398 THE POPULAR SCIENCE MONTHLY. 



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 crys- 

 tals. 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 devel- 

 opment, 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 equiv- 

 alent 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 develop- 

 ment. 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 bo 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 discrimi- 

 nate 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 



