The ferrotype and how to make it - The Grian Press

COLLODION. 59
the same time perfectly tough and easily re-
movable from the plate if desired. These pro-
portions are, however, more satisfactory during
the season of cold weather, as it is found that the
film sets too quickly in hot weather, when evapo-
ration is so much more rapid. The surprising
improvement caused by the addition of a certain
quantity of alcohol, is referable to causes partly
chemical and partly mechanical, for, on examining
the films, it will be found in a sample containing
I alcohol, and occasionally, when a larger propor-
tion is used, that on immersion in the bath the
iodide of silver is formed on the surface, and can
be entirely removed by friction without destroy-
ing the transparent Collodion film below, while
in those Collodions containing a larger propor-
tion of alcohol (one-half or even more) the
iodide of silver is wholly in the substance, and
in this state possesses the utmost of sensitiveness.
This difference of condition is owing to the very
sparing solubility of ether in water, which, in the
first case, prevents the entrance of the nitrate of
silver into the film, consequently the iodide and
silver solutions meet on the surface, but on the
addition of alcohol in a greater proportion, its
solubility enables the two to interchange places,
and thus the iodide of silver is precipitated
throughout the substance- of the Collodion film
in a state of the utmost division.
It is most important, in the selection of ether
and alcohol, to form a solvent for gun-cotton to