The result of all these experiments is that there are essentially two
modifications of silver bromide, the one being obtained by precipitation
in aqueous, the other in alcoholic solutions. The first, on account of
the position of the maximum of sensitiveness for the solar spectrum, he
calls blue sensitive, the other, for the same reason, indigo sensitive.

It is of no consequence whether the aqueous or alcoholic solution in
which the silver bromide is formed contains gelatin or collodion, or
whether the precipitation is effected with excess of bromide or of
silver nitrate. It makes no difference whether the solution is hot or
cold, or whether the silver bromide is treated with ammonia or
whether it is boiled or not. The only necessary condition is that in
precipitating indigo sensitive silver bromide the solutions must contain
at least 96 per cent of alcohol. From aqueous alcoholic solutions blue
sensitive silver bromide is precipitated.

Besides the difference of sensitiveness toward the solar spectrum, these
modifications of silver bromide exhibit other characteristic differences
in properties which indicate beyond a doubt that they are two
essentially different modifications of the same substance. Among these
are, 1st. Their unequal divisibility in gelatin or collodion solutions.
The indigo sensitive silver bromide cannot be distributed through a
gelatin solution, while the blue sensitive modification does so very
readily. 2d. Their unequal reducibility; the blue sensitive silver
bromide being reduced with much greater difficulty than the indigo
sensitive variety. 3d. Their different action toward chemical and
physical sensitizers. 4th. Their different action toward photographic
developers. 5th. Their different action under the influence of heat.
The blue sensitive variety if heated under water has its sensitiveness
perceptibly increased, while the other is not changed by such treatment.