These dry plates are, when required, placed in the camera and exposed to the image formed by the lens. The effect of this is that a latent or invisible image is impressed on the film of the plate, which may be afterwards rendered visible by development. What is the change that occurs to produce this effect? Chemists and scientists are as yet divided upon the subject.
The theory that till recently has met with general acceptance, and which Captain Abney gives in his work, is that the silver bromide is reduced by light to sub-bromide (Ag,Br). The formation of this sub-bromide depends, of course on the active rays from different parts of the object.
Another theory is that electricity is sufficient to account for the formation of the latent image. .
C. J. Leaper says / that the weight of indirect evidence (necessarily indirect from the diffi culties surrounding the problem to be solved) seems to point to the formation by light of silver oxybromide It is well known that bromine is evolved during exposure,§ and on the assumption that oxygen is at the same time absorbed, we may express the change by the following equation :— 4AgBr+ Br.
Carey Lea, in America, has recently published his investigations in what he terms the "photo-salts of silver," which he has prepared, and which he states are producible from the haloid salts of silver (the chloride, bromide and iodide) by the action of light, as well as by purely chemi cal means, his photo-bromide consisting of silver sub-bromide in combination with silver bromide, the sub-bromide forming but a small portion of the whole. This photo-bromide is insoluble in strong nitric acid, whilst the sub-bromide itself is very soluble in it, and this theory receives strong confirmation in other ways ; it appears to be the one most probably correct" The latent image produced, we have next to consider the action of the developing agent. All developers are energetic oxygen absorbers, it being upon this property that their power of developing is based. The commonest developer is the pyrogallic acid.
Pyrogallic acid is an erroneous term for what is not an acid at all, but more correctly pyro gallol—its chemical name is tryhydroxy-benzene. It is a powerful and greedy absorber of oxygen.
It is only active in an alkaline medium. Combined With ammonia, its action on the silver sub bromide is to reduce the silver to the metallic state, ammonium bromide being formed thus— When ammonium bromide is used with this developer as a restrainer, its action appears to con sist in diminishing the amount of silver sub-bromide capable of being dissolved by the ammonia. If strong ammonia is used more silver bromide is dissolved. If strong pyro solution is used a more rapid reduction of the bromide will take place, resulting eventually in fog.
Another developer largely employed is the ferrous oxalate, composed of ferrous sulphate, mixed with potassic oxalate, forming ferrous oxalate and potassic sulphate, the ferrous oxalate being dissolved by the excess of potassic oxalate.
This also acts as a powerful reducing agent, becoming oxidized to ferric salt.
+Fe23r6+2Fe,(C,04)9.
Hydroquinone or paradihydroxybenzene is another favorite developing agent. It is closely allied to pyro, its formula being On development it becomes oxidized to quinone.
• Besides the various reducing agents used as developers, the alkalies—ammonia potash and soda and their carbonates—are used as accelerators. Ammonia is most energetic, but is liable to produce stains.
The latent image having been developed and become visible, the next operation is to fix the image, or, in other words, to make it permanent by dissolving out the silver bromide unacted Upon by the light. If this were not done it will be obvious that the image would be reduced on exposure to the light. The fixing salt usually employed is sodium hyposulphite. Potassium cyanide, common salt, ammonia, and other salts can also be employed. Hyposulphite of soda, or, as it is more correctly termed, sodium thiosulphate, is considered to be sodium sulphate in which one atom of oxygen has been replaced by sulphur.
Na,S,0,.
The action of this salt upon the photographic film is one of double decomposition.
Na,S,O, + 2AgBr=2NaBr + Ag,S,0„.
The silver hyposulphite formed combines with excess of hypo forming the soluble double salt.