Page:Popular Science Monthly Volume 10.djvu/692

672 What most strikes one is the invariably greater hygroscopic power of wool than of linen; the maxima and minima of flannel and linen being respectively 175 and 111, 75 and 41.

Observations 5 to 8 show that linen changes the quantity of its hygroscopic water at a proportionately quicker rate than flannel. The two pieces were for twelve hours in the cellar, when linen absorbed 111, flannel 175; immediately after, for four hours, in a cold place, where linen lost 18 per 1,000 of its absolutely smaller amount of water, while the flannel lost only 15 per 1,000; but during the next three hours linen lost only 2, but flannel 12 per 1,000.

When (Obss. 9 to 15) the pieces had come from the cold lecture-room into a warmed room, linen again ceased giving off water at a much quicker rate than flannel.

The accelerated rate, only in an opposite direction, took place again (Obss. 15 to 18) when the temperature in the room sunk from 65° to 59°.

All bodies become more hygroscopic with a sinking temperature, but the absorption of water and increase of weight, as well as the contrary process, take place proportionately quicker with linen than with flannel.

The more the air in any material is displaced by water, the less it keeps us warm, the quicker it conducts the heat; hence the frequent injury resulting from wet clothes, and the striking discomfort produced by a damp cold. You all know how comfortable we can feel in a walk, when the air is cold and dry, and how differently we feel when it is damp, although not colder. Then our clothes also get much damper, and conduct more of our heat away.

This is not to be underrated. We have seen in the table that 1,000 parts of flannel took up in the cellar 157 parts of water. Take the weight of a whole woolen clothing as ten pounds, and you see that it may absorb one and a half pound of hygroscopic water, which requires about 1,680 caloric units from our body to be evaporated.

Linen and flannel bear the same relation toward water they are wetted with as toward their hygroscopic water. Linen is quickly wetted and soaked, wool more slowly, but linen cannot take up the same quantity. Spilled water has certainly taught you this many times, when you wanted to take it up. It is the same in evaporation, which is also much quicker from linen. Two equal pieces of linen and flannel, weighing each 1,000 grammes, put into water and wrung out till they no longer yield a drop of water, keep back respectively 740 and 913 per 1,000.

But a much greater difference exists in the intensity of evaporation from wet linen and from wet flannel, during equal periods, in a heated room.