410 MR. W. T. DAVID: RADIATION IN EXPLOSIONS OF COAL-GAS AND AIR. 



(i.) The greater the energy density in the ether (which depends among other 

 things upon the transparency and the volume of the gas) the greater will be the 

 average vibratory energy of a molecule as compared with its translational energy, 

 though, of coiirse, the average vibratory energy of the molecule will only increase 

 slowly with the energy density in the ether. 



(ii.) The smaller the time of description of free-path (or, in other words, the greater 

 the density of the gas) the nearer will the average vibratory energy of a molecule 

 approach a value which is proportional to the absolute temperature of the gas. 



The average value of the vibratory energy of the radiating molecules of a gas thus 

 appears to be a function, not only of the absolute temperature of the gas, but also of 

 the value of the energy density in the ether, the rate at which the molecules emit 

 radiation, the time of description of free-path (inversely as the density of the gas), 

 and the rate of partitioning of energy during collisions. 



The result given on p. 381, that the rate at which radiation is emitted by the 

 gaseous mixture is a maximum some time before the attainment of maximum 

 pressure, shows that the vibratory energy of the radiating molecules is a maximum 

 some time before the mean temperature of the gaseous mixture attains its maximum 

 value. Prof. HOPKENSON'S experiments show that no portion of the gaseous mixture 

 during explosion has such a high value as at the moment of maximum pressure,* so 

 that it is very probable from this result that the violence of combustion during 

 explosion causes a considerable part of the energy of combination to pass into the 

 form of internal vibrations of the CO 2 and steam molecules. Part of the energy in 

 these vibrations is lost by radiation, but the greater part is transformed into 

 rotational energy and translational or pressure energy. 



I desire to thank Prof. HOPKINSON for his kind interest in these experiments and 

 for the encouragement and advice which he has so kindly given me during the 

 progress of the work. The experiments described in this paper were all carried out 

 in the Engineering Laboratory at Cambridge. 



* ' Roy. Soc. Proc.,' A, vol. 77, p. 389. 





