
    a list of the substances by which it is decomposed, 90;
    changes by use, 91;
    effect of these changes on sensitiveness, 97;
    on intensity, 102;
    care required to prevent it from yielding foggy pictures, 104;
    a caution against the too frequent addition of alkali, 204;
    quantitative testing of the Bath, 371.
  Nitrite of Silver, adds to rapidity of development, 102;
    tends slightly to produce fogging, 104;
    solarizes the high lights, 111.
  Nitric Acid, its preparation and properties, 355;
    its oxidizing powers, 12;
    impairs sensitiveness of Collodion film, 93;
    lessens rapidity of development, 98;
    tends to prevent fogging, 104;
    sometimes usefully employed for glass Positives, 110;
    contra-indicated for Negatives, 116;
    its accumulation in the Nitrate Bath explained, 94;
    mode of removing it, 90;
    cannot exist in contact with Acetate of Silver, 116;
    produces stains on cloth, 215;
    mode of determining the strength of Commercial Nitric Acid, 186;
    table of strength of Nitric Acid of different densities, 378.
  Nitro-Sulphuric Acid, explained, 77;
    process for making by mixed acids, 186;
    by Oil of Vitriol and Nitre, 190;
    should not be used cold, 83.
  Nomenclature, chemical, 315.
  Norris, Dr., his dry Collodion process, 298.
  Notation, chemical, 318.

  Organic bodies, chemistry of, 324.
  Oxide of Silver, preparation and properties, 17;
    dissolves in the Nitrate Bath, rendering it alkaline, 88;
    properties of its solution in Ammonia, 362;
    preparation of ditto, 247;
    its solution in Nitrate of Ammonia used in Photography, 249.
  Oxymel, keeping process, 291;
   preparation of Oxymel, 360.
  Paper, Photographic, selection of, 240;
    peculiarity of English papers, 241.
  Paper, sensitive, for printing. _See_ Sensitive Paper.
  Perchloride of Iron, preparation of toning Bath with, 160.
  Permanence of Positives, mode of testing, 169.
  Photographic image, chemical composition of, 140;
    action of destructive tests on, 145.
  Photographic properties of Salts of Silver, 18;
    of Iodide of Silver upon Collodion, 74.
  Photographic researches by the Author, 140.
  Photography, historical sketch of, 6;
    the term explained, 61.
  Portrait lenses, theory of their construction, 59;
    rules for their use, 227;
    mode of finding chemical focus, 229.
  Portraits, drapery for, 66;
    directions for taking, 220;
    the position of the Camera, and other points of importance, 228;
    the time of exposure, 221.
  Positive printing, on Albuminized paper, formulæ for, 241;
    on plain paper, formulæ for, 245;
    on Ammonio-Nitrate paper, formula for, 246, 258;
    by development, formulæ for, 259;
    manipulatory details of printing, fixing, toning, washing, and
      mounting, 251;
    process of toning by Sel d'or, theory of, 134;
    practice of, 267;
    reasons for the want of correspondence between different formulæ, 257;
    use of Chloride of Gold in toning, 132, 271;
    theory of the preparation of the sensitive paper for Positives, 122;
    theory of the process of fixing, 129;
    of toning by Gold, 132;
    the Author's researches, 140;
    rationale of the printing process, 120;
    composition of the image, 140;
    fading of Positive prints, 160;
    destructive action of Sulphur on, 145;
    of oxidizing agents on, 148;
    of Chlorine, acids, boiling water, etc., on, 151;
    of combustion of coal-gas on, 153;
    effect of damp air on, 153;
    theory of mode of washing Positives, 162;
    comparative permanency of prints, 166;
    mode of testing permanency, 169.
  Positives, definition of, 106;
    Collodion Positives, theory of production of, 108;
    formulæ for solutions for, 201;
    development of, 111, 221;
    Collodion and Nitrate Bath best adapted for, 109;
    mode of whitening by Bichloride of Mercury, 112;
    solution for whitening, 207;
    mode of backing up, 226;
    spots and markings on, 284;
    mode of printing Positives on Collodion, 272.
  Positives, enlarged, mode of printing, 272.
  Practice of Collodion process, 183.
  Preservative processes for Collodion plates, 289.
  Printing, Photographic, theory of, 120;
    practical details of, 240.
  Prism, refraction of light by, 51;
    diagram of formation of spectrum by, 47;
    explained, 54.
  Prismatic spectrum, 47, 61.
  Protonitrate of Iron, preparation of, 206;
    a feeble developer when free from excess of Sulphate of Iron, 98;
    theory of its mode of action, and rules for its use, 112;
    cannot be prepared in quantity by adding Nitrate of Potash to Sulphate
      of Iron, 314;
    sometimes requires the addition of Nitrate of Silver, 206.
  Protosulphate of Iron, its preparation and properties, 29;
    its characteristics as a developer for Collodion Positives, 111;
    not well adapted for developing Collodion Negatives, 117;
    preparation of the solution for Positives, 205;
    mode of applying it to the plate, 221;
    to remove iron stains on glass, 215.
  Pyrogallic Acid, its preparation and properties, 28;
    solution for developing glass Positives, 205;
    for Negatives, 211;
    cannot be used without Acetic Acid, 105;
    less adapted for developing paper pictures, 178;
    requires addition of Nitric Acid when used for Positives, 111;
    superior to Sulphate of Iron for developing Negatives, 117, 144;
    mode of obviating the brown discoloration of developing solutions, 212.
  Pyroxyline, its nature and properties, 75;
    preparation of, by Mr. Hadow's formula, 186;
    by a rule-of-thumb mixture of the acids, 188;
    by the Oil of Vitriol and Nitre process, 190;
    details of immersing, washing, and drying, 191;
    the glutinous variety produced by cold acids, 83;
    recapitulation of the effects of varying the strength of the acid
      mixture, 193;
    spontaneous decomposition of Pyroxyline, 166.

  Reduction of metallic oxides by developers, theory of, 26;
    of Silver salts by developers, theory of, 30;
    practical details of reducing Silver compounds to metallic state, 372.

  Salts nature of, 310.
  Salts of Silver, their preparation and properties, 12;
    their Photographic action, 18;
    theory of their reduction by developer, 30;
    directions for obtaining metal from, 372.
  Sel d'or, toning process by, its theory, 134;
    its practical details, 267;
    its advantages, 271;
    gives permanent prints, 167.
  Sensitiveness, term explained, 92;
    conditions favourable to, 97.
  Sensitiveness of Collodion film, causes influencing, 92;
    superior sensitiveness partially explained, 74;
    preservation of sensitive film, 288.
  Sensitive paper, theory of preparation of, 22, 122;
    its darkening by light described, 123;
    preparation of Albuminized paper, 241;
    of plain paper, 245;
    of Ammonio-Nitrate paper, 246, 258;
    of paper for Negative processes, 259;
    causes which affect the sensitiveness of Positive paper, 123;
    which alter the colour of the image, 126;
    spots and markings on, 285;
    a large excess of Nitrate of Silver essential, 124;
    the paper should not be kept too long, 130, 286.
  Serum of Milk, preparation of, 262, 355;
    used in Negative printing process, 262.
  Shadbolt, Mr., his Honey keeping process, 289;
    employs artificial light in Micro-Photography, 237.
  Silver, properties of, 362;
    estimation of, in Nitrate Baths, 371;
    recovery from waste solutions, 372;
    reduction from Chloride, 374;
    stains, removal of, 377.
  Size, mode of removing, from paper Positives, 255.
  Solar spectrum, 47, 61.
  Soluble paper. _See_ Pyroxyline.
  Specific gravity of liquids, mode of finding, 375.
  Spherical aberration, 56.
  Spirits of Wine, preparation and properties, 330;
    not always sufficiently strong for Collodion, 84;
    mode of rectifying, 196;
    sometimes contaminated with, fusel oil, 96.
  Spots on Collodion plates, 279;
    on paper Positives, 285;
    on prints obtained by development, 266;
    on preserved Collodion plates, 293.
  Stains, Silver, removal of, 377.
  Stereoscope, invention of, 67;
    theory of, 68;
    Wheatstone's, 69;
    Brewster's, 70.
  Stereoscopic Photographs, rules for taking, 71;
    practical details of, 232.
  Stops, theory of use of, 57;
    simple mode of making, 228;
    position of the stop often important, 230.
  Strength of acids, tables of, 378.
  Subchloride of Silver, its preparation and properties, 15;
    decomposed by fixing agents, 141.
  Suboxide of Silver, its properties, 18;
    forms compounds with organic matters. Citric Acid, Albumen, etc., 21.
  Substitution, laws of, explained, 78.
  Sulphate of Iron. _See_ Protosulphate of Iron.
  Sulphate of Quinine, absorption of chemical rays by, 65.
  Sulphuric Acid, table of strength of, 378.
  Sutton, Mr., theory of Sel d'or toning process, 134;
    practical details of, 267;
    Negative printing process, 262;
    preparation of Serum of Milk for, 355.
  Symbols, use of, 318.
  Syruped Collodion film, 289.

  Talbot, Mr., his discoveries, 9;
    theory of Calotype process, 176.
  Taupenot, M., his Collodio-Albumen process, 181;
    practical details of, 294.
  Temperature, its effect upon development of Collodion film, 102;
    upon fogging, 105;
    upon keeping Collodion, 210;
    upon action of fixing Bath for paper Positives, 130;
    upon Hypo toning Bath, 136.
  Test-papers, use of, 376.
  Toning Bath for Positives, with Sel d'or, 134, 267;
    with Hyposulphite and Gold, _see_ Fixing and Toning Bath;
    with Chloride of Iron and Hyposulphite, 160.
  Toning of Positives, term defined, 121;
    may injure the stability of the proof, 154;
    points to be kept in view to avoid fading, 167;
    manipulatory details of, 253;
    by Sel d'or, 267.
  Transparencies, mode of printing, 273.

  Varnishes for Collodion Photographs, 226.
  View Lenses, directions for using, 230.
  Vocabulary of Photographic chemicals, 327.

  Washing Positive prints, rules for, 162;
    details of, 255.
  Waxed paper process, theory of, 178.
  Weights and Measures, table of, 379.


PRINTED BY JOHN EDWARD TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS.


       *       *       *       *       *


Transcriber Note

Minor typos have been corrected. Images moved to prevent splitting
paragraphs.












*** END OF THE PROJECT GUTENBERG EBOOK A MANUAL OF PHOTOGRAPHIC CHEMISTRY, INCLUDING THE PRACTICE OF THE COLLODION PROCESS ***


    

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