396 
POPULAR SCIENCE REVIEW. 
plates lias been brought during the past year, that all baths, 
washings, preservatives, and organifiers may now be entirely 
dispensed with, the sensitive emulsion being so composed as to 
contain within itself everything that conduces to the rendering 
the sensitive film complete. 
A practical difficulty that long existed in the preparation of 
a sensitive emulsion lay in hitting so exactly the combining 
equivalents of the salts used in sensitising the collodion as to 
leave neither of them in excess. The bromide of silver being 
formed in the collodion by the decomposition of nitrate of silver 
and a soluble bromide, there was a difficulty, well-nigh amount- 
ing to impossibility, in so combining them as not to allow either 
to predominate. To make a combination in water is easy ; but 
not so is it in a thick, viscid liquid like collodion. If the silver 
be left in excess, fogging of the negative is certain to follow, 
unless a restrainer like mineral acid be added ; if, on the other 
hand, the bromide preponderate, the plate is insensitive in pro- 
portion to that excess ; and hence for some time it was customary 
to have a much larger proportion of bromide present than was 
necessary, and after coating a plate with an emulsion of that 
kind to confer sensitiveness upon it by washing with water, so as 
to effect the removal of the free bromide which acted so power- 
fully as a retarder. A further necessity for having the free 
bromide removed was foimd in the fact that when an image is 
impressed upon a film containing it, unless that latent image 
be quickly developed, it is rapidly destroyed by the soluble 
bromide. 
It has already been said that a result of the decomposition 
arising from immersing a salted collodion plate in a nitrate of 
silver bath is not only the formation of bromide of silver, but 
also of the nitrate of the base of the haloid salt. The presence 
of this nitrate in a wet process is of minor consequence, but far 
different is it when it exists in a collodion film that is to be 
dried ; for on crystallising out, as it must necessarily do if pre- 
sent in a moderately large quantity, it disintegrates the film ; 
and even if quite innocuous in a chemical sense, its presence is 
fatal in a physical point of view. 
The maleficent influence of the crystallisable salt resulting 
from the decomposition had previously been noted by Mr. J. 
King, of the Bombay Civil Service, during a brief visit to this 
country ; and when making experiments with gelatine instead of 
collodion as a vehicle for the sensitive bromide, he, by a happy 
application of the principle of dialysis, succeeded in effectually 
removing every crystallisable compound, as will be presently 
shown. The method subsequently adopted by Mr. Bolton in 
effecting a similar removal on behalf of collodion was very 
complete, inasmuch as he not only eliminated the crystallisable 
