true fibre, chemically termed fibroin; and secondly, an envelope composed of a substance or substances, chemically termed sericin, and often “silk-glue” or “silk-gum.”  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.  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