A manual of photographic chemistry : including the practice of the collodion process

Preface To The Third Edition

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 difiers 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 Pliotographic Printing are entirely re-written, and include the whole

VI

PEEFACE.

of tlie 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 Pharmacopoeia' 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

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 pubhcation 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 CoUodion" 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 Qth, 1857.

EERATA.

Page 24, line ^,for conditions read condition. Page 115, line 32, for Iodide read Iodine. Page, 194, line 15, /or p. 88 read p. 188.

Chapter II. The Salts Of Silyer 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

X

CONTENTS.

Chapter III. On The Development Of An Invisible Image

Page

Simple experiments illustrating tlie 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 underand 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 Eixing 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. —

CONTENTS.

xi

Page

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

Sectionl. — Collodion. — Pyroxyline — itsvarieties — 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 11. — 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 Proxii

CONTENTS.

Page

cess. — 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 M13

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

CONTENTS.

xiii

Page

of damp air upon Positive Prints. — The change in properties of the fixing Bath by constant use 140

Section IV. — The Fading of Photographic Prmts. — 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 fihn. — 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 11.

PRACTICAL DETAILS OF THE COLLODION PROCESS.

Chapter III. Manipulations Op The Collodion Process

Section I. — 3Ianipulations 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 talcing 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 hy 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 hy Bevelopment. — A process on Chloride and Citrate of Silver. — On Iodide of Silver. — On Bromide of Silver 259

Section III. — The Seld'Or Process for toning Positives. — Preparation of the toning Bath. — Manipulatory details . . 267

CONTENTS. XV

Page

Section IV. — On Printing enlarged Positives, Transparencies

etc., wpon Collodion 272

Chapter V. Classification Of Causes Of Failure In The Collodion Process

Section I, — Imperfections in Negative and' Positive Collodion Photographs. — Pogging. — 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

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 xvi

CONTENTS.

CHAPTER II. Page

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. lire's tables of the strength of Sulphuric and Nitric Acid of different densities.—Table of Weights and Measui'es 371

A MANUAL

OF

PHOTOGRAPHIC CHEMISTRY.

Introduction

In attempting to impart knowledge on any subject, it is not sufficient tliat the writer sliould 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, intreating of any science, such as that of Photography, which may be said to be comparatively new and unex-

B

INTEODUCTION.

plored, there is great danger of erroneously attributing effects to tlieir wrong causes ! Perhaps none but lie 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 inlinitesimally 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 formulae, 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-MATTEE OF EACH.

The title given to the Work is "A Manual of Photographic Chemistry," and it is proposed to include in it a

INTRODUCTION.

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, wdth 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 Photo2:raphy, 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 jjroperties 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 Photo-

INTEODUCTION.

graphy — 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 YI. 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 YII. continues the same subject, describing the classification of Collodion Photographs as Positives and J^egatives, with the distinctive peculiarities of each.

Chapter YIII. 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 YL, which describes the preservation of the sensitiveness of Collodion plates and the mode of operating upon films of Albumenized Collodion.

INTRODUCTION.

In Part III. will be found, in addition to a statement of the laws of cliemical 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 1.

HISTOKICAL SKETCH Or PHOTOGEAPHY.

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, Avithout 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 17.77. 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

HISTORICAL SKETCH OF PHOTOGRAPHY. 7

by Light availabie 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 JN^itrate 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 difiused 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 afiected by the subdued light of that instrument.

In the year 1814, however, twelve years subsequent to the publication of Wedgwood's paper, M. Niepce, 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. Niepce, which was termed " Heliography," or " sun-drawing," the use of the Silver Salts was discarded, and a resinous substance, known as " Bitumen of Judsea," 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 sol-

S HISTOEICAL SKETCH OF PHOTOGEAPHY.

vent, tlie shadows dissolved away, and tTie ligJits were represented by the unaltered resin remaining on the plate.

The Discoveries of 3f. Daguerre. — MM. iNiepce and Dagnerre appear at one time to have heen 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 m 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 M.ulti])lying Photographic Impressions ^ and other Discoveries of Mr. Fox Talbot. — The first communication made to the Hoyal Society by Mr. Fox Talbot,

HISTOEICAL SKETCH OF PHOTOGKAPHY. 9

in Jamiary, 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 Talhoty^e or Calotype dates from Tebruary, 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

10 HISTORICAL SKETCH OF PHOTOGEAPHY.

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 J ohn 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. Niepce 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 goldbeater'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 Pi/ro-gaWic 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 11.

THE SALTS OE SILYER EMPLOYED IN PHOTOGEAPHT.

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, 80^ 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.