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earlier, and till quite lately common, application of the 

 nitrate of silver bath solution, only differed from what was 

 called the wet plate process in interposing an interval 

 between the sensitizing of the plate, by dipping it into the 

 bath, and its exposure in the camera, which with wet plates 

 had necessarily to follow very speedily upon the former. 

 This was achieved by coating the sensitized plate,- after 

 washing it in water, with a preservative, generally composed 

 of some familiar household decoction. Thus tea, coffee, 

 beer, albumen, and gum have each been used as a pre- 

 servative. 



But the other group of collodion dry plate processes was 

 dependent upon a different reaction ; one, namely, in which 

 the sensitive silver salts were formed in the collodion by the 

 application of a solution of silver nitrate to it, before coating 

 the plate, and not (as with the tirst mentioned) by dipping the 

 plate, when coated, into a bath. This was termed the 

 preparation of an emulsion, and this process it is which, 

 with the substitution of gelatine for collodion, has given us 

 the nearly perfect material now at the command of pho- 

 tographers. I propose to trace briefly the preparation of a 

 collodion emulsion, with the view of comparing it with that 

 in which gelatine is employed. 



The first step was the impregnating (if I may be 

 allowed the expression) of the solution of pyroxyline and 

 ether in alcohol, which constitutes collodion, with a soluble 

 bromide ; that is a compound of bromine and some other 

 element from which it will readily part company in favour 

 of the silver contained in silver nitrate — in its turn a readily 

 decomposed compound of that metal. This was done by 

 dissolving the bromide in a known proportion in plain col- 

 lodion, and then adding to the solution thus formed a given 

 weight of silver nitrate, also dissolved in alcohol. The 

 immediate result of this addition was the formation of 

 bromide of silver, the element previously in combination 

 with the bromine deserting it, to unite with the nitrate left 

 free by the separation of the silver, and forming a new 



