The second necessity is that the excrementitious matters should be rapidly absorbed, as they are being constantly poured out from the skin, so that the material must be absorbent in the highest degree, otherwise the fluids remain in contact with the skin to irritate it, and the atmospheric air confined between the undergarment and the skin rapidly becomes surcharged with gases and moisture, and so acts like a warm jacket around the skin. Heat radiation is interfered with and the skin becomes extremely sensitive to changes of temperature and to drafts. Wool is a poor conductor of heat and a great absorber of water. Its powers of hygroscopic absorption is at least double in proportion to its weight, either cotton or linen, and this property is an important one. During perspiration the evaporation from the surface of the body is necessary to reduce the heat which is generated by exercise. When exercise is finished, evaporation still goes on, and, if unchecked, to such an extent as to chill the body. If dry woolen clothing is put on after exertion, the vapor from the surface of the body is condensed in the wool, and gives out again the large amount of heat which had become latent when the water was vaporized, and from this cause alone a woolen covering feels warm during sweating. In the case of tightly woven cotton and linen fabrics, the perspiration passes through them, and evaporates from the external surface without condensation; the loss of heat then continues. In addition to this, the texture of wool is warmest from its poor conducting powers, and it is less easily penetrated by cold winds. The more readily material conducts heat, the cooler it feels. The property of the conduction of heat is in proportion to the closeness of the weave and the amount of air which it contains. For this reason all loosely woven fabrics feel warmer than closely woven ones, on the same principle that the more layers of clothing there are, the more layers of air will be retained between them. The recent methods of weaving cotton and linen fabrics more loosely have materially reduced their general defects as underclothing in cold weather, and if cotton or linen is worn next the skin it must be so woven as to give both thickness and porosity to the fabric. For the majority of people, during the wet and cold seasons woolen undergarments are the best. If cotton is worn, it must be loosely woven, so that it may entangle a large quantity of air within its meshes. The underclothing should fit tolerably closely the outlines of the figure, without impeding the movements. Combination undersuits, which reach from the neck to the ankles, are the best; the weight should be adapted to the season of the year, and should be changed with the weather, regardless of the date of the calendar.