120 HOW WE USE HEAT 



themselves to the body. Linen can be washed in hot 

 water without injury, but it is costly and so is not used 

 as much as cotton. Silk both absorbs and loses water 

 rapidly, but it is expensive and hard to wash. Rayon 

 which is made largely from wood pulp is used extensively 

 for underwear. It absorbs water readily and loses it 

 rapidly, and consequently makes good undergarments. 



Clothing for Winter and Summer. The human body is 

 a self-regulating machine in which the body temperature is 

 normally kept at 98.6 F. Underneath the skin is a fine 

 network of blood vessels from which heat is passed off 

 through perspiration. When we do hard work, the blood 

 becomes heated and circulates more rapidly. The blood 

 vessels of the skin get larger, and give heat off to the sweat 

 glands, which pour out perspiration. This in turn evapo- 

 rates, cooling the skin, and this cools the blood ; thus our 

 temperature is kept constant. Evidently, then, in winter 

 we need underclothes which will not let heat out. Under- 

 clothes which do not hold moisture are best because wet, 

 sticky undergarments cool us by conduction if it is cold, 

 and keep us uncomfortably hot by preventing evapora- 

 tion if it is warm. It does not make very much differ- 

 ence what kind of materials are used, provided the under- 

 clothes are porous. Woolen underclothes are best for 

 winter, because the curly fibers make them porous, and 

 because they absorb moisture and give it up slowly, thus 

 preventing the skin from being cooled too rapidly. We 

 have seen that dark substances absorb heat and light- 

 colored substances reflect heat ; therefore, to wear dark 

 clothes in winter and light clothes in summer is scien- 

 tifically correct as well as more comfortable. If you take 

 two test tubes, place a thermometer in each tube, then wrap 

 a piece of white cloth around one and a piece of dark cloth 

 of the same weave around the other, leave both tubes in 

 the sun side by side for a few minutes, and then read the 



