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, pass independently off
to the surface. Other glands terminate under the skin in the hair
follicles, which follicles or hair sockets contain or enclose the hair
roots. These glands terminating in the hair follicles secrete an oily
substance, which bathes and lubricates as well as nourishes the hair.
With respect to the origin of the hair or wool fibre, this is formed