60 HOMOIOTHERMISM 



artificially covering the body with clothing, by taking 

 advantage of shelter in homes, by warming the food and 

 drink taken into the body, and by heating the atmosphere 

 with which the body comes in contact, man protects him- 

 self against a too great loss of heat and decreases to a 

 large extent the necessity of relying upon the involun- 

 tary, and subconscious, means of temperature regulation. 



Evaporation of Moisture. — When the temperature of 

 the body is like that of the environment of an animal, the 

 heat lost by radiation and conduction will be exactly bal- 

 anced by the heat absorbed. As the environment attains 

 a higher temperature than the body a means of heat 

 elimination by conduction or radiation become inade- 

 quate and thermolytic means are employed; i.e., the 

 evaporation of water from surface of the organism. 



The quantity of heat lost by evaporation of moisture 

 depends upon several factors : 



1. Humidity of circumjacent atmosphere. 



2. Temperature gradient between body and environment. 



3. Velocity of movement of circumjacent atmosphere. 



4. Effective area of moist surface exposed. 



5. Color of moist surface exposed. 



The amount of moisture in the air affects its cooling 

 powers. If the air is saturated, it can obviously contain 

 no more moisture and consequently no water is lost by 

 the organism. Also moist air is a much better conductor 

 of heat than dry air and heat lost by conduction is at its 

 maximum in a humid atmosphere. The former factor 

 plays an important part when the surroundings are 

 warmer than the animal, the latter acts when the envi- 

 ronment is colder. 



The temperature gradient between the surrounding 

 medium and the animal limits the amount of heat lost by 

 evaporation. The greater the gradient, the stronger the 



