Negatively Electrified Corpuscles by Hot Bodies, 261 



produced by the motion o£ corpuscles inside molecules from 



which they never become detached. The electromagnetic 



effect will evidently be of much the same character, whether 



the velocity of a corpuscle is reversed by a collision or by 



swinging* round a closed orbit under the action of a central 



force. If the orbits of the corpuscles in the molecules are 



circular, the calculation of the amount of energy radiated 



from them is very simple. A corpuscle moving with an 



1 e 2 f 

 acceleration f emits radiant energy at the rate q*^? where 



V is the velocity of light. If the corpuscle moving with a 

 velocity v describes a circle of radius r, 



f= — = — n if fi/r* is the force on the corpuscle divided 

 in by its mass. 



and this is proportional to the rate at which the corpuscle is 

 emitting energy. Thus this rate is proportional to the kinetic 

 energy of the particle raised to the power m 2n/n — l: and if 

 we assume that the kinetic energy of the corpuscles is pro- 

 portional to the absolute temperature 0, the rate of radiation 



from the corpuscles varies as 6 n - K If the force on the corpuscle 

 varies inversely as the square of the distance n = 2, the 

 rate of radiation will be proportional to the fourth power of 

 the absolute temperature. To calculate the rate at which 

 energy comes out of the body we require to know the law of 

 absorption ; if the corpuscles are moving with different 

 velocities, the character of the radiation emitted by a cor- 

 puscle will depend upon its velocity; if the absorption does 

 not depend upon the character of the radiation, the rate at 

 which energy is emitted from the body is proportional to the 

 fourth power of the absolute temperature (assuming w = 2); 

 but this is not the case if the absorption depends upon the 

 character of the radiation. If, for example, as in the case of 

 Rontgen rays, the greater the velocity of the corpuscles the 

 more penetrating the radiation they originate, a larger pro- 

 portion of the radiation from the quicker corpuscles would 

 emerge from the body than of that from the slower corpuscles, 

 and the rate of escape of the radiation would increase more 

 rapidly than the fourth power of the temperature ; while if 

 the law of absorption went the other way it would vary less 

 rapidly. Although the calculation of the amount of radiation 



