In the process of Trebouillet and De Besancele, the cellulose, which may be in the form of paper, cotton, or linen, is twice nitrated--first in the acid mixture employed in a previous operation; and secondly, in a fresh mixture of 3 parts sulphuric acid of 1.83 specific gravity, and 2 parts concentrated nitric acid containing nitrous acid. After each nitration the mass is subjected to pressure, and is then carefully washed with water, to which, at the last, a small quantity of ammonia or caustic soda is added to remove the final traces of acid. The impregnation of the pyroxyline with the camphor is effected in a variety of ways. The usual proportion of the constituents is 2 parts pyroxyline and 1 part camphor. In Trebouillet and De Besancele's process, 100 parts of pyroxyline are intimately mixed with from 40 to 50 parts camphor, and moulded together by strong pressure in a hot press, and afterwards dried by exposure to air, desiccated by calcium chloride or sulphuric acid. The usual method is, however, to dissolve the camphor in the least possible quantity of alcohol, and sprinkle the solution over the dry pyroxyline, which is then covered with a second layer of pyroxyline, and the whole again treated with the camphor solution, the addition of pyroxyline and camphor solution being repeated alternately until the requisite amount of celluloid mixture is obtained. 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.