22 HOMOIOTHERMISM 



rents or winds than if the air is quiet. In the absence 

 of air currents a layer of air will form an envelope 

 around the animal, decreasing the temperature gradient 

 and preventing rapid conduction of heat. Humidity is 

 obviously a factor only in the terrestrial animals. Water 

 is a better conductor of heat than air. The more humid 

 the atmosphere, the more rapid will be the gain and loss 

 of heat if other factors do not change. The humidity of 

 the atmosphere also influences the rate of evaporation 

 of water from the surface of an animal's body. The 

 maximum rate of evaporation will take place in an at- 

 mosphere containing no water vapor and the minimum 

 in an atmosphere saturated with water vapor, assuming 

 that temperature and other factors remain constant. 

 Evaporation has a marked cooling effect on animals. 



Radiation. — The transmission of heat by radiation is 

 fundamentally different from that of conduction. Radia- 

 tion implies vibratory movements of particles or elec- 

 trons depending on their kinetic energy. If the tempera- 

 ture of the particles is increased, the velocity of the 

 vibrations will increase. Conduction requires molecular 

 continuity. In other words, one body must be in contact 

 with another in order to have transmission of heat energy 

 by conduction. One body may affect the thermal state 

 of another body by radiation without being in contact 

 with it, and also without affecting appreciably the inter- 

 vening medium. Radiation is not dependent upon a con- 

 tinuous medium, such as air or water. In fact, it takes 

 place quite readily in a vacuum. 



The heat gained or lost by an animal through radia- 

 tion depends on the area of its surface, color, and tem- 

 perature. It has been proved experimentally that the 

 absorptive and emissive powers of a body are equal. 

 Therefore if the temperature of an animal is higher than 



