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
semi-transparent, the beautiful pearly lustre being due to the
smoothness of the outer layer and its reflection of the light. In the
silk fibre there are two distinct parts: first, the central portion, or,
as we may regard it, the true fibre, chemically termed _fibroin_; and
secondly, an envelope composed of a substance or substances, chemically
termed _sericin_, and often "silk-glue" or "silk-gum." Both the latter
and _fibroin_ are composed of carbon, hydrogen, nitrogen, and oxygen.
Here there is thus one element more than in the vegetable fibres
previously referred to, namely, nitrogen; and this nitrogen is contained
in all the animal fibres. The outer envelope of silk-glue or sericin can
be dissolved off the inner fibroin fibre by means of hot water, or warm
water with a little soap. Warm dilute (that is, weak) acids, such as
sulphuric acid, etc., also dissolve this silk-glue, and can be used like
soap solutions for ungumming silk. Dilute nitric acid only slightly
attacks silk, and colours it yellow; it would not so colour vegetable
fibres, and this forms a good test to distinguish silk from a vegetable
fibre. Cold strong acetic acid, so-called glacial acetic acid, removes
the yellowish colouring matter from r