70° for twenty-four hours. The thick cakes are once more cut into plates
of the desired thickness, and placed in a chamber heated from 30° to 40°
for eight to fourteen days, whereby they become thoroughly dry, and are
readily made into various articles either by being moulded while warm
under pressure, cut, or turned. Occasionally other liquids, e.g., ether
and wood spirit, are used in place of alcohol as solvents for the camphor.

Celluloid readily colours, and can be marbled for manufacturing purposes,
&c. It is highly inflammable and not explosive even under pressure, and
may be worked under the hammer or between rollers without risk. It softens
in boiling water, and may be moulded or pressed. Its specific gravity
varies slightly with its composition and with the degree of pressure it
has received. It is usually 1.35. It appears to be merely a mixture of its
components, since by treatment with appropriate solvents the camphor may
be readily extracted, and on heating the pyroxyline burns away while the
camphor volatilises.

The manufacture of pyroxyline for the purpose of making celluloid has very
much increased during recent years, and with this increase of production
improved methods of manufacture have been invented. A series of
interesting papers upon the manufacture of pyroxyline has been published
by Mr Walter D. Field, of New York, in the _Journal of the American
Chemical Society_[A] from which the following particulars are taken:--

[Footnote A: Vol. xv., No. 3, 1893; Vol. xvi., No. 7, 1894; Vol. xvi., No.
8, 1894. Figs. 19, 20, 21, 22, and 23 are taken from Mr Field's paper.]

~Selection of the Fibre.~--Cotton fibre, wood fibre, and flax fibre in the
form of raw cotton, scoured cotton, paper, and rags are most generally
used, and give the best results. As the fibres differ greatly in their