In the second place the clothes prevent a too rapid escape of heat from the body, and by keeping a layer of warm air in contact with the skin, they preserve the body heat. Again, the various materials used to clothe the body vary much as to the readiness with which they conduct heat; accordingly we speak of good and bad conductors of heat. A bad conductor, such as wool, will keep the heat of the body from escaping to the sir, and thus forms warm clothing, while a good conductor like cotton will lead away the heat quickly and prove cooler. As said before, the texture of the material--that is, the size of its meshes--which allows air to pass more or less freely through it, also exercises a greater effect upon clothing. No healthy clothing is absolutely air-proof, the access of the air through it being necessary to our health and comfort. Thus oil-skin and mackintosh, which are air-tight as well as water-tight, make most people feel very uncomfortable. In addition to their texture or permeability to air, and to their conducting or non-contracting powers, fabrics also vary according to their hygroscopic qualities. By hygroscopic is meant the power of absorbing moisture; thus a thin flannel is one of the coolest materials we can have, for it absorbs perspiration; while linen, which is non-hydroscopic, when moist allows the fluid to evaporate rapidly, and thus cools the body too quickly, and therefore dangerously. Hence flannel is a most suitable fabric in which to take exercise, as there is less danger of taking a chill. There are four chief materials to be considered in connection with clothing, namely--cotton and linen, which belong to the vegetable kingdom, and silk and wool, which are obtained from the animal world. These four, either in their own form or else in combination with each other, such as merino, constitute most of our wearing apparel. Cotton is the fine, soft, downy material of a hairy nature which is found on the seeds of a certain plant, the cotton plant, which belongs to the mallow family. Its fibres are flattened in shape, and are twisted at intervals. The form of the fibres has an important effect in the action of cotton material on the skin. Being of a flattened shape, they have sharp edges, which in delicate skins are apt to cause irritation. Cotton wears well, it is not absorbent of moisture nearly to the same extent as linen, nor does it conduct away the heat of the body so quickly as the latter, hence it is a warmer material than linen. On the other hand, it does not retain the heat against the body like wool, and is an appropriate material for dress in hot climates. In merino there is a mixture of about one-fifth to one-half part of wool with cotton.