Evaporation from the skin is very rapid, and may lead to too sudden cooling of the body. A person who, after exercise that has produced free perspiration, stands in a current of cool air, is apt to take cold. The dryer the atmosphere, the more rapid is the cooling process. The most uncomfortable and oppressive atmospheric condition is that in which the air is heavily laden with moisture and the temperature high. Evaporation from the body is then slow, and the sensation of heat is oppressive. It is a mistaken notion that clothing keeps the cold out. Its purpose is to keep the heat in, or, in other words, to prevent the rapid radiation of heat. We speak of warm clothing and of cool clothing. That clothing is warm which retards the giving off of heat from the body. The Indian wraps a blanket about his body to keep it warm; we wrap a blanket around a piece of ice to keep it from melting. Any clothing that prevents the rapid escape of heat from the body is said to be a bad conductor, and is called warm. Woolen textures rank first among dress materials as poor conductors, and are therefore best adapted for winter clothing. Silk and cotton come next. Linen is a good conductor of heat; that is, it carries off the heat from the surface of the body very rapidly, and produces a sensation of coolness; therefore, all dress materials made from flax are said to be cool. =Materials.=--The principal materials used for clothing are wool, cotton, linen, and silk. These differ greatly in weight, texture, warmth, porosity, power to absorb moisture, and in other less important qualities. In a climate so changeable as that which prevails in most parts of the United States, the body, and especially the trunk, should be protected at all seasons from sudden chill by the use of under garments containing wool. Even in the warmest weather it will be found that a light woolen fabric absorbs the perspiration, and is more agreeable to the skin than cotton. The wearing of a flannel band, eight to ten inches in width, buttoned around the waist next to the skin, will prove an excellent protection to the kidneys and the abdominal region. Silk is light and soft, and as it retards the giving out of heat from the body, is worn for under garments, especially by those to whom wool is irritating or otherwise unpleasant. Rubber cloth is useful for rain coats, but as it prevents evaporation of the perspiration, it increases the liability to chill, and renders the wearer uncomfortable except in cold weather. =Animal Heat.=--The bird is warmer than the air in which it moves; the fish possesses a higher temperature than the water. As before remarked, chemical changes are constantly going on in the system which give rise to this result. Even plant life is subject to this law. A delicate thermometer placed among a cluster of geraniums about to burst into flower will show a temperature a degree or two higher than the surrounding air.