that of the gelatine layer. The mixture is poured from a small
teapot, at the opening of which has been adapted a bent glass tube about
three-sixteenths of an inch in diameter, between the rod and the lath, so
that by a simultaneous motion, one can equalize the gelatine as it is
poured on. When the gelatine is set the paper is hung up to dry. In
drying, the gelatine contracts, and, necessarily, causes a deformation of
the tissue, which curls up at the edges and loses its planimetry. To
prevent this, while the gelatine is almost dry, the tissue is placed under
pressure until quite desiccated. Dumoulin advises to apply on the film,
while still soft and tacky, a wooden frame, which, by adhering to it.
keeps the tissue perfectly plane as it dries.

[Chardon's method of coating]

_Sensitizing.--_The tissue is sensitized in a bath of potassium bichromate.
The degree of concentration of the bath, which varies from 2 to 5 per
cent. of water, is important. The tissue sensitized in a weak bath is
less rapidly acted on by light and yields more contrasts than when imbued
in a concentrated one. The former should consequently be employed for
printing weak negatives, and the latter for those which are intense. A
bath compounded with 30 parts of potassium bichromate, 1,000 parts of
water and 2 parts of aqueous ammonia, is used for printing negatives of
the ordinary intensity, the tissue being, then practically of the same
sensitiveness, a silvered paper insolated to obtain a print not
over-exposed. For intense negatives the ammonia should be discarded and
replaced by the same quantity of chromic acid.

The time of immersion has also a certain influence on the results. The
less the tissue is allowed to absorb the solution the less sensitive it
is, but also the more the tendency of the half tints to be washed off
during the development. Generally the tissue should remain immersed until
it lies flat and the edges just commence to curl up, unless white and
black impressions are desired, but even then it is preferable to operate
as said above, using a bath at 2 per cent.

For use the bichromate bath should be cooled down to 15 deg. C. (59 deg.
Fahr.), and much lower in summer, say 10 deg. C. (50 deg. Fahr.), and kept
at about this temperature by placing pieces of ice around the tray. At 20
deg. C. (68 deg. Fahr.) the prints are more or less granulated; above this
the gelatine is softened and the reticulati