The iodide of potassium in the film produces with the nitrate of silver in the nitrate bath a double decomposition, iodide of silver and nitrate of yotass being formed. The latter salt appears to be inert in the venous operations.
On exposure to light, the iodide of silver and free nitrate in the film are reduced, and an image formed, which, in the collodion process, is too faint to be visible.
The developer, when mixed with the free nitrate on the plate, forms an unstable compound, extremely liable to be decomposed, and only preserved for a few minutes from spontaneous decomposition by the organic acid which is added to it. When this is poured over the reduced image, the reduced particle,s act as centres of attraction, which first decompose the pyro-gallo-nitrate contact with them, and then cause it to accumulate atom by atom upon them, in this way causing the image produced by light to acquire fresh material, and become visible and intense.
The fudng solution dissolves the iodide of silver, forming, in the case of cyanide of potassium, a double cyanide of silver and potassium together with iodide of potassium, and, in the case of hyposulphite of soda, a double hyposulphite of silver and soda, together with iodide of sodium.
The exact chemical composition of the blacks of the image is not known. The red substance formed in the early stage of the develop ment is probably an organic subsalt of silver, and the black substance precipitated at the end of the development either metallic silver, or a compound of metallic silver with carbon. If an unvarnished negative be dried before the fire, and then rubbed lightly with a piece of leather, the material of the image exhibits exactly the appearance of polished metallic silver. This is no proof, however, that a small quantity of carbon may not be present, because in the case of steel, (which is a carbide of iron,) the metallic appearance and lustre is perfectly preserved. It is not unlikely that, in the decomposition of gallo-nitrate, or pyro-gallo-nitrate of silver, a compound of meta-gallic acid with suboxide of silver is first produced, and afterwards reduced either to metallic silver, or carbide of silver. The matter is, however, at present very obscure.