Wool is a much more hygroscopic fibre than cotton or any of the other vegetable fibres, usually it contains about 18 per cent. of water, but much depends upon the atmospheric conditions that prevail. This water is contained in the wool in two forms: (1) as water of hydration amounting to about 81 per cent., and (2) as hygroscopic water. Experiments have shown that when a piece of dried wool is exposed to an atmosphere saturated with water vapour it will absorb 50 per cent. of its weight; cotton under the same conditions will take up 23 per cent.; flax, 27.5 per cent.; jute, 28.5 per cent., and silk, 36.5 per cent. Heated to about 100 deg. C. it parts with nearly the whole of its water and becomes hard, horny and brittle, exposed to the air, the dry wool again absorbs water and is restored to its former condition. When heated to 100 deg. C. wool becomes somewhat plastic, so that whatever form is then imparted to it it will retain when it becomes cold, this property is very useful in certain processes of finishing wool fabrics, making hats, etc. #Chemical Composition.#--In the natural or raw state each wool fibre is surrounded by a considerable amount of foreign matter, so that in treating of its chemical constitution it is necessary to distinguish between pure wool and the raw fibre. The incrusting substance is technically known as "Yolk," or "Suint," and is principally composed of a kind of natural soap, consisting of the potash salts of certain fatty acids, together with some fats which are incapable of saponification. The amount of yolk present upon different samples of wool varies greatly, the finer varieties containing, as a rule, a larger proportion than the coarser, and less valuable sorts. The variation in the relative amount of pure fibres and yolk is (p. 007) well shown in the following analyses which, however, do not by any means represent extreme cases. ANALYSES OF RAW MERINO WOOL. DRIED AT 100 deg. C. No. 1. No. 2. Moisture 6.26 10.4 Yolk 47.30 27.0 Pure Wool 30.31 59.5 Dirt 11.13 3.1 ------ ------ 100.00 100.00 Yolk consists very largely of two complex substances which have been termed wool perspiration and wool fat. The former is composed of the potash salts of fatty acids, principally oleic and stearic acids; the latter of the neutral carbohydrate, cholesterine, with other similar bodies. The wool perspiration may be removed by a simple washing with water, and on the Continent forms a valuable source of potash salts, since the ash after ignition contains 70 to 90 per cent. of potassium carbonate. The wool fat is insoluble in water, but dissolves readily in ether, benzene, carbon disulphide, etc. It is also removed from the wool by a treatment with alkali, and it is not easy to explain the action in the case, since the wool fat is not a glyceride, and will not form a soap, but is probably emulsified by the wool perspiration.