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.
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