344 REGULATION OF TEMPERATURE 



(ii) 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 environ- 

 ment 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) 4 

 =310 4 -290 4 =216 xlO 6 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 black-skinned 



animals. This has been confirmed experimentally by Leslie, who 

 used a differential air thermometer as a very delicate method for 

 the detection of very small differences in temperature. 



III. Evaporation of Moisture. 



As the temperature of the air and of the body approach the same 

 value, the heat lost by the body by radiation and conduction will 

 be exactly balanced by the heat absorbed from the environment 

 by the body. If the atmosphere attain a higher temperature 

 than the body, then these means of heat elimination will become 

 inadequate, and the body temperature should increase synchron- 

 ously with that of its surroundings. That this is normally not 

 so is common knowledge. Many interesting instances have been 

 known of persons submitted to high temperatures, and an examina- 

 tion of some of these cases may lead us to a knowledge of the 



TABLE LVIII. 



TIME TO KILL SPORES or HAY BACILLI AT VARIOUS 

 TEMPERATURES. 



