CLOTHING 



71 



FIG. 62. 



Distribution of Heat Obtained 

 from Food. 



thus a complex engine, using its changes to produce heat, 

 and to enable it to do work. 



It has been calculated that about 3 / 4 of all the heat produced in the 

 body is used to heat the body (Fig. 62). A day's work requires about 

 3 /ie, respiration about 1 / 60 , and the heart 

 about 1 /ie. While the oxidation changes 

 are much greater in some organs than 

 in others, the heat is carried away by 

 the blood as fast as it is produced. 

 Hence the temperature does not vary 

 in any two parts as much as half a 

 degree Fahr. The skin is, of course, 

 cooler than the rest of the body, both 

 because it is exposed to the air, and 

 also because it is cooled by the constant 

 evaporation of the perspiration (cf. 

 70). The excess heat of the body acts 

 like the heat of a stove in turning 



perspiration into steam. Ordinarily the perspiration is evaporated 

 as rapidly as it is formed, and we do not notice it. This is insensible 

 perspiration. Sensible perspiring, or sweating, takes place only when 

 water is given off by the perspiration glands more rapidly than it can 

 be evaporated by body heat. 



75. Clothing. Man protects his body against sudden 

 loss of heat by clothing. The best clothing for this pur- 

 pose is that which is made of a non-conductor (cf. 65), 

 and permits little heat to escape. Of the common non- 

 conducting materials the best is wool. Wool prevents 

 heat from coming to the body as well as its escape from 

 the body. Hence firemen, who are obliged to work where 

 it is hot, use woolen clothing to keep cool. Linen and 

 cotton are better conductors of heat than wool is, and so 

 are better for summer clothing than for winter clothing. 



