aw 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 fibroin 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 fibroin; 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.
Chlorine and bleaching-powder solutions soon attack and destroy silk,
and so another and milder agent, namely, sulphurous acid, is used to
bleach this fibre. Silk is easily dyed by the aniline and coal-tar
colours, and with beautiful effect, but it has little attraction for the
mineral colours.

_Wool_.--Next to silk as an animal fibre we come to wool and different
varieties of fur and hair covering certain classes of animals, such as
sheep, goats, rabbits, and hares. Generally, and without going at all
deeply into the subject, we may say that wool differs from fur and hair,
of which we may regard it as a variety, by being usually more elastic,
flexible, and curly, and because it possesses certain features of
surface structure which confer upon it the property of being more easily
matted together than fur and hair are. We must first shortly consider
the manner of growth of hair without spending too much time on this part
of the subject. The accompanying figure (see Fig. 5) shows a section of
the skin with a hair or wool fibre rooted in it. Here we may see that
the ground work, if we may so term it, is four-fold in structure.
Proceeding downwards, we have--(first) the outer skin, scarf-skin or
cuticle; (second) a second layer or skin called the _rete mucosum_,
forming the epidermis; (third) papillary layer; (fourth) the corium
layer, forming the dermis. The peculiar, globular, cellular masses below
in the corium are called adipose cells, and these throw off perspiration
or moisture, which is carried away to the surface by the glands shown
(called sudoriparous glands), which, as is seen,