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POPULAR SCIENCE REVIEW. 
sium and nitrate of silver, each previously dissolved in water, 
are added, and after a sufficient time is allowed to enable them 
to re-act upon each other the solution is poured into a dialyser, 
which is placed in a vessel of warm water. In a few hours the 
whole of the crystalline salts will he found to have passed through 
the septum of the dialyser. Here, then, is a means bv which a 
gelatine film may he loaded with bromide of silver, free both 
from excess of either of the two salts employed to effect the 
decomposition, as well as from the nitrate of potash which re- 
sults from that decomposition. 
Only a short time has elapsed since the foregoing discovery, 
but during that time great progress has been made. Mr. E. 
Kennett has further simplified the preparation of the gelatino- 
pellicle by making itself the septum, and doing away with a 
separate dialyser. He mixes with the gelatine the necessary salts, 
pours the whole out into a flat dish, and when the gelatine has 
set, but not become desiccated, he merely places it in a vessel of 
cold water, by which everything of a crystallisable nature is 
removed ; after which the sensitive colloidal body is dried, cut 
up into shreds, placed in opaque packets, and may be trans- 
ported to any part of the world, ready to be converted into a 
highly sensitive emulsion by the addition of warm water. 
A singular fact, for the elucidation of which no tenable 
hypothesis has yet been brought forward, is that a gelatin o- 
bromide emulsion film is extremely sensitive, much more so 
than one of collodion. The quality of negative obtained on 
plates so prepared is most excellent, while the atoms of silver 
of which the image is composed, when examined under the 
microscope, partake even still more of the nature of a stain than 
a collodion emulsion negative. This process is still in its 
infancy, but from its having been brought to such a state of 
perfection during the brief period of its existence it is safe to 
predicate that further improvements will rapidly be effected. 
The way by which we successfully develope gelatine plates is 
similar to that already described for collodion plates, omitting 
the application of alcohol and substituting for it a rinsing with 
water. 
Although gelatine at present possesses such a marked advan- 
tage over collodion in respect of sensitiveness, it is difficult to 
work with it in hot weather ; and the emulsion must be used 
soon after it is made, otherwise putrefaction will set in ; and it 
is unfortunate that the addition of any of those antiseptics 
which prevent putrefaction affects the excellence of the emul- 
sion. Salicylic acid is being tried for this purpose as well as 
some preparations of camphor ; but while we hope much from 
their agency, our experiments with them are not sufficiently 
advanced to warrant the results being recorded in this article. 
