Radiation from Hot Gases. 267 



same number and kind of radiating molecules does not depend 

 upon the temperature alone, even after correcting the radia- 

 tion for absorption. This implies that the vibratory energy 

 which gives rise to the radiation is not solely dependent upon 

 the gas temperature (which is proportional to the translational 

 energy). It seems to depend upon the lateral dimensions and 

 density of the gas as well *. An explanation in terms of the 

 kinetic theory of gases which has been suggested by the 

 writer to account for this may be briefly stated thus : — A 

 molecule as it describes its free-path loses energy owing to 

 the emission of radiation and gains energy owing to the 

 absorption of radiation energy from the aether f . The rate at 

 which it emits radiation is a function of its vibratory energy, 

 say f(V), and the rate at which it absorbs energy is a 

 function of the density of radiation energy in the aether, say 

 <£(E). During the free-path, therefore, 



f=<KE)-fO0, 



and the vibratory energy, V, will increase or decrease 

 according as the rate of absorption of energy, <£(E), is 

 greater or less than the rate of emission of radiation, f (V). 

 During collision with another molecule there will be a trans- 

 ference of energy between the vibratory degrees of freedom 

 and the rotational and translational degrees of freedom, and 

 the vibratory energy of the molecule will tend to take up 

 during this time a value, which we will call V , such that 

 the energy in each of the vibratory degrees of freedom 

 equals that in each of the translational and rotational degrees 

 of freedom. Daring collision, therefore, the vibratory energy 

 of the molecules will tend to take up a value which is pro- 

 portional to the absolute temperature of the gas, but during 

 the free-path there will be a considerable departure from 

 this value if the density of radiation energy in the aether is 

 above or below a certain value, and the time of description 

 of free-path is not very short. If t is the time of de- 

 scription of free-path the gain in the vibratory energy, V, 



* Phil. Trans. A. vol. ccxi. (1911; pp. 402 & 406. 



t The molecule gains energy from the aether in two ways : — (i.) The 

 absorption of energy by its vibratory degrees of freedom, and (ii.) a 

 direct transfer to its translational degrees of freedom due to the pressure 

 of radiation. Of these (ii.) is possibly small in comparison with (i.) in 

 the case of thermal radiation from carbonic acid gas and steam, and is 

 neglected in this discussion. 



T2 



