The chemical theories assume the decom position of the silver bromide, and that bromine is given off ; and if the formula for silver bro mide be written Agarx in which x merely repre sents a given number of atoms of each element, the latent image could be described as in which y is merelya certain number of bromine. In support of this theory it must be noted that it is by no means so easy to destroy the latent image ; nitric acid, potassium cyanide, f erri cyanide, and acid bichromate do not entirely destroy it, and it is possible to fix an exposed plate, and yet physically develop it afterwards so as to obtain a good image.
Other chemical theories are that metallic silver is produced, but this has been proved to be untenable, as strong nitric acid would dissolve silver, but does not destroy the latent image. Another chemical theory is that a subsalt is formed, but the latter forms a solid solution with the silver bromide in varying proportions.
At present, at any rate, no definite conclusion as to the nature of the latent image can be formed, and the position is probably best summed up in the words of Dr. Joly in his presidential address to the Photographic Convention in 19o5 " The latent image is built up of ionised atoms, or molecules, the result of the photo electric effect upon the illuminated silver haloid, and upon these ionised atoms the chemical effects of the developer are subsequently directed. It may be that the liberated electrons ionise molecules not directly affected, or it maybe that in their liberation they disrupt complex mole cules built up in the ripening of the emulsion.
With the amount that we have to go upon, we cannot venture to particularise. It will be said that such an action must be in part of the nature of a chemical effect." In connection with this subject it should be pointed out that Dr. Schafer, of Berlin, has been able to obtain photo-micrographs of the image on silver bromide, and these prove that there is some sort of thread formation or protrusion of a filament from an exposed sensitive grain, which would certainly point to disruption of the com plex ; but this might agree with both the phy sical and chemical theories, as in the former case it would represent the disruption of the particle, and in the latter case the extrusion of the sub bromide.
Meldola has assumed the formation of an oxy haloid, but the objection to this is that the latent image can be formed under gases and in the presence of substances which preclude entirely free oxygen, which would be necessary for this formation.
From a series of experiments on development with the indoxyl compounds, Homolka advances the theory that the latent image is an equi molecular mixture of sub-bromide and per bromide of silver, which is formed according to the equation :— 3AgBr = silver bromide The existence of per-bromide has also been tenta tively established by Lumiere and Seyewetz, but further proof is required.