296 



that the first step in ''fixing" is always the formation of the 

 silver sodium salt given in equation vi., and that further 

 treatment in the sodium thiosulphate solution turns this over 

 to the silver sodium salt given in equation vii., which salt 

 is the soluble one. 



It will now be seen, that after ''fixing," it is necessary 

 to get the print free from : — 



1. Free sodium thiosulphate. 



2. The silver sodium salt in equation vii. 

 2. The sodium bromide. 



It is also quite possible that a certain amount of the insoluble 

 silver sodium thiosulphate may remain in the film, not having 

 been turned over to the soluble salt. 



Now, for a moment, consider another reaction^ which is- 

 the basis of "reduction" — i.e., reduction of density — in. 

 photographic silver images. 



Wlien a silver image is placed in a solution of sodium 

 thiosulphate and potassium ferricyanide, the following re- 

 actions take place (9) : — 



Ag, + 2K3F6 (CN) e = Ag,K,Pe (CN), + K.Fe (ON), (viii.) 

 Ag,K,Fe (CN) , + SNa.S.O, = Na,K,Fe (CN) , + Ag.Na, 



The potassium ferrocyanide formed in the first equation is 

 soluble in water, a.nd the silver potassium ferrocyanide dis- 

 solves in the sodium thiosulphate solution according to the 

 second equation, giving the soluble silver sodium salt. Hence 

 the density of the silver image is reduced. 



Now it was found that, after "fixing," three soluble 

 salts, and possibly one insoluble salt, had to be considered 

 as associated with the silver image. In both the ordinary 

 sulphide, and the copper methods of toning, potassium ferri- 

 cyanide is employed. Heiice it will be readily seen from 

 equations viii. and ix., that if any sodium thiosulphate 

 remained, that "reduction" of the silver image could go on, 

 and so be responsible for the trouble. Prints have behaved 

 unsatisfactorily, however, especially with the copper method 

 of toning, in spite of most prolonged washing, which ^would 

 certainly be credited with having removed the soluble salts. 

 If prints are taken from the wash water at intervals, where 

 washing is accomplished by keeping the prints moving in 

 running water, and these prints are then toned, it is found 

 that the tones improve as the period of their washing^ increases. 

 Washing in excess of one and one-half hours, however, seems 

 to have no further effect, so far as concerns its influence on 

 the resulting tones. It would appear, therefore, that some 



