Materials of which clothing is made differ very greatly in their ability to accomplish the object just mentioned, some being comparatively poor conductors of heat and hence fulfill the desired function admirably, while others, for opposite reasons, are of comparatively little value for this purpose. In general it may be said that structures of animal origin, such as wool and silk, are much poorer heat conductors than those obtained from the vegetable world, and as a consequence the former are justly held in much higher esteem as material for clothing than the latter. It should not be forgotten, however, that the protective value of a fabric also depends upon the manner in which it is woven, since those that are loosely constructed are much warmer, other things being equal, than those that are put together more closely; this depends upon the fact that in the former there are innumerable small cavities between the fibers in which air is contained, and as this substance is a very poor conductor of heat, it follows that a garment made loosely and containing many such chambers is warmer than where the number is less. It may well be the case that a fabric constructed of a material which is a poor conductor of heat and closely woven may be actually cooler than another composed of a substance which is a much better conductor of heat but of a loose texture.
The efficiency of different materials of which clothing is made also depends upon their capacity to absorb water. This may be done in two ways: the water may simply collect between the fibers, in which case it may be in a large measure removed by wringing, or it may be actually absorbed into the substance composing the fabric, and, as a consequence, the latter, even though containing much moisture, do not appear damp. Fabrics made from vegetable materials, as cotton or linen, have little power of actually absorbing water, and hence they become wet on the slightest addition of moisture, while on the other hand those of animal origin have the capacity of absorbing water, and appear dry even after the addition of this substance in considerable amounts. A person, therefore, dressed in cotton fabrics will find after active perspiration has begun that his clothing quickly becomes moist, while if he have on woolen garments this will not occur. It is particularly noteworthy that water is gradually removed by evaporation from animal fabrics, which causes a general cooling without producing a chill; it is therefore readily understood that woolen clothing is much to be preferred where active exercise is being taken.