1878, when, it having been found that silver bromide precipitated in
a gelatine solution and boiled for several hours becomes much more
sensitive to light, that the remarks of Stas was recalled. Today these
observations have become of the greatest importance to practical
photography. They have led to the preparation of the silver bromide
gelatin emulsion and the silver bromide gelatin plates, which are twenty
times more sensitive than the silver iodide collodion plates, and have
become indispensable when impressions are to be taken in a dim light.

The extraordinary sensitiveness of silver bromide in gelatin seemed the
more remarkable since it was known that silver bromide in collodion is
only moderately sensitive. The explanation was sought for in various
directions, but as the result of numerous investigations it appears
that the chief cause of the difference is the presence of different
modifications of silver bromide. From a consideration of the work
already done on the subject, Vogel suspected that silver bromide
precipitated in an aqueous colloidal liquid would have notably different
properties from silver bromide precipitated in an alcoholic colloidal
solution. Silver bromide was prepared in many different ways. Emulsions
were made in bromide solutions containing gelatin or collodion (the
former aqueous, the latter alcoholic), some with the aid of heat, others
without. Part of the emulsion was then poured upon plates kept at a
moderate temperature and dried. The remainder was boiled or treated with
ammonia before being applied to the plates. He also precipitated silver
bromide in dilute gelatin or collodion solutions, allowed it to settle
completely, washed the precipitate, and mixed it with a new portion
of gelatin or collodion before applying it to the plates. Finally he
precipitated pure silver bromide, in the absence of all colloids, by
means of pure aqueous or alcoholic solutions of bromides and attempted
to bring this upon plates, using gelatin or collodion as a cement.