Parasitic skin diseases are greatly favoured by the presence of a dirty skin, which affords a suitable soil for the development of the parasites. (See also page 275). =Uses of Soap.=—Soap is produced by an action of an alkali on an oil. The alkali displaces glycerine from the oil, and forms an alkaline stearate, which is soap. Soft soap is chiefly stearate of potassium; hard soap is stearate of sodium. There may also be present the alkaline salts of oleic and palmitic acid. Soft soap is not used for washing the skin, as it is too irritating. All soaps contain a slight excess of soda; the greater this excess, the more irritating is the soap to delicate skins. Hard soaps may be also made with potash, if the fat employed be a solid one; but such soaps are rather softer than ordinary hard soaps, and more caustic. Cocoa-nut oil is used in making marine soaps, because, unlike all other kinds, it is not rendered insoluble by brine, and so will form a lather with sea-water. Normal soaps contain from 15 to 35 per cent. of water. “Liquoring” a soap consists in adding 5 to 25 per cent. of soluble silicates. By this means the soap may be made to hold 70 per cent. of water, which is obviously very wasteful. In washing the skin, the water washes away a considerable amount of epidermis, and the saline matters which have collected. For the oily sebaceous secretion soap is required. The alkali in soap combines with the oily matter, forming an emulsion which carries away with it a quantity of the dirt which previously blocked the orifices of the sebaceous and sweat ducts. When the skin is rubbed by the towel after washing, the softened epithelium, and with it any remaining dirt, are rubbed off, leaving the skin clean, and able to perform its normal functions. =The Use of Baths.=—The primary object of bathing is cleanliness. A secondary consideration is the pleasure derived from bathing. Baths are especially necessary for those persons who lead sedentary lives. When the skin is kept in an active condition by exercise, it to some extent cleanses itself. Thus, a farm labourer who has a weekly bath, may be really cleaner than a person of sedentary habits, who has two baths per week. Baths are classified according to temperature as follows:—Below 70° Fahr. they are described as _cold_; tepid up to 85°; _warm_ up to 97°; and _hot_ over this temperature. It is important in deciding the temperature of a bath not to trust to one’s sensations; the only accurate measure is by the thermometer. A cold morning tub in the summer will commonly contain water at 55° to 60°; while the same in winter will be down to 40°, or occasionally to 32°.