" 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 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.
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