Excretion. 135 



The Body gives off Heat. In considering the regulation 

 of the temperature of the body, we must bear in mind that 

 the body is surrounded by air that is almost always con- 

 siderably cooler than itself. The body is therefore almost 

 always giving off heat. Our clothes do not warm us ; we 

 warm them, and they keep us from warming the air too 

 fast, i.e. they keep us from losing too much heat. Indoor 

 air in winter is kept at about 70 F. by artificial heat. 

 This air does not warm us ; we, being about 30 F. 

 warmer, are warming it. 



Ways of giving off Heat. -The skin gives off heat by 



1. Radiation: heat is given off in every direction. 



2. Conduction: whatever we touch that is cooler than 

 our bodies is warmed. We warm chairs, clothing, etc. 



3. Convection: the air in contact with the skin is 

 warmed and rises. Our bodily heat is thus carried off by 

 convection. 



4. Evaporation: the evaporation of sweat is the most 

 important factor in regulating the heat of the body. Any 

 liquid in evaporating absorbs heat. The cooling effect of 

 alcohol, ether, or cologne on the skin is due to the fact 

 that heat is taken from the skin in converting the liquid 

 into a gas. 



Experiment in Evaporation. With a medicine dropper put a drop 

 of cologne on the back of the hand. Note two facts : (i) it produces a 

 cooling effect ; (2) the liquid quickly disappears. 



Practical Applications of this Principle. We sponge a 

 feverish patient to reduce his temperature. The cooling 

 effect is due not so much to the coolness of the water itself 

 as to the absorption of heat from the skin in evaporating 

 the water. We sprinkle the floor in hot weather and thus 

 cool the air of the room. 



