No clothing, of course,--not even fur,--has any warmth in itself; it simply has the power of retaining, or keeping in, the warmth of the body that it covers. The best and most effective way of retaining the body warmth is to surround the body with a layer of dead, or still, air, which is the best non-conductor of heat known. Hence, porous garments, like loosely-woven flannels, blankets, and other woolen cloths, are warm because they contain or hold large amounts of air in their spongy mesh. The reason why furs are so warm is that their soft, furry under-hairs, or "pelt" as the furriers call it, entangle and hold an enormous amount of air. The fur of ordinary sealskin, for instance, is about half an inch deep; and _ninety per cent_ of this half-inch is air. If you wet it, its fur "slicks down" to almost nothing, although the most drenching wetting will not wash all the air out of it, but still leaves a dry layer next to the skin. The fur of mink skin, coon skin, or wolf skin, is an inch thick; and nearly eighty per cent of this thickness is air. The great advantage for clothing purposes of wool over fur is that the wool is porous through and through, while the fur is borne upon, and backed by, a layer of leather--the skin of the animal upon which it grew--which layer, after tanning and curing, becomes almost absolutely air-tight. As a matter of fact, furs are worn mostly for display and are most unwholesome and undesirable garments. The only real excuse for their use, save for ornament in small pieces or narrow strips, is on long, cold rides in the winter, and among lumbermen, frontiersmen, and explorers. They hold in every particle of perspiration and poisonous gas thrown off by the skin, and if worn constantly, make it pale, weak, and flabby; and the moment we take them off, we take cold. For outer garments and general wear, nothing yet has been discovered equal to wool, particularly at the cooler times of the year. But for under wear, in the hotter seasons and climates, wool has certain disadvantages. It is likely to be rough and tickling to most skins, which makes it uncomfortable, especially in warm weather. It is also difficult and troublesome to wash woolens without shrinking them; and, as soon as they do shrink, not only do they become uncomfortably tight, but the natural pores in them which make them so valuable close up, and they become almost air-tight. Finally, when loaded with perspiration, woolens easily become offensive, so that they must be frequently changed and washed; and as they are also high in price, it is easily seen that there are practical drawbacks to their use.