18 



pound, readily decomposed, and containing phosphorus and sulphur, 

 which elements, possessing as they do a high degree of affinity for 

 silver, probably assume an important part in the decompositions which 

 ensue. 



In the collodion process the same salts of sUver are employed, but 

 gallic acid is no longer applicable as the developing agent : other sub- 

 stances are therefore resorted to, viz. pyrogallic acid, which reduces 

 silver as a black powder, — or sulphate of iron, which precipitates it in a 

 white crystalline condition ; thus by the first agent we obtain the result 

 required for negative impressions, and by the latter positive pictures of 

 great delicacy and beauty. But wQl the substitution of these reducing 

 agents for the gallic acid account for the e.xtraordinaiy and dispropor- 

 tionate rapidity of the collodion process ? Probably not. We must then 

 seek for the solution of this problem in the nature of the organic matter 

 subjected to these conditions; for the cause of dissimilarity is not 

 apparent in the nature of the chemicals employed. Indeed the decom- 

 position in all the silver processes is capable of a ver^' general expres- 

 sion, as follows : — 



Salts of silver, as the iodide, bromide, chloride, &c., are partly 

 reduced by light ; further reduced, and the structure changed by 

 developing agents, as mercury, gallic acid, pyrogallic acid, sulphate of 

 iron, &c. ; while the remaining salts of silver are dissolved in the 

 fixing bath of hyposulphite of soda, or cyanide of potassium. 



To the chemical constitution of albumen (C40, H31, N^, 0^, PSj), 

 reference has already been made ; its composition is highly complex, 

 and it is readily subject to decomposition. Pure paper, which is one 

 of the forms of lignine {C24, Hjo, Ooq), is a much more simple and stable 

 compound, and probably undergoes no decomposition during the 

 photograpliic process ; but the poi'ous character of its texture affords a 

 kind of mechanical force, called catalysis, which, as in the case of 

 spongy platinum and burnt charcoal, assists in overcoming the resis- 

 tance of chemical affinity, and thus contributes to the reduction of the 

 metal. In this view of the case we might expect a less raj^id though 

 similar action from paper as that obtained from albumen. Pure 

 lignine, however, is not to be obtained in the form of paper, and the 

 starch or size employed in its manufacture (as well as extraneous sub- 

 stances introduced), causes considerable variation in the sensitiveness of 

 the paper under different processes. Many modifications of the calo- 

 type have therefore been adopted, and their success depends in some 

 measure on the choice of the paper most suitable to the particular 

 process. 



