RADIATION OF HEAT 439 



temperature, fold much more severe thcUi is experienced in Hritain may 

 be enjoyed in the dry, Canadian climate. 



For example, a temperature of — 40° may he borne if the air be dry and 

 still, while a iiiucli wanner but damp and windy atmosphere may "" cut to 

 the bone." 



8. Ingestion of hot or cold food or of heat-producing food normally produces 

 an almost negligible variation in rectal temperature. 



What we want to get at i^. the mechanism whereby the body 

 temperatnre of the higher mammal is kept fairly constant despite 

 variations of outside and " inside " temperature. In this respect, 

 the ordinary laws of cooling-surface phenomena, are obeyed. 

 Heat may be lost by : 

 I. Radiation. 



II. Conduction and convection. 



Ha. Ingestion and excretion. Inspiration and expiration. 



III. Evaporation of moisture. 



I. Radiation 



The transmission of heat by radiation differs essentially from 

 conduction and convection. The particles of a body have a 

 vibratory movement depending on their kinetic energy. Increase 

 of temperature will, therefore, cause increase in the velocity of 

 these movements. In conduction, some portion of another body 

 is heated b\' contact with the warm body. This second body 

 passes the heat on, i.e. there is molecular continuity. By radiation, 

 on the other hand, one body can effect the thermal state of another 

 body not in contact with it without sensibly affecting that of the 

 intervening medium. Radiation is not dependent upon the 

 presence of air. It takes place quite readily in a vacuum. This 

 is manifest when we consider how we derive heat from the sun 

 whose radiant heat is transmitted through the ether at a very 

 high velocity (about 186,000 miles per second) by means of 

 transverse wave motion. 



Radiant heat may be detected and measured by means of a 

 suitable thermopile provided with a collecting horn or by means 

 of Leslie's differential air thermometer. The amount of heat lost 

 by radiation depends on : 



(i) The area and colour of the surface. It has been proved 

 experimentally as well as deduced mathematically that the 

 emissive and absorptive powers of a body are equal. A perfectly 

 black body absorbs all the heat energy that falls on it, and therefore, 

 since a body cannot absorb more than is incident on it, the absorp- 

 tive power of such a body is unity. The emissive power of a body 

 is the ratio of the quantity of heat radiated per square centimetre 



