PHOTOGRAPHIC PROCESSES, 89 



The blackening of the plate with ammonia, after 

 whitening with mercury, is carried on in the open 

 air, at all events in another apartment, as it would 

 spoil all subsequent manipulation with the wet plate 

 process. 



It has been recommended to add citric acid to 

 the alum solution when pyro is used and oxalic acid 

 with ferrous oxalate, to remove any staiu due to the 

 developer on the resulting negative. There should 

 be no stain after development, and we cannot too 

 strongly condemn the use of acids in any of the 

 solutions used subsequent to development. Without 

 entering into the theoretical grounds of this advice, 

 we simply state that nothing tends more, except 

 traces of hypo, to impair the permanency of the 

 negative. 



