temperature from 16 deg. to 18 deg. C. (60 deg. to 64 deg. Fahr.), it will require from
12 to 15 minutes.

It will be observed that the time of exposure also depends on the
thickness of the paper used for the original. If, however, the weather
is dark, it requires from 30 to 50 minutes for the exposure. It will be
observed that if the temperature is above 25 deg. C. (about 80 deg. Fahr.), the
sheets of glass should be kept very cool and be less dried; otherwise,
when exposed the sheets are instantly metallized, and the reproduction
cannot take place. The same inconvenience takes place if the temperature
is beneath 5 deg. C. (41 deg. Fahr.). In this case the sheets should be kept
warm, and care should be taken not to expose the frame to the open air,
but always behind a glass window at a temperature of from 14 deg. to 18 deg.
C. (about 60 deg. Fahr.). The time necessary for the exposure can be
ascertained by taking out one of the many pieces of glass, applying to
the sensitive surface a vitrifiable color, and observing whether the
color adheres well. If the color adheres but slightly to the dark, shady
portions of the image, the exposure has been too long, and the process
must be recommenced; if, on the contrary, the color adheres too well,
the exposure has not been sufficient, the frames must be closed again,
and the exposure continued. When the frame has been sufficiently
exposed, it is taken into the dark room, the sensitized pieces of glass
laid on a plate of glass or marble with the sensitive surface turned
upward, and the previously prepared vitrifiable color strewed over it by
means of a few light strokes of a brush. This powder does not adhere to
the parts of the picture fully exposed to light, but adheres only to the
more or less shady portions of the picture. This operation develops
on the glass the image as it is on the paper. Thirty to 40 grammes
of nitric acid are added to 1,000 grammes of wood-spirit, such as is
generally used in photography, and the prepared pieces of glass are