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 the most perfectly soluble nitrates of
any of the flaxes. Cotton gives a glutinous collodion, and calico a fluid
collodion. One of the largest manufacturers of pyroxyline in the States
uses the "Memphis Star" brand of cotton. This is an upland cotton, and its
fibres are very soft, moist, and elastic. Its colour is light creamy
white, and is retained after nitration. The staple is short, and the twist
inferior to other grades, the straight ribbon-like filaments being quite
numerous. This cotton is used carded, but not scoured. This brand of
cotton contains a large quantity of half and three-quarter ripe fibre,
which is extremely thin and