One factor influencing the suitabilty of fabrics for clothing is resistance to the passage of water vapor. Measurement of the low resistances of woven fabrics requires a different technique from that for the high resistances of waterproofing and packaging materials, since the resistances are close to that of air itself, which may be used as a convenient comparison standard. The resistance of a woven fabric depends on the kind of fiber, the thickness, and its tightness of weaving. With glass and vinyon, the resistance increases rapidly with density, showing that the main pathway through these materials is the air spaces. With cotton, viscose rayon, and wool, the tightest weaving produces resistances only four to eight times that of air, whereas those of ordinary fabrics range from two to four times that of air. This indicates that significant amounts of water vapor travel through the substance of the fibers themselves, which is confirmed by ex periments on viscose film (cellophane). Nylon and cellulose acetate are intermediate. The effects of resistant fibers are seen when the fraction of the volume occupied by fiber exceeds 40 percent; below this level all fabrics are closely similar, since the low-resistance pathway through the air spaces is sufficiently available.

It is frequently stated that one fabric dries faster than another, usually with reference to house hold conditions of drying on a line. However, the data of this report show that by and large all fabrics dry at the same rate under these conditions (rate being expressed as weight of water evaporating per unit area per unit time), but that the time of drying depends upon the amount of water originally held, so that some fabrics dry sooner than others. The main portion of the drying time shows this constant rate of drying, although there is a final period which is short compared to the main drying period, during which the rate of drying decreases. The water-holding capacity of a fabric depends upon how the fabric is supported (vertically or horizontally) and upon the mechanical treatment given to remove water. For simple drainage in the vertical position, the water-holding capacity of small samples is more closely correlated with fabric thickness than with fabric weight. The kind of fiber—i.e., wool or cotton-and large differ ences in moisture affinity, as shown with Vinyon, also affect the water-holding capacity. How ever, these factors have almost no effect on rate of drying, since this is controlled by the resistance of air layers to the passage of heat. The thickness of these air layers is sufficient under ordinary conditions to smooth-over the fabric surface irregularities, even when these are rather large, as in rib knits or in cellular or waffle fabrics. Increasing the hairiness of the surface in order to give more area for evaporation is sometimes proposed as a means of increasing the drying rate, but results show that this is without effect except for extremely high, open naps, for which the effect is the opposite-the thickness of the layers of still air is increased and the drying retarded.

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Wool protein, like other fibrous proteins, is composed of long, flexible molecular chains.This flexibility appears to be the basis of the long-range elasticity of wool fibers. The wool fiber is distinguished from other textile fibers by the presence of covalent disulfide cross links between these main chains. Rupture of these links by chemical means decreases the strength of the fiber without necessarily affecting the• elastic recovery.Rebuilding the covalent linkages largely restores the original properties of the fiber.Wool appears to be analogous to rubber in several respects. Thus the stress-strain, solubility, and swelling characteristics are greatly influenced by the extent of cross linking in the two materials.