120
THE LATE COLONEL RUSSELL'S
of the present time, and there can be no doubt that the success of
Russell's dry plates largely contributed to its introduction.
Important as was the advancement rendered possible by the
use of tannin as a preservative, this is by no means the only note-
worthy photographic achievement with which our late member must
be credited. Only a year after the introduction of his dry plate
method, Col. (then Major) Russell made known the use of ammo-
niacal solution of pyrogallol (pyrogallic acid) as a developing agent,
a discovery which marks a distinct epoch in the history of the art,
and of which the practical utility makes itself felt at the present time.
Prior to the year 1862 the developers in use by photographers were
of an acid character. Gallic acid mixed with silver nitrate had been
used by Fox Talbot for developing his paper prints as far back as
1840. Pyrogallol was known to be a more powerful developer than
gallic acid both by Liebig and Regnault, in 1851. The use of ferrous
sulphate, the active substance in the "iron developer" of the old
collodion process, was known to the late Robert Hunt, in 1844.
When dry plates first came into vogue acid development had to be
resorted to as the only known method of bringing into visible
existence the invisible image formed by the action of light on the
photographic plate.
For certain chemical reasons, which need not be entered upon in
detail here, an acid developer is eminently unsuited for dry plate
work. The early experimenters in this field were not only at a dis-
advantage as regards the comparative insensitiveness of their plates,
but this slowness of action could not be compensated for by using a
stronger developer, because the free silver nitrate, which is an essen-
tial constituent of an acid developer, would be reduced, and metallic
silver deposited all over the film; that is to say, the plate would
become fogged. It is, of course, impossible to trace the precise
origin of the idea which led to the use of alkaline pyrogallate. It
appears that experiments had been made by Anthony, in New York,
in 1862, having for their object the increased sensitiveness of dry
plates by exposing them to ammonia vapour before use, and about
the same time Glover attempted to increase the sensitiveness by the
action of ammonia vapour after exposure and prior to development.
Whether Col. Russell was aware of these experiments there are at
present no means of ascertaining, but it is not improbable that his
attention may have been directed thereby to the use of ammonia.
The fact that dry plates could be developed by means of a neutral