cotton, or let us say cellulose particles, is treated with dilute
vitriol, pressed or squeezed, and then roughly dried. That cellulose
then becomes mere dust, and is simply beaten out of the intact woollen
texture. The cellulose is, in a pure state, a white powder, of specific
gravity 1·5, _i.e._ one and a half times as heavy as water, and is quite
insoluble in such solvents as water, alcohol, ether; but it does
dissolve in a solution of hydrated oxide of copper in ammonia. On adding
acids to the cupric-ammonium solution, the cellulose is reprecipitated
in the form of a gelatinous mass. Cotton and linen are scarcely
dissolved at all by a solution of basic zinc chloride.

[Footnote 1: _J.S.C.I. = Journal of the Society of Chemical Industry._]

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

[Illustration: FIG. 3.]

[Illustration: FIG. 4.]

_Silk._--We now pass on to the animal fibres, and of these we must first
consider silk. This is one of the most perfect substances for use in the
textile arts. A silk fibre may be considered as a kind of rod of
solidified flexible gum, secreted in and exuded from glands placed on
the side of the body of the silk-worm. In Fig. 4 are shown the forms of
the silk fibre, in which there are no central cavities or axial bores as
in cotton and flax, and no signs of any cellular structure or external
markings, but a comparatively smooth, glassy surface. There is, however,
a longitudinal groove of more or less depth. The fibre is