water, containing in every 12 ounces of solution 11/4 ounces of the salt.

The next material we require is a soluble bromide. I have here specimens
of various bromides which can be employed, such as ammonium, potassium,
barium, and zinc bromides; as a rule, however, either the ammonium or
potassium salt is used, and I should like to say a few words respecting
the relative efficiency of these two salts.

1. As to ammonium bromide. This substance is a highly unstable salt.
A sample of ammonium bromide which is perfectly neutral when first
prepared will, on keeping, be found to become decidedly acid in
character. Moreover, during this decomposition, the percentage of
bromine does not remain constant; as a rule, it will be found to contain
more than the theoretical amount of bromine. Finally, all ammonium salts
have a most destructive action on gelatine; if gelatine, which has
been boiled for a short time with either ammonium bromide or ammonium
nitrate, be added to an emulsion, it will be found to produce pink
fog--and probably frilling--on plates prepared with the emulsion. For
these reasons, I venture to say that ammonium bromide, which figures so
largely in formulae for gelatine emulsions, is one of the worst bromides
that can be employed for that purpose, and is, indeed, a frequent source
of pink fog and frilling.

2. As to potassium bromide. This is a perfectly stable substance, can be
readily obtained pure, and is constant in composition; neither has it
(nor the nitrate) any appreciable destructive action on gelatine. We
prepare, then, a solution of potassium bromide in water containing in
every 12 ounces of solution 1 ounce of the salt. On testing it with
litmus paper, the solution may be either slightly alkaline or neutral;
in either case, it should be faintly acidified with hydrochloric acid.