on greater; at 25 deg. C. (75
deg. Fahr.) it may dissolve.

The addition of alcohol to the bichromate bath--sometimes recommended to
harden the film and allow it to stand a higher temperature, and to hasten
the desiccation of the tissue--is objectionable, for the spirits tend to
reduce the bichromate, which is transformed into the green salt, and,
therefore, a partial or complete insolubilization of the gelatine is the
result.

Aqueous ammonia added to the sensitizing solution has for its object to
permit one to keep the sensitive tissue for a somewhat longer period, but
it renders it less sensitive. If enough be added to turn the solution
yellow weak prints are obtained.

The bichromate bath should be renewed often. It does not keep owing to the
presence of gelatine and other organic matters which it dissolves and
which cause the reduction of the chromic salt even in the dark. The
tissue prepared in such a bath is not very sensitive and the image
develops with difficulty, and even cannot be developed at all.

As said above, the tissue is well sensitized when its edges commence to
curl up. It is then removed from the bath by drawing it on a glass rod
fixed at the end of the tray, and placed, prepared side down, on a
slightly waxed glass plate, rubbing it with an India rubber squeegee to
remove the superflous liquid, when it is hung up to dry.

While wet the bichromated tissue is insensitive; the sensitizing can
therefore be made by daylight, but the drying should of course be done in
the dark room, that is in a room lighted by a candle or the sunlight
filtered through a deep orange window glass.

_Caution.--_The soluble bichromates are very poisonous. By absorption they
produce skin diseases not without danger and very difficult to cure. Hence
when handling the wet tissue the fingers should be protected by India
rubber tips, and any yellow, stains on the hands should be rubbed with a
dilute solution of aqueous ammonia, and the hands well rinsed in water.

_Drying.--_When the tissue dries rapidly it adheres well on the support
upon which it is applied for developing and yields brilliant images which
are easily cleared. On the other hand, were it allowed to dry slowly the
adherence would not be so complete, the image dull and developing with
difficulty. They may even refuse to develop at all from the
insolubilization of the gelatine.

In winter and in the cool days of spring and autumn, the gelatine dries
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