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 inside the follicle by the exuding therefrom of a plastic liquid or lymph; this latter gradually becomes granular, and is then formed into cells, which, as the growth proceeds, are elongated into fibres, which form the central portion of the hair. Just as with the trunk of a tree, we have an outer dense portion, the bark, an inner less dense and more cellular layer, and an inmost portion which is most cellular and porous; so with a hair, the central portion is loose and porous, the outer more and more dense. On glancing at the figure (Fig. 6) of the longitudinal section of a human hair, we see first the outer portion, like the bark of a tree, consisting of a dense sheath of flattened scales, then comes an inner lining of closely-packed fibrous cells, and frequently an inner well-marked central bundle of larger and rounder cells, forming a medullary axis. The transverse section (Fig. 7) shows this exceedingly well. The end of a hair is generally pointed, sometimes filamentous.