(1) Wool from the sheep is really a soft and elastic hair, composed of fibres three to eight inches long, and about 1∕1000 inch thick. The finer and shorter wools are used for fine cloth, the longer and coarser for “poplins,” “worsted pieces,” etc. Flannel is a woollen stuff of rather open and slight fabric. Wool is irritating to delicate skins, and may be so much so, that it cannot be worn next the skin, whether as flannel, worsted, or merino. In these cases, it may be worn outside a linen or gauze vest, and so all its advantages secured. It is one of the worst conductors of heat, and ought always to be worn in winter; while even in summer, it ensures a greater immunity from chill after perspiration than any other material. (2) _Cashmere_ is made from the down found about the roots of the hair of the Thibet goat. Imitation cashmere is made of various materials mixed together. [Illustration: FIG. 56. MICROSCOPICAL APPEARANCE OF FIBRES OF A—COTTON. B—SILK. C—LINEN. D—WOOL.] (3) _Alpaca_ is obtained from the fleece of the llama, alpaca, and vicuna. It is longer than the fleece of the sheep, the fibres, which are soft and strong, averaging six inches in length. It is commonly made up with cotton or silk. (4) _Mohair_ is the hair of a goat inhabiting the mountains near Angora. It is woven into an almost waterproof cloth, and used in making plush, braid, etc. 2. _Hair_ derived from the horse or cow differs from the hair usually called wool, in the greater solidity of its structure, which makes it ill adapted for clothing. Its chief use is in the manufacture of felts, of which hats are made. 3. _Leather_ is a kind of natural felt, very close and firm in its texture. It is used in this country chiefly for boots, but in some colder climates also for coats, etc. It is impervious to moisture, like sealskin, and is consequently not very healthy. The same objection applies to _chamois-leather underclothing_, which is non-porous, and consequently keeps the skin hot and clammy; also, it cannot be washed without becoming stiff on drying. This necessitates wearing the material after it has become impregnated with perspiration. 4. _Silk._ The thread spun by the silk-worm is composed of filaments 1∕2000 inch wide, and is the strongest and most tenacious of textile fabrics. Its thread is three times as strong as a thread of flax of the same thickness, and twice as strong as a thread of hemp. Its fibres are round like those of linen, but softer and smaller; it gives an agreeable sensation of freshness to the skin even more than linen. It is a worse conductor of heat than cotton or linen. Its great disadvantage for wearing next the skin, apart from its expense, is that it irritates delicate skins. _Satin_ is silk so prepared as to form a smooth, polished surface.