The chemistry of photography

Front matter

THE

SCOVILL

PHOTOGRAPHIC

Preface

The object of this book is to assist in the abolition of ^‘Photography by Pale of Thumb.”

The study and practice of the “ light science” (as CutJibert Bede playfully styled it forty years ago) ought to be educational as well as recreative. But even from the point of view of the man who cares nothing for photography save as a means of making pictures, it cannot be doubted but that the student who thoroughly understands his tools, and who is able to give “ a reason for every rule,” will turn out a far higher percentage of technically successful work than one to whom all chemicals are merely “ portions of matter” ; and to whom every change, every reaction is “ magic or mystery.”

Familiarity with our modern miracles has, it is true, bred contempt in the minds of many ; but every true craftsman tries to get below the surface of things, and to understand the nature of his materials and the way in which they act and interact.

In my account of many of the chemical processes employed in photography, I have treated the subject historically as well as scientifically. For I think that we ought to recall as often as possible the names of the many workers who have helped to place photography on its present high footing as “ the handmaid of all the sciences ” ; and besides, it will generally be found tliat the pith of a matter is most forcibly presented in the wmrds of the man who was the actual discoverer.

I wish to take this opportunity of expressing the special debt which I owe to photography for the many kind friends with whom it has brought me into relation in both hemispheres; and I wish to thank those friends for their kind appreciation of my labors. If this book — used either as a text-book or as a work of reference — should prove in any way useful to the daily increasing fraternity of the “lovers of light,” I shall esteem myself amply repaid for the time and pains (not inconsiderable) which I have expended upon its

preparation. WILLIAM JEROME HARRISON.

Birmingham, England, hlovember, 1892.

Introduction

There can be little doubt but that tlie photographers of twenty years ago were better chemists than the photographers of to day. Our work is made easy for us by the manufacturers of pure chemicals and excellent dry plates ; but the labors of our predecessors were not without their recompense; and although we have gained much in convenience by being no longer under the necessity of preparing our own chemicals, and studying the behavior of the silver bath, yet we have also, to a large extent, lost the love of experiment and research which enabled the last generation of photographers to make such remarkable advances.

No one wdll deny that a knowledge of chemistry is absolutely necessary to the proper comprehension of photographic processes. Photography is, indeed, but a branch of applied chemistry, so far as most of its manipulations are concerned ; and the photographer who is ignorant of this science is only a worker by rule of thumb.” But chemistry itself has advanced very rapidly — has been almost revolutionized, indeed — during the last quarter of a century. It was with pleasure, therefore, that I acceded — a year or two ago — to the request of the editor to contribute to the Photographic Times a series of articles upon modern chemistry, considered more especially in its relations to photography : these articles were followed by others in which photographic materials and processes were treated from a chemical point of view ; and the present book consists of these contributions, with such alterations and additions as have been found necessary.

The Chemistry of Photography.

CIIAPTEE I.

MATTER AND FORCE.

Existence of Matter. — To all substances which are able to affect any of the senses we give the name of Matter. Thus matter is a general name for everything which helps to compose the crust of the Earth and its liquid and gaseous envelopes — the ocean and the air. Then there is matter external to our Earth. Our eyes tell us of the existence of countless heavenly bodies — the sun, moon, and stars — and these are connected together by a mysterious kind of matter called the ether, of which we know very little, but which tills all the space between.

General Properties of Matter.— the fact that matter always appeals to the senses, we recognize its existence by the fact that it possesses certain general properties, or properties which are common to all kinds of matter.

Thus all matter has loekfit. A cubic foot of air weighs nearly 1 J ounces ; and a cubic foot of the lightest kind of matter known — the gas called hydrogen — weighs nearly tliirty-five grains.

Pimsibility — or the capability of being divided— is another characteristic of matter. Some solids possess this property in a very marked degree ; thus gold can be beaten out so thin that it would take three hundred thousand leaves laid one upon another to make a thickness of one inch. A grain in weight

THE CHEMISTRY OF PHOTOGRAPHY.

of the odoriferoHS solid called musk will scent a room for years^ with little, if any, perceptible diminution in weight. How extremely small the particles must have been, which, proceeding from the musk, spread through the air and so caused the odor !

But it is wBen solids are dissolved in liquids, or heated till they change into gases, that the most remarkable facts about the divisibility of matter become known. A very small lump of sugar will give a perceptibly sweet taste to a quart of water. There must be tiny pieces of sugar in each drop of the water, though they are so small as to be invisible. As much strych- ]iine as can be taken up by the point of a needle will give a distinctly bitter taste to the whole of a pint of water; and a single drop of any of the aniline dyes will color a large quantity of the same liquid.

Extension^ or Enpenetr ability ^ expresses that property of matter in virtue of which it takes up space, or occupies room. Two things cannot be in the same place at the same time.

Porosity implies that all substances possess pores or hollow spaces. Solids vary in their porosity, from those like sponge — which is highly porous — to glass, in which the pores are very few and small. The porosity of liquids can be shown by dissolving a small quantity of salt or sugar in a given bulk of water contained in a glass vessel. When the solid has all dissolved, it will be found that the liquid containing it occupies no more space than it did before the solid was added. We can only explain this by supposing that the solid becomes divided into extremely small parts, and that these tind room in the pores of the liquid.

The porosity of gases can be jaroved by putting a few grains of solid iodine into a glass flask and then carefully corking it. When the flask is gently heated the iodine will be converted into a violet vapor which will fill the flask. Thus the flask will now be full of two gases — air, and iodine vapor — and this can only be by the latter finding room in the pores of the former.

Special Properties of Matter. — But while all matter has certain properties — such as weight, extension, etc. — incommon^

MATTER AND FORCE.

each kind of matter is distinguished by possessing certain properties, which the other kinds do not. Thus the color of matter varies greatly ; and in hardness, solubility, and many other points there is gi’eat diversity. It is the province of the chemist to make himself acquainted with these similarities and dissimilarities of matter, for he is thereby enabled to recognize the different kinds of matter, and to distinguish them one from another.

The Three States of Matter. — Matter can exist in three states — the solid, the liquid, and the gaseous. Moreover it is possible by the addition or subtraction of heat, to change the state of almost every substance. Thus, taking water in the liquid state, we can by raising its temperature to 212 deg. F., convert it into water-gas (or steam) ; while by withdrawing heat until the temperature is reduced to 32 deg. we can cause it to assume the solid state known as ice.

Matter is indestructible. It is impossible to destroy or entirely get rid of any portion of matter whatever. We may break, burn, or dissolve a substance, or change its appearance in many ways, but we cannot destroy it. We can prove this by showing that the weight of the original sulistance is always present. Thus when a candle burns, the matter forming the tallow of which it is composed is not destroyed. Tallow is mainly composed of the two elements hydrogen and carbon. The hydrogen and the carbon each unite with the oxygen of the air to form, respectively, water- vapor and carbonic acid gas. These gaseous products mingle with the air and usually escape unseen, while the candle disappears. But if, by chemical means, we arrest the escaping products, we shall find that their weight will exceed that of the candle, for they will include all the matter which formed the candle, and in addition some oxygen gas from the air. Thus we do not destroy the matter composing a candle by the act of burning it, we merely change it into other substances — water- vapor and carbonic acid gas.

The Structure of Matter. — The discoveries of modern science have led us to believe that matter consists of exceedingly small parts called molecules. When we take a lump of sugar and

THE CHEMISTRY OF PHOTOGRAPHY.

grind it in a mortar we obtain numerons particles of sugar. Although these particles are small, they are perfectly visible to the eye. But shake up some of these sugar particles in water and they become invisible ; the sugar dissolving^ as we say, in the water. It is in the water, but it is now in pieces so small — called molecides — that they cannot be distinguished even by the aid of the most powerful microscope. Thus we regard every portion of matter, whether solid, liquid, or gaseous, as composed of an enormous number of small parts or molecules.

Although we cannot see these molecules, yet mathematicians and physicists have attempted to estimate their size, and Sir William Thomson states that “if a drop of water could be magnified until it appeared the size of the Earth, then the molecules of which it is composed would be visible, and would be of a magnitude somewhere between that of a grain of sand and cricket balls.”

It is needless to say that we can never hope to attain such a power of magnification, and therefore we can never hope to see the molecules. Yet, thanks to the researches of the chemist and the physicist, we are as sure of their existence, and can reason upon their powers and properties with as much certainty as if we could handle and see them individually.

Molecular Motion, — No molecule forming part of any portion of any kind of matter is ever at rest. In a solid the motions of the molecules are necessarily much restricted by the force of cohesion, but in a liquid they have more freedom and can roll over one another; while in a gas the course of any molecule is only checked by its concussion with its neighbors, or by the sides of the vessel or room in wdiich the gas happens to be contained.

Molecular motion is usually of a vibratory or to-and fro motion, like that of the prongs of a tuning-fork. But the vibrations of the molecules do not affect our senses in the same w^ay as the vibrations of the tuning-fork. The impressions which are produced by molecular motion are those of heat and light.

When a piece of iron feels cold it is l>ecause the molecules of

MATTER AND FORCE.

the iron are vibrating very slowly as compared with the molecules of which our skin is composed. When the same piece of iron feels hot, it is because the rapidity of motion of its molecules has in some way or other been greatly increased. Lastly, when the iron becomes red, or even white hot, it is due to the fact that the iron molecules are vibrating with almost inconceivable rapidity, and are then able to produce waves of light in the ether which lies between the iron and our eyes. These waves in the ether traveling through the intervening space, at last reach the retinal expansion at the back of the eye, wdiere they excite fresh vibrations in the optic nerve, and these traveling along that nerve to the brain, produce there the sensation of light.

Changes 'Due to Molecular Motion. — The effect of heat upon a body is to throw its molecules into more rapid vibration. This motion causes each molecule to push away its neighbor, with the result that the body expands or becomes larger.

If the heat be increased beyond a certain point, we have learnt that solids are changed into liquids, and liquids into gases. A still further increase of heat may produce a still more remarkable change. By heating many substances it is possible to convert them into two or more new substances of very different appearance and properties. Thus, if a little of the red precipitate ” of the drysalters’ shops be strongly heated in a glass tube, it entirely disappears and we have in its place two substances known as ^‘oxygen” and mercury.’’ The exj)lanation of such changes belongs to the science of chemistry.

The Forces of Nature. — Although we cannot conceive of force” apart from ‘‘matter,” nor of the existence of matter unacted on by force, yet we believe them to be radically different. Force is without weight — a body wdien hot weighs no more than when cold — and is unable to affect tlie senses except through the medium of matter.

The forces of nature have been classified as follows :

I. — The Physical Forces.

Gravit}^

Electricity,

Magnetism,

Heat,

Sound,

Light,

Cohesion.

THE CHEMISTEY OF PHOTOGKAPHY.

These physical forces are able to act at a distance. To the action of gravity and of light, for instance, no limits can be set ; the effects of these forces extend far beyond even our solar system.

Sound can travel by means of vibrations of the air, but not by the ether, and earthly sounds are, therefore, confined within the limits of our atmosphere.

Cohesion is unlike the other physical forces in being unable to act save at comparatively small distances, but even these distances are wide, as compared with those over which the chemical force is able to exert its influence.

The physical forces do not commonly change the properties or composition of the matter upon which they act. F or example? apiece of iron may be allowed under the influence of gravity to fall from any height ; it may be melted by heat / after cooling it can be magnetised by causing a current of electricity to flow round it ; yet, after all these physical forces have acted upon it, the iron is still unchanged in its nature and properties.

II. The Chemical Foece. — The chemical force stands apart from all the other forces in two respects. It is quite unable to act at even extremely small distances ; indeed it seems necessary to bring the molecules of one body into actual contact with those of another before chemical action can take place between them. For this reason it is hardly ever possible to get two solids to unite chemically ; one or, better, both must be brought into the liquid or the gaseous state.

Then the chemical force always produces a change in the composition, and usually in the properties of the matter upon which it acts. When sulphuric acid, for example, is poured upon sugar, violent chemical action takes place, gases are given off, and a black porous solid remains.

CHAPTER IT.

THE CHEMICAL ELEMENTS.

Definition of Chemistry . — Chemistry is the science which studies the composition of matter, and which endeavors to explain the processes by which changes are effected in its composition.

So great a mass of chemical knowledge has now been accumulated that a lifetime might be spent in the study of a single section of chemistry. In these articles we shall only consider those ])rinciples and facts of chemistry which bear more especially upon photography.

The Two Great Divisions of Chemistry.

The science of chemistry is divided into two main parts ; viz : —

I. Inorganic Chemistry, dealing with substances belonging to the Mineral Kingdom, i. e.^ with matter having no parts or organs ; and, —

II. Organic Chemistry, which treats of compounds many of which exist ready formed in animals or plants — organized beings.

Organic Chemistry is also known as the chemistry of the Carbon Compounds, because Carbon forms a part of every substance of which this division of chemistry treats.

Nature of the Chemical Eh ments. — We are acquainted with about seventy substances, each of wdiich we believe, in the present state of our knowledge, to be composed of one kind of matter only, and that different from all other kinds. These seventy substances are known as the chemical elements. Thus an element may be defined as a simple substance^ composed of one kind of matter only.

Gold is a good example of an element. Out of pure gold no one has succeeded in getting anything but gold.

It is impossible to change any one element into any other

THE CHEMISTRY OF PHOTOGRAPHY.

element. This was the mistake made by the alchemists^ who in the Middle Ages tried so hard to transmute the baser metals, such as lead, into the precious metals gold and silver. The molecules of lead are totally different in their properties from the molecules of gold, and it is equally impossible to change lead into gold, or gold into lead.

Still there is no reason why the number of the elements should not any day be increased or diminished. Within the last ten years six new elements have been found, though all these are of rare occurrence. On the other hand, it is just possible that the continued researches of chemists may end in proving that some of the substances we now consider elementary, are really compounds of two or more other elements. It has even been suggested that there is only one true element, of which all the others are composed in varying proportions and under varying conditions ; but we are still very far from being able to prove this.

TABLE OF THE CHEMICAL ELEMENTS.

Name.

Symbol.

Atomic

Weight.

Name.

Symbol.

Atomic

^Aluminium

AI.

*Gold

Au.

Antimony

Sb.

■^Hydrogen

H.

Arsenic

As.

Indium

In.

113.4

*Barium

Ba.

*IODINE