188 PHYSIOLOGY AND HEALTH. 



The amount of hydrogen consumed is not so easily deter- 

 mined ; but it is supposed by learned chemists that a great 

 proportion of the animal heat is given out by it. 



439. If so much heat be daily added to the body, the 

 same amount must be carried off in some way, otherwise it 

 will warm the body too much, and cause distress. But it 

 does not increase ; when the body is at its usual temperature, 

 it does not become any warmer, although so much heat is 

 continually added to it. This quantity must, then, find its 

 way out, through the surface and through the passages. 

 Whatever goes from the body, carries some of its heat. 

 However cold may be the air which we inhale, it becomes 

 warm within the lungs, and is then exhaled at the tempera- 

 ture of the body. 



440. The skin is the main avenue of the heat outward ; 

 and through this it is continually passing away, both winter 

 and summer. When the air is considerably colder than the 

 body, it is very plain to every one that heat goes off by 

 transpiration through the outward surface, in order to main- 

 tain an equilibrium with the surrounding atmosphere; and 

 thus the internal temperature does not rise. But when the 

 air is as warm as, or even warmer than, the body, this transit 

 of heat outward is not so manifest, yet it is equally certain. 



441. The skin not only affords a passage-way for the heat 

 to go out, as through any dead substance, but it has an active 

 power to furnish the means of carrying off the surplus heat, 

 when it would otherwise accumulate in the body. The skin 

 is constantly preparing and throwing the perspiration upon 

 its surface, where it is usually converted into vapor and ab- 

 sorbed by the atmosphere. This change of the perspiration 

 from a fluid to a gaseous form from water to vapor is 

 effected by the addition of heat, ( 416, p. 177,) which is 

 absorbed from the body, and therefore cools it. This perspi- 

 ration is most abundant in warm weather, when the air can 

 absorb the most, and causes the greatest cooling when it 

 is most needed. It is a common, and by no means an un- 

 founded notion, that one is cooled and refreshed, in summer, 



