EXCRETION. 137 



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. keep us from losing too much heat. Indoor heat in 

 winter in the cooler parts of the United States 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 the sweat is a much 

 more important factor in heat regulation. Any liquid, in 

 evaporating, absorbs heat. The cooling effect of alcohol 

 or ether on the skin is due to the fact that heat is taken 

 from the body in converting the liquid into a gas. 



Experiments in Evaporation. Let the teacher, with a medicine 

 dropper, place a drop of alcohol, ether, or cologne on the back of the 

 hand of each pupil. Notice two facts: (i) It produces a cooling 

 effect. (2) The liquid soon disappears. To prove that it is not 

 merely that the liquid is cool, try the following : Tie a piece of cheese 

 cloth around the bulb of a thermometer ; dip the bulb into a dish of 

 alcohol or ether, and note its temperature (if these are not at hand, 

 gasoline serves very well, or even water, though the evaporation is 

 slower) ; then lift the bulb out of the liquid, and note any change in 

 temperature. The evaporation of the liquid takes heat from the bulb, 

 and causes the thermometer to register a lower temperature. We 

 sponge the face and hands of a feverish patient to reduce the amount of 

 heat. We sprinkle the floor in hot weather, and, by the absorption 

 of heat in evaporating the water, cool the air of the room. 



Heat and Exercise. When we exercise, we produce 

 more heat : we sweat more ; more heat is taken from the 



