THERMAL PHENOMENA OF THE BODY 213 



heat can be transferred across a space in which there is neither 

 solid, liquid, nor gas, and in which conduction and convection are 

 consequently impossible. The most familiar and striking example 

 of radiation is the transfer of heat from the sun to the earth, since 

 there is no atmosphere in the greater part of the more than ninety 

 millions of miles of space which separate us from that intensely 

 heated body. 



Any detailed consideration of radiation belongs to the domain 

 of physics rather than physiology and would be out of place 

 here. It is enough for our present purposes to understand that, 

 whether a solid body be in an atmosphere of air, or in a trans- 

 parent liquid, or even in a vacuum, it transfers or loses heat by 

 direct radiation to colder objects about it. From an open fire heat 

 may be transferred by conduction to andirons or walls in direct 

 contact with it; or by convection through heated air currents to 

 the chimney top ; or, finally, by radiation to persons standing in 

 front of it. In the latter case the heating is chiefly by radiation, 

 since there is no contact with the fire, and such air currents as 

 exist are mostly composed of cool air sucked towards and into 

 the chimney by its draft. It is for these reasons that open fires 

 are said to " roast people in front and freeze them behind." Con- 

 versely, the human body, if warmer than its surroundings, may 

 lose heat by conduction, convection, and radiation to cooler objects 

 in the vicinity. 



The practical importance of these facts is seldom realized. It 

 often happens that the air in contact with the skin is of the proper 

 room temperature ; and yet, if one is sitting too near a cold wall 

 or window, enough heat may be lost by radiation from the skin 

 to the cold wall, through the warm air, to chill the skin materially, 

 causing a loss of heat and a " cold." 



Laws of conduction and radiation. For our purposes the two 

 most important factors which determine the loss of heat by con- 

 duction and radiation are (1) the difference in the temperature of 

 the two objects and (2) the distance between them. In general, 

 the greater the difference of temperature, the more heat will be 

 lost from the warmer to the colder object ; thus the skin loses 

 heat rapidly by these means when surrounding objects are at 

 F., but only slowly when they are at 90. It is also clear that 

 as soon as the temperature of surrounding objects and of the 



