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 _fibroïn_; and
secondly, an envelope composed of a substance or substances, chemically
termed _sericin_, and often "silk-glue" or "silk-gum." Both the latter
and _fibroïn_ 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 fibroïn 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 raw silk without dissolving the
sericin or silk-gum. By heating under pressure with acetic acid,
however, silk is completely dissolved. Silk is also dissolved by strong
sulphuric acid, forming a brown thick liquid. If we add water to this
thick liquid, a clear solution is obtained, and then on adding tannic
acid the fibroïn is precipitated. Strong caustic potash or soda
dissolves silk; more easily if warm. Dilute caustic alkalis, if
sufficiently dilute, will dissolve off the sericin and leave the inner
fibre of fibroïn; but they are not so good for ungumming silk as soap
solutions are, as the fibre after treatment with them is deficient in
whiteness and brilliancy. Silk dissolves completely in hot basic zinc
chloride solution, and also in an alkaline solution of copper and
glycerin, which solutions do not dissolve vegetable fibres or wool.