Page:Popular Science Monthly Volume 22.djvu/412

398 you can dab it out, and, if you want to keep it back, you can put a little water over the place. There is no process like the paper process to please an artist. Now, what is the meaning of the development in this process? This morning I was in my laboratory, and I saw lying on the bench a feeble negative which I had badly developed, and which I had fixed with hyposulphite of soda. On taking it up, I found the salt had crystallized over the surface in the most beautiful manner; and I do not think I could point out to you anything which would give you a better idea of what development is than those crystals. When you have silver precipitated from a solution by any means whatever, you have it always in a crystalline form, and, as all crystals possess polarity, so crystals of silver possess polarity; and where one silver particle is deposited, there another silver particle will deposit. I look upon this as a physical development; we have a crystalline action going on during development, and nothing else. The iodide of silver is altered into a subiodide, and this, like the pole of a magnet, attracts the precipitating silver, and from that time, where the silver is deposited, other crystals of silver are deposited. That is what I call physical development.

There is another kind of development which some call chemical development; it is shown by a change in the color or material of the substance acted upon, and not by a building-up process, such as we have just had illustrated. The process may be illustrated in the development, by means of silver nitrate, of a picture which has been printed on nitrate of uranium. The picture is formed by silver oxide reduced by the particles of uranium nitrate which have been acted upon by light, and by nothing else. The silver oxide reduced is an exact equivalent of the uranium salt which has been acted upon by light. This differs from the previous process in that the gallic acid, in the one case, reduces the silver solution to the state of metallic silver; and, in the other case, the uranious image itself reduces it to the state of silver oxide.

Another mode of development, called chemical development in Germany, may, I think, more properly be termed alkaline development. Its theory is, that when you have a strongly oxidizing agent in the presence of an alkali and a silver compound, solid or in solution, the last will be reduced to the metallic state. Such an oxidizing agent we have in pyrogallic acid, and the alkali generally used is ammonia. Now, this kind of reduction is evidently useless, unless it can discriminate between a compound which has been acted upon by light and one which has not. When pyrogallic acid is used, in order to make the discrimination, something more has to be added as a restrainer to cause the reduction inducing the change to take place only in the part acted upon by the light. A solution of the bromide of an alkali is generally used for this purpose. Without a restrainer, the tendency is for those parts to be first reduced, but the action extends to that