The only necessary condition is that in precipitating indigo sensitive silver bromide the solutions must contain at least 96 percent 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. 2nd - Their unequal reducibility; the blue sensitive silver bromide being reduced with much greater difficulty than the indigo sensitive variety. 3rd - 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.

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