Page:Popular Science Monthly Volume 23.djvu/814

794 air. When the atmosphere is much agitated the cold penetrates more readily through the furs, and also through our overcoats, as all know who have been much exposed to cold winds. According to M. Krieger's experiments, the loss of heat through the skin is doubled when the fur is shaved off, and tripled when the skin is varnished. These facts bring us to the conclusion that the goods called impermeable are generally anti-hygienic, because they impede the aeration of the garments beneath them. They are good for protection against rain, but they excite perspiration and prevent its evaporation, and are very uncomfortable in pleasant weather.

Another very important property in cloths is their hygroscopicity; they condense moisture from the atmosphere and become impregnated with it the more speedily as the air is more nearly saturated with vapor, and consequently less capable of favoring evaporation. The condensation, which is equivalent to a kind of dew, is increased when the temperature is diminishing. According to M. Courier's researches, the water absorbed by a cloth may be divided into two parts: one part which is not perceptible to the touch and can not be pressed out—the hygrometric water proper; and the other part, that which fills the pores and can be wrung out, and which M. Coulier calls interposed water. According to his experiments, wool is more hygroscopic than hempen cloth, and linen than cotton. Dr. Pettenkofer compared the hygroscopic qualities of a piece of linen and a piece of flannel having equal surfaces and nearly equal weights. Having been previously dried at the boiling-point of water, the two pieces of goods were exposed together in places more or less moist, and the variations in weight they went through after several hours of exposure were measured. It was found that wool was nearly twice as hygroscopic as linen. Similar differences between different materials may be observed when they are wet by immersion. Linen gets wet much more speedily than wool, but the wool really absorbs the most water.

The quantity of water that cloths are capable of absorbing is evidently more considerable than is commonly supposed. A woolen coat weighing five or six kilogrammes may take up nearly a litre of water, and this will add a kilogramme to its weight. We see also that cloths absorb more moisture when the temperature is low than at ordinary summer heat. Wet garments conduct heat better than dry ones, and consequently give much less protection against chills; hence the danger of cold combined with dampness. But wool, although it is more hygroscopic than linen, protects better against the effects of humidity because of the slowness with which it absorbs and gives off water, and because of its indestructible porosity.

As we fill up the meshes and pores of a tissue, it becomes less permeable to air, and goods with close meshes, like linen, cottons, and silks, feel this effect much more quickly than woolen goods. As Dr. Pettenkofer remarks, the elasticity of the fibers counts for much in