The mass, which sinks together in transparent lumps, is worked for about an hour between cold iron rollers, and then for the same period between rollers which can be gently heated by steam.  The layer of celluloid surrounding the rollers is then cut away and again pressed, the resulting cake, which is now about 1 cm. thick, being cut into plates of about 70 cm. long and 30 cm. broad.  These are placed one above the other, and strongly pressed together by hydraulic pressure at a temperature of about 70 deg. 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 deg. to 40 deg. 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 structure, they require different methods of nitrating.  The cotton fibre is a flattened hollow ribbon or collapsed cylindrical tube, twisted a number of times, and closed at one end to form a point.  The central canal is large, and runs nearly to the apex of the fibre.  Its side walls are membraneous, and are readily penetrated by the mixed acids, and consequently the highest nitration results.  In the flax fibre the walls are comparatively thick, the central canal small; hence it is to be presumed that the nitration must proceed more slowly than in the case of cotton.  The New Zealand flax gives