440 THE REGULATION OF TEMPERATURE 



of surface to the quantity radiated per em.- of a " perfectly 

 black body under the same conditions. (Expt. 74, p. 556.) 



TABLE LXVII 



Radiating and Absorbing Po\vp:rs of Bodies 



Lamp black . . . . . . .1-0 



Cinnabar . 

 White paper 



White lead 

 Polished silver 



0-28 

 0-9 

 0-21 

 0-03 



(ii.) Temperature of the surface. There is a connection between 

 the amount of radiation from, and the temperature of, a body. 

 The total heat-loss by radiation is proportional to the difference 

 between the fourth powers of the absolute temperatures of the 

 body and its environment as well as to the area and nature of the 

 surface. If body and environment differ little in temperature, the 

 heat lost by radiation will thus be comparatively small. For 

 example, if the temperature of the air were 17° C, the heat 

 radiated from a " perfectly " black man at 37° C. would be 

 (273 + 37)4 „ (273 + 17)* = 310* — 290* = 216 X 10^ per unit 

 surface. 



Of course a perfectly black man does not exist, nor yet a perfectly 

 white man. If black skin has a radiation-coefficient of 0-98 and 

 white skin of 0-7, then the difference in radiating power would be 



expressed by — ^ = 1-4, a trivial advantage to the black-skinned 

 man in a tropical climate. 



II. Conduction 



By conduction is meant the loss of heat from a body at a higher 

 temperature to one at a lower temperature by passage from 

 particle to particle, for example, the passing of heat along a poker, 

 one end of which is in the fire. The amount of heat lost by the 

 body in this way depends on several factors. 



(i.) Surface Exposed. 



(a) Area. The loss of heat varies directly with the area of the 

 surface exposed. For example, the flow across 2 sq, metres is 

 double that across 1 sq. metre. The area of surface of a child of 

 2 years weighing 10 kg. is 5,120 sq. cm., and of a man of 60 kg. is 

 14,079 sq. cm. That is, the area per kg. for a child of 2 years is 

 512 sq. cm., and for a man is 234 sq. cm. It is obvious that weight 

 for weight the child will lose more heat than the man. 



