The photographic image : a theoretical and practical treatise of the development in the gelatine, collodion, ferrotype and silver bromide paper processes

Preface

We have endeavored to give in this work all the theoretical and practical instructions necessary to guide one in the operation of developing the photographic image, and we have described the processes employed to finish the cliches which is the complement of the operation.

We were led to write it for the reason that in the text-books published since a few years the development, although well described, is not treated, we think, with the importance it merits. It does not suffice to give formulas and to explain their use in some cases, their applications should be illustrated by examples, from those examples simple rules should be deducted and the special action of each of the chemicals employed must be pointed out in order that one understands

PREFACE.

the results to which give rise the modifications made in the constitution of the developing solution.

Not too many instructions can be given, for the development is the most difficult operation of photography, the value of the negative cliches and, as a consequence, the perfection of the pictures depending from the manner the latent image has been developed.

Indeed, it is among many amateurs and even professional photographers a common error to think that it can be done in a sort of mechanical or automatic way as it has sometimes been presented. There are probably not two subjects which can be well photographed by operating exactly in the same manner; and, to meet the various cases which present themselves, not only should the operator have studied the relation of the exposure-time to the lighting, or oppositions of lights and shades, and that of the development to the lighting, but, to the latter end, have a perfect knowledge of the action of the chemicals constituting the developing solution on the parts of the photo film impressed by light.

PREFACE

Therefore we have written a purely theoretical chapter on the behavior of the chemicals employed, which we consider as a very important one and of which we recommend the lecture even to those having no notion of chemistry : for the theory is the base of practice — it explains the phenomena, it guides the operator ; without knowing the principles one may occasionally produce excellent works, but generally the results are imperfect, and whenever one meets with failures, he is unable to trace them to their real cause.

We present this work to those interested in the art of photography, trusting it will smooth the path to the beginners and be useful to those who desire not to work in a mechanical manner.

P. C. D.

New York, October, i8gi.

Introduction

PAGE

Action of light— Reversal — Solarization — The relation

of the exposure to the development, - - - vii

Part I

Chaptek I. Chemistry and behaviour of the reagents employed in the development. Salts of iron — Pyrogallol — Hydroquinone — Eikonogen ~ The hydrates and carbonates of the alkali-metals — Action of the alkalies on the reagents — Their special action on the photo-film — The sulphites — Their action on the reduction of the silver salts — The restrainers— The accelerators — Summary of the actions of the chemicals, - - - - 3

Chapter II. The dark room illuminationf - - 50

Chapter III. The exposure-time. Influence of the exposure on the character of the picture— The exposure in relation to the lighting — Normal exposure defined— Example— Table of exposure-times — Photometers — The pendulum, - - - 5t iv. CONTENTS.

PART II. The Development in the Gelatine Pkocess

Chapter I. Generalities on the development. The development regulated both by the exposure-time and the lighting of the subject— Influences of rapid and slow developments— Two important rules — Examples — The effects of rocking — Evalescense— When to stop the development — Rule to determine it — Local and general intensities — Rule on which is based the development, - - 69

Chapter II. The development with pyrogallol. First example: The development of plates normally exposed, or nearly so — Second example : The development of plates under exposed — Third example : The development of plates over exposed — Fourth example: The tentative development — The development according to the lighting — Example o/— The development of orthochromatic plates — Local development — The development of cliches in line, 83

Chapter III. Summary of the rules regulating the exposure-time and the development, - - - - 94

Chapter IV. Fixing. The theory of — Generalities —Berkeley's fixing bath, 100

Chapter V. Intensification. The chemistry of — Generalities — How to obtain various degrees of intensity — Intensifying with silver, - - - 108

CONTENTS.

V.

Chapter VI. Reduction of intensity. The processes usually employed — Their defects — Rational method of reduction — How to treat cliches too intense in the lights, 123

Chapter VII. Causes of failures, . - - - 133

Chapter VIII. The development with hydroquinone. Generalities— Formulas for various exposure-times— Tentative development, . . - . 144

Chapter IX. The development with eikonogen. Generalities—Formulas for various exposure-times, - 152

Chapter X. The development with ferrous oxalate. Generalities — Examples, 159

Chapter XI. The development with pyrocatechin, hydroxylamine, and jjaramidophenol, . - - 165

PAET III.

The Development in the Collodion and Silver Bromide Printing Processes.

Chapter I. The development in the wet collodion process. Theory — Generalities — Normal, strong and weak developers — Action of — Developer for line-works — Re-development — Intensification— Causes of failures, 173

Chapter IT. The development in the dry collodion process. Physical and chemical developers — Tentative development— Fixing, - - - - 190 vi.

CONTENTS,

Chapter III. The development of ferrotypes. Generalities— Spiller, Keith, and the writer's formulas — Modifications to obtain good whites — Sulphuric acid objectionable — Modus operandi— Fixing, - 195

Chapter IV. The development of gelatino-bromide paper. Generalities — Development with ferrous oxalate and eikonogen — Causes of failures and remedies, 203

Chapter V. Varnishing, 211 INTRODUCTION

When a photo-plate is exposed in the camera obscura for a period of a few seconds, then treated by a reducing agent, the developer, one obtains a negative cliche representing the luminous image formed at the focus of the lens. The light has therefore impressed the silver salt, but the impression is not visible and cannot by any known means be detected before the development.

"What is this primary action of light on the silver haloid : is it purely physical, or one causing a partial or entire reduction ? It is an open question.

Although experiment shows that the ultimate luminous action results in a more or less complete dissociation of the constitutive elements, the successive changes occurring viii.

INTRODUCTION.

during the period of exposures in the camera^ which give rise by development to a negative, then to a positive, then to a negative, then again to the same series of transformations, as well as the evalescence of the luminous impression, sustain the hypothesis of a purely physical action : the silver haloid changing of state by displacement of the molecular centers and the caloris producing the work necessary to effect the chemical changes occurring in the development.

From the behavior of light on the silver haloids it results that the change goes on increasing to a certain limit which, in one way, coincides with the maximum of action producing, by development, the greatest opacity of reduction, then as the luminous action progresses the reversal commences and causes a reduction which becomes less and less opaque. This phenomenon is termed solarization. It was first observed by Moser, of Konigsberg.

The solarization is the cause of many

INTRODUCTION^.

defects ; thus, the sky in landscape, for example, is devoid of opacity in the negative and, as a consequence, represented by tinted whites in the positive. Furthermore, if the sky was cloudy, the clouds will not be reproduced : the half lights always disappear in the solarized high lights to which they become equal in intensity.

There are necessarily various degrees of solarization, and it is on this account that the reversal is a phenomenon useful in photography. It permits one to reproduce the ensemble of the object ; without it, it would be impossible to photograph any object with gradation from light to shade, for not only the less refrangible rays act with much less energy than the others, but even the weak lights from the same ray does not, in proportion, impress the photo-film as rapidly as the unmitigated ray. In fact the photo-film is impressed by the white light reflected from the surface of bodies, and the colored rays reflected by the body itself. Hence the rule to over expose.

z.

INTRODUCTION.

During the period of over exposure the reversal necessarily commences in the high lights, that is on the parts of the photo-film upon which are projected the most luminous rays of the object, while, the action progressing on those which correspond to the half lights or tints, the latter acquire a great tendency — almost as great as that of the high lights — to be acted on by the developer, the result being, therefore, when the subject is lighted by great oppositions of lights and shades a perfect, or, at least, as possible a perfect picture as one can obtain, and if the subject is normally lighted, a picture wanting in contrasts and solarized in the high lights, that is, devoid of half tints, the whole having been equallized, so to speak, during the reversal of the primary action on the parts in question.

On the other hand, if the exposure time is short, the half lights, being in proportions to the high lights little impressed* or not at

* The effects of solarization are much attenuated by dying the photo film, (orthochromatic plates).

INTRODUCTION, all, fail to develop or do not take a proportionate and sufficient opacity, and the result is a picture with exaggerated contrasts unless the subject be uniformly lighted when the picture will be improved by the greater oppositions between the lights and shadows resulting thereof.

As it is seen the character of the picture can be altered by varying the time of exposure. In practice this is sometimes of difficult application, and had we not at our disposal the means to annul, or, more correctly, to attenuate the defects arising from over or short exposure-times, it would be a matter of chance to obtain perfect pictures. These means we find them in the manner of conducting the development and of compounding the developing solutions either to force the half lights to develop before the high lights gain intensity or to cause the latter to intensify to a certain extent before the former make their appearance. In a word the operator by a proper management of the chemicals can

INTRODUCTION.

obtain at will, so to say, eitlier softness and harmony, or vigor and brilliancy.

Hence the perfection of the picture depends not only of the exposure-time but also, and principally, of the development. This is the most important operation of photography. It requires judgment and artistic taste io produce photographs of faultless beauty, and much experience to overcome the difficulties which present themselves. We hope that the beginner guided in his first essays by the general rules given in this book with examples of their applications will soon master them.

PART I.

Chemistry of the Keageots.

Illumination of the Dark Room.

The Exposure Time.

Chapter I

Chemistry and Behavior of the Compounds Employed to Develop the Photographic Image.

SALTS OF IKON.

The following iron salts are employed in the development of the latent image : Ferrous sulphate, ferrous oxalate, and ferrous citrate.

Ferkous Sulphate, Fe S O4. Syn: jprotosulj)hate of iron, green vitriol, copperas. Ferrous sulphate crystallizes with 7 molecules of water ; it therefore contains per cent 5468 parts of the anhydrous salt.

The crystals are greenish-blue when the salt crystallizes in an acid s:lution, and emerald green if the solution contains ferric sulphate.

It is not venemous. Its test is styptic; its reaction strongly acid. All the oxidizing agents transform it into ferric sulphate.

THE PHOTOGRAPHIC IMAGE.

It is insoluble in absolute alcohol. Alcohol at 50^ dissolves about 3 per cent, of it.

100 parts of f 10° C. dissolve 61 ) parts of

\ 15° C, " 70 \ water at ( 33° C. " 151 ) ferrous sulphate.

100 Darts of f 164 parts of water at 10° C.

ferrous sulphate 1 << «< 24° 0*

dissolve in [ 66 *' " 43° c!

Exposed to the air the crystals effloresce and become covered with an ochreous coating of basic ferric sulphate, (Fe203)2S04. This oxidation is prevented by the presence of sugar, glucose, etc.; the oxygen acts then on the organic substance by means of the sulphate which serves of a vehicle.

The solution of ferrous sulphate oxidizes in the air by absorption of oxygen with formation of basic ferric sulphate and normal ferric sulphate. Saccharine substances and sulphuric acid prevent these actions.

Ferrous sulphate precipitates the soluble salts of silver and gold to metal.

Ferric Sulphate, Fe2(S04)3. Syn. : iron sesquisulphatej MonseVs salt. Ferric sulphate is

CHEMI8 TRY AND BEHA VIOR, cfcc. 5 yery soluble in water. It dissolves metallic silver forming argentic and ferrous sulphate. This property is utilized in photography to reduce the intensity of negatives. The solution saturated by the aid of heat deposits, in cooling, very fine crystals of metallic silver, ferric sulphate being regenerated, thus :

Ferrous Oxalate, FeC204, is obtained by dissolving iron in oxalic acid, or by exchange of sases by treating ferrous sulphate with neutral potassic oxalate :

It crystalizes in small, brilliant, yellow crystals. It reduces the salts of silver, but being scarcely soluble in water it is employed united to the potassic oxalate with which it forms a soluble double salts, KaFe (0204)2. 100 parts of a saturated solution of potassium oxalate^ dissolve 12 parts of the salt in question.

The use of ferrous oxalate as a developer was suggested by Mr. Oarey Lea in 1877. The

* Potassium oxalate dissolves in 3 parts of cold water,

THE PHOTOGRAPHIC IMAGE.

developing solution is prepared bj adding 1 part of a solution of ferrous sulphate at 30 per cent of water to 3 parts of a solution of neutral potassium oxalate at the same strength.

The reduction of silver haloids by ferrous oxalate is effected by the dissociation of the elements of water. The chemical actions occurring during the reduction are as follows :

1st, The ferrous oxalate is oxidized with formation of ferric oxalate and ferric oxide in displacing the oxygen, hydrogen being set free :

2nd, Then the hydrogen in the nascent state reduces the silver salt by combining with bromine :

AgBr+H=HBr+Ag :

3rd, And hydrobromic acid uniting with the ferric oxide forms ferric bromide and water :

6HBr+Fe203=re2Bre+3 H2O.

The ultimate result is represented by the following equation :

CHEMISTRY AND BEHAVIOR, &c.

With potassium ferrous oxalate, potassium bromide is formed :

The chemical actions taking place during the development with pyrogallol, hydroquinone and other developers are similar : the reduction of the silver haloid is effected through the agency of hydrogen dissociated from water uniting to the haloid.

Fereous Citrate, re3(C6H507)2 , is prepared by double decomposition for photographic use. It was selected by Eder and Pizzighelli for the development of emulsion with silver chloride^ as acting less energetically than ferrous oxalate. It is now little used. Three stock-solutions should be at hand to compound the developer, viz.;

A. Ammonium citrate solution. Dissolve by the aid of heat 600 grains of citric acid in 4J ounces of water, and neutralize by aqueous ammonia ; drive out the excess of ammonia by heating, then dissolve 400 grains of citric acid and add water to make up 9 ounces.

* This emulsion is used for transparencies. Negativea can also be made with the same.

THE PHOTOGRAPHIC IMAGE.

B. Ferrous sulphate solution. 1 to 3 of water with a few drops of sulphuric acid to prevent oxidation.

C. Sodium cJdoride Solution. 1 to 30 of water. To develop, these solutions are mixed in the

following proportion :

Sodium chloride is an energetic restrainer. It should be employed judiciously. The ferrous sulphate solution employed in greater proportion than that above given retards the development.

By diluting the developer less intensity is obtained, which in case of under exposures, etc., is advantageous.

Gallic acid added to the developer produces a sepia tone and acts as an accelerator. To the above solution 5 parts of a saturated solution, can be added.

Potassium Feekocyanate, (FeCye). It crystallizes with 3 molecules of water in yellow prisms or tables insoluble in alcohol, soluble in 4 parts of cold water. Its taste is sweetish at first, then salted and bitter. It is not poisonous.

Chlorine, bromine, dilute nitric acid, all the oxidizing agents capable of displacing potassium transform it into ferricjanate :

The silver salts — chloride, bromide, nitrate — decompose it with formation of the double cyanide of potassium and silver under the influence of heat.

K4(reCy6)+2 AgCl=FeCy2+2 KAgCy2+2KCL

Potassium ferrocyanate was recommended by Mr. A. L, Henderson in conjunction with the pyrogallol developer. It is also employed with hydroquinone and eikonogen. Per se it possesses no reductive property. Some authors assert that it causes a softer image to be developed, others, amongst which Mr. E. Himly who studied his action with hydroquinone, that the addition of a few drops of a solution of the salt at 30 per cent of water is advisable to obtain more contrasts. The author of thip book has ascertained that it effectively prevented fogging and consequently acted as a weak restrainer, but thinks that in

Id 'I HE PRO TOGRAPHIC IMA G^.

the whole it can be discarded, moreover, any change advisable in the character of the negatives is easily effected by compounding the developing solution with more or less reagent or more or less alkali and the usual restrainer KBr.*

Potassium Feericyanate, K6(re2Cyi2), crystallizes without water of crystallization in rhomboidal crystals of a ruby-red color, unalterable in the air and soluble in 2 J parts of cold water, the solution under the influence of light depositing a blue powder with formation of ferrocyanate. In presence of organic matters it is likewise reduced.

It attacks zinc, copper, lead, mercury, silver which it transforms into ferrocyanate, being itself reduced to a similar compound. This

* We have used as an energetic developer :

A. Sodium sulpiiite, . . 40 parts. Potassium ferrocyanate, .20 Hydroquinone, . . 10 •* Water, . . . 480 *'

B. Sodium hydrate, . . 30 parts. Water, . . . 480 "

Equal volumes for rapid exposures.

Chemistry AND behavior, dc.

action is utilized for the reduction of over dense cliclies (Farmer's process).

PYROGALLOL.

Pyrogallol, aHA=C6H3(OH)3. Syn : Trioxyhenzine, pyrogallic phenol, j^y^'ogallic acid. Pyrogallol is venemous, causing death at the dose of from 2 to 3 grams in acting as phosphorous by absorption of oxygen from the blood.

It occurs in prisms or in long needles which are perfectly white.

It blackens in damp air, the action being more rapid in the light. The products of the decomposition are brown-black matters whose constitution has not been ascertained, plus oxalic and acetic acid and carbon dioxide :

Its taste is faintly bitter, its odor nul,* its reaction neutral.t It melts at 115° C, boils at

* The commercial pyrogallol has generally an empyreumatic odor. If it deposits a black substance it contains metagallic acid which is insoluble.

t Litmus paper turns slightly red, but assumes its original blue color in drying, exactly as when reddened by carbonic acid.

THE PHOTOGRAPHIC IMAGE.

210^ C, and at 250^ C, splits into metagallic acid and water, thus :

Pjrogallol is soluble in 2| parts of water at 15^ C, 1| part of alcohol at 90^ and in 1| part of ether. The aqueous solution turns black in the air by absorption of oxygen, depositing black flocks and evolving carbon dioxide. Sulphurous acid, the alkaline sulphites, citric, oxalic, and formic acids, traces of nitric acid prevent this decomposition. The alcoholic solution is more staple and keeps for a long time, it being little altered.

In presence of the alkalies, of the alkaline carbonates and aqueous ammonia, pyrogallol rapidly absorbs oxygen, becomes yellowish, then brown and lastly brown-black with formation of carbon dioxide (carbonic acid), acetic acid, oxalic acid, etc., and traces of hydrogen dioxide.

It does not decompose the carbonates. Boiled with potassium hydrate pyrogallol is decomposed into carbon dioxide, acetic and oxalic acids. Added to lime water, the liquid assumes a fine red tint which turns brown.

CHEMISTRY AND BEHAVIOR, &g

This action is \ery delicate. It serves in analyses.

Hydrochloric acid has no action on pyrogallol.

Nitric acid transforms it into oxalic acid.

Nitrous acid and the nitrites even in very small quantities turn brown its solution. This action serves to detect infinitesimal traces of these compounds.

Chlorine decomposes pyrogallol in precipitating a black substance and forming hydrochloric acid.

Bromine forms an insoluble bromized derivative BrsfCeHsOs) which first becomes deep red in presence of the alkalies and then turns red-brown in the air.

Ammonia precipitates from the ethereal solution of pyrogallol, white crystals of a compound termed ammonium pyrogallate CfiHsNH^, which rapidly oxidizes in the air with formation of pyrogallein.

Potassium permanganate decomposes it, liberating carbon dioxide with effervescence in concentrated solution. The permanganate is decolored.

Heated with anhydrous phthalic acid, it is

THE PHOTOGRAPHIC IMAGE.

converted into gallein, which treated by hot sulphuric acid yields a very staple green coloring matter, the coerulein employed in orthochromatic photography.

Pyrogallol unites with gelatine and casein. It gives with ferric chloride a red coloration turning black progressively. It colors the skin and hairs brown-black.*

It reduces the salts of silver, gold and platinum. In silver nitrate solutions it dissociates the metal, depositing a red compound termed purpurogallein, C20H10O9, which oxidizes rapidly.

Pyrogallol is obtained from gallic acid which under the influence of heat splits into carbon dioxide and the compound in question : C6H3.CO,(OH)3=CeH3(OH)3+CO,.

In manufactures it is obtained from nutgalls which yields about 12 per cent of it. The dry extract is placed in an iron vessel heated to 180^ C. on a sand bath and covered with a receiver into which pyrogallol sublimates and condenses in prisms.

* Stains on the hands can be removed by a dilute solution of sulphuric acid 1.100, or of hydrochloric acid 5.100 or bv citric acid or any clearing solution.

HYDEOQUINONE.

Hydkoquinone, C6H602= C6H4(0H)2. Syn : Quinolj Paradioxyhenzene. Hydroquinone is a diatomic phenol isomeric witli catechol and resorcinol. It is formed by the action of di oxidizing agents such as sulphur dioxide, hydriodic acid, stannic acid, etc., on quinone which takes up a molecule of hydrogen, C6Hi02+H2=C6H602, or by oxidizing aniline with potassium bichromate. It is by the latter process that it is produced in manufactures, thus : one part of aniline and 8 parts of sulphuric acid are dissolved in 30 parts of water and the mixture allowed to cool. Two and a half parts of pulverized potassium bichromate are then little by little added, taking care to keep cold the solution, whereby among other products hydroquinone is formed together with quinone. A current of sulphur dioxide now passed through the solution transforms the quinone into hydroquinone which is taken up by ether. The ethereal solution rises on the surface and, the aqueous liquid being drawn off, the ether by evaporation leaves a residue which is dissolved in a

THE PHOTOGRAPHIC IMAGE.

small quantity of hot water, and from this solution treated by sulphur dioxide, decolorized with animal charcoal, boiled, filtered, and cooled, hydroquinone is deposited.

Hydroquinone crystallizes in orthorhombic prisms without taste and odor, and without action on test paper. When the crystallization is slow the crystals are lemon-yellow and contain 27.2 of water per 100. But if formed rapidly, they are very white and take up only 12.8 parts of water.

It is soluble in ether and alcohol. Water, at 15° C. dissolves 6 per 100 of it, and 9.5 parts at the temperature of 30° C. It melts at 169° C, and when gradually heated it sublimates easily as brilliant scales without alteration. It is on this form that it is generally found in commerce.

It reduces in the cold the salts of mercury, silver, gold and platinum with formation of quinone by elimination of hydrogen :

According to H. Eeele, 8 parts of hydroquinone reduce 68.22 of silver oxide.

The reductive property of hydroquinone is

CHEMISTRY AND BEHAVIOR, &c.

increased by the hydrates and carbonates of the alkali metals, it being transformed by oxidation into quinone, green hydroquinone and brown substances analogous to those formed by the oxidation of pyrogallol. The solution is first tinged yellow, then turns red-brown and possesses tinctorial properties. Fer se hydroquinone does not reduce the silver haloids.

Hydroquinone is not precipitated by lead acetate. It dissolves without alteration in normal and acid sulphites solutions from which it deposits as yellow crystals containing sulphurous acid. These solutions are almost inalterable in the air and keep well even in presence of an alkali.

Nitric acid transforms hydroquinone into oxalic acid.

Chlorine, ferric chloride, potassium bichromate, silver nitrate and other oxidizing agents convert it into quinone and an hydroquinone crystallizing in needles with a beautiful green lustre. This compound. Green hydroquinone, C6O5O2, is soluble in ether and alcohol. Sulphur dioxide resolves it into colorless hydroquinone. Treated by ammonium hydrate and the alkalies it rapidly absorbs oxygen, being converted into a red-brown coloring matter.

Hydroquinone forms derivatives with the haloids. Its use in photography was suggested by Captain W. de W. Abney, in 1880.

EIKONOGEN.

/-'SO.Na C,oH5< OH

This compound is the sodium salt of Amido ^-Naphtol ^-monosulphonic acid, discovered in 1881, by professor Eaphael Meldola.

It crystallizes with 2 molecules of water in semi opaque yellowish-white crystals having a shining lustre. Its taste is sweetish and peculiar, its smell prickly. It slowly evolves acid fumes in being decomposed.

It is insoluble in alcohol, dissolves with difficulty in cold water which, however, retains at common temperatures from 7 to 8 parts per cent when the solution has been effected by heat. According to a well known law of chemistry, it is precipitated from its solution \>j the addition of a more soluble salts, sodium

CHEMISTRY AND BEHAVIOR, &c. l9 sulphite, carbonate, etc. We found that in an aqueous solution containing per cent 5 parts of normal sodium sulphite only 4 parts of eikonogen can be dissolved. As pyrogallol and hydroquinone, eikonogen possesses tinctorial properties when oxidized, the dyed substance, gelatine, being greenish by reflection and pink by transparency.

Eikonogen reduces silver nitrate in acid solutions, but per se it has nq action on the haloid salts of that metal without the intervention of an alkali. HowevQr, the reduction takes place, although slowly, yielding very clear negatives, in presence of sodium sulphite, which may be explained by the alkaline reaction of the salt.

With an alkali to form a more reductive compound, eikonogen reduces almost instantaneously the silver haloids impressed by light ; but notwithstanding such a powerful reducing action, by equal weight, it absorbs four times less oxygen than hydroquinone (H. Reeb) and still less than pyrogallol. The energy of its action is therefore soon impaired, and this explains why during the development the image, which rapidly appears in all its details slowly intensifies, and why it should be employed in larger quantity than the other reagents to obtain strong negatives.

According to Mr. H. Eeeb, 8 parts of hydroquinone make the work of 33 parts of eikonogen, these quantities reducing 100 parts of silver nitrate, or the equivalent of silver oxide, with the same dose of alkali : 40 parts of potassium carbonate or 23.53 parts of sodium hydroxide.

HYDEOXYLAMINE OE OXYAMMONIA.

This base discovered by Lossen, in 1865, is considered as ammonia in the constitution of which a molecule of hydroxyl is substituted to one atom of hydrogen, thus :

It is formed by the action of hydrogen on nitrogen oxide. This mode of formation is similar to that of ammonia when the oxide and hydrogen in excess are simultaneously passed on heated platinum sponges, thus :

All the salts of hydroxylamine, like those of ammonium, are decomposed by the alkalies which set free hydroxylamine.

Hydroxylamine possesses powerful reducing properties. It reduces the salts of aluminum, zinc, iron, lead, silver and gold, and transforms manganese dioxide into monoxide.

It converts mercuric chloride into calomel and, in excess, precipitates it to metallic state.

Hydroxylamine has not been isolated. It is known only in solution or combined to acid radicals. A pure solution is best obtained from the sulphate treated by calcium hydrate :

It is decomposed by the alkalies into nitrogen and ammonia, the action progressing so much more rapidly as the solution is more cencentrated :

3 NHsO^N^+NHs-l-.S HA

The chloride and nitrate of hydroxylamine

THE PHOTOGRAPHIC IMAGE.

are soluble in water and in alcohol. The sulphate is precipitated by alcohol from its aqueous solution.

Hydroxylamine procipitates cuprous oxide from cupric sulphate in hot solutions.

As a developer in the gelatine process, hydroxylamine was suggested by Spiller and Egli, either alone or in conjunction with pyrogallol. It yields very clear negatives. The ultimate action is represented by the following equation :

2 NH3O + 4 AgBr + 4 KOH= 4 KBr +N2O+ 5 OH,+Ag,.

In the arts it is employed in calico printing to form reserves.

For our purpose it is obtained from the chlorhydrate treated by an alkali, and acts while in the nascent state with great energy.

The chief advantage of hydroxylamine as a developer is not to absorb oxygen from the air. Its solution is therefore inalterable. It is little employed on account of the evolution of nitrous oxide, N2O, which gives rise to blisters in the gelatine process.

OHEMIBTEY AND BEHAVIOR, &c. 23

THE HYDKOXIDES (HYDKATES) AND CAKBONATES OF THE ALKALI- METALS.

Potassium, sodium, ammonium, and lithium hydroxides are employed in the development of the latent image. Their action consists to increase the reductive power of the reagent by exalting its affinity for oxygen, but see further on.

Whatever be the hydroxide employed the ultimate result is the same .