Not lightly have we approached the task of writing an account of the chemical changes involved during the exposure and subsequent development of the ordinary dry plate. At the very start we are baffled. The latent image is invisible, and though, without doubt, it contains less bromine than the ordinary bromide of silver, still the loss of weight is so small that all the efforts of chemists in devising more and more ingenious methods for detecting this loss have hitherto been fruitless. The unhappy chemist is deprived of his balance, the one wholly reliable source of evidence. He can only fall back on indirect methods of reasoning, based on analogy. And he finds no analogy to help him. He dare not point to the compounds of silver with fluorine, for his brethren would at once send him an elementary text book on the non-metals, with a blue-pencilled paragraph to the effect that the lowest member of a family of elements always differs so widely from the other members, that no analogies can safely be drawn from its behaviour.

Defeated here, a gleam of hope flashes across his mind. He remembers that there is gelatine on a photographic plate. But this fitful gleam vanishes at once, leaving him in utter darkness. We do not know anything about gelatine. Most Organic Chemistry books are silent concerning gelatine: others give only a few lines, which tell him very little more than every carpenter knows. Humbly he goes to the gelatine manufacturer and asks him for information. But the manufacturer knows no more about gelatine than the chemist does - the manufacturer, in fact, cannot depend on two batches of gelatine made under identical conditions, turning out alike.

The Latent Image

Only one positive statement can be made concerning the latent image, that a smell of bromine accompanies its formation; and one conclusion may apparently be safely drawn. From the smell of bromine emitted by a plate after an exposure to light, we conclude that a certain amount of dissociation of the silver bromide has taken place. Two theories are current in explanation of this. The first theory states that half the bromine splits off, leaving a substance more complex in structure than the original silver bromide.

4AgBr= Ag4Br2 + Br2 4AgBr = 2Ag2Br + Br2

Similarly, with the chloride and the iodide of silver, and to these hypothetical halides have been given two names - (1) sub-halides, (2) photo salts. According to the second theory, all the bromine leaves the silver bromide, leaving a latent image of metallic silver in one of its allotropic forms. 2AgBr = 2Ag + Br2. Neither of these theories can be proved. Such a substance as (Ag2Br)n 1 has never been prepared, nor does the chemistry of silver point to its existence. A fluoride of the constitution (Ag2F)n is known. But fluorine is often a diad, whereas the valency of the other three halogens is always an odd number.

On the other hand, the substance forming the latent image seems to differ very widely in properties from any allotropic form of silver yet prepared. To cite only one difference: dilute nitric acid does not destroy the latent image, even on warming the solution.2

The fact that bromine water does destroy the latent image points equally well, either to the presence of metallic silver, or of the sub-halide.

1 Possessing the properties of the latent image.

2 All the allotropes of silver are unstable, passing readily into the ordinary form, which is dissolved by nitric acid.

Ag+Br = AgBr

Ag4Br2 + Br2=4 AgBr

Nor does it help matters forward very much for the supporters of either theory to maintain that the properties of silver sub-bromide, or of silver, are profoundly modified by the presence of gelatine. This is doubtless true. But if we ask the theorist "what next? " he has no other resource, owing to his ignorance concerning gelatine, than to invent for it a number of properties, which seem to him to fit in with his theory. We will leave him to it.

On the whole, as Luther has shown, the metallic silver theory is simpler. The cardinal objection to the sub-halide theory is that it began life as a blind guess, which gained currency by reason of its plausibility.

To conclude, we can only assert of the latent image that it consists of some unknown substance, which is acted on by the developer at a different rate to the unchanged silver halide, and on that account acts as a catalyst. We cannot say whether it is acted on more, or less, rapidly, but shall find this point merely enhances the pleasure of sketching the theory of development.

Action Of Light On The Plate

Before proceeding to discuss development, we should like to clear the air a little as to the chemical effect of light. The actinic power of light is so often spoken of as if it were something not-to-have-been-expected, some new manifestation of energy. And this while two facts concerning light arc common knowledge. We have but to mention the name of Clerk Maxwell and the reader at once remembers that light is an electrical phenomenon.

One expects some chemical change to take place when an electrical disturbance is propagated through an ionised solution.1

1 Research has shown part of the silver halide on the photographic film to be in solid solution, and slightly ionised.

The other well-known fact is that heat is developed when light is absorbed by a medium. This local heating will have two effects. It will first increase the number of free ions, thereby aiding electrolysis. Secondly, it will tend to vaporise any bromine or iodine that may be liberated, and by thus removing it from the sphere of chemical action, render recombination with the silver impossible.