RADIATION OF HEAT 343 



of heat than air. A moment's consideration of the feeling of cold 

 before entering cold water and during the process will convince 

 one of this. We have already mentioned the difference in the 

 cold sensation caused by a dry cold wind compared with that pro- 

 duced by a wind of even a much higher temperature but laden 

 with moisture. 



II. 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 by contact with the warm body. This second body 

 passes the heat on. There is molecular continuity. By radiation, 

 on the other hand, one body can affect 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 

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

 of a body is the ratio of the quantity of heat radiated per sq. cm. 

 of surface to the quantity radiated per cm 2 , of a "perfectly ' 

 black body under the same conditions. (Expt. 53, p. 424.) 



TABLE LVII. 

 RADIATING AND ABSORBING POWERS OF BODIES. 



Lamp black 1-0 



Cinnabar - 0-28 



White paper - 0-9 



White lead - 0-21 



Polished silver - 0-03 



