Hermann W. Vogel has made a comparative study of the properties of
silver bromide, obtained by precipitation in an aqueous solution of
gelatin, and those of the same compound prepared by precipitation in an
alcoholic solution of collodion. In 1874 Stas called attention to six
modifications of silver bromide. One of these, granular bromide of
silver, obtained by boiling the flocculent precipitate for several days
with water, he stated, was the most sensitive to light of all substances
known; exposure for two or three seconds to the pale blue flame of a
Bunsen burner being sufficient to blacken it. Important as this fact was
for photographers it was not applied for years, and it was only in
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.
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.
A direct transformation of one modification into the other has not yet
been accomplished. The effect of the light upon these substances is
incipient reduction, and we might hence suppose that the more reducible
indigo sensitive variety would be the more sensitive to light. But
this is not the case, because it is not chemical reducibility, but the
absorption power for light that is of the greatest importance. Now the
blue sensitive silver bromide has a greater absorption power than the
indigo sensitive variety, and hence its greater sensitiveness. Silver
chloride prepared by methods similar to those used in making the two
forms of bromides was also found to exist in two modifications. One is
designated as ultra violet sensitive, the other as violet sensitive
silver chloride.--_Amer. Chem. Jour_.
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