264: Mr. W. T. David on 7 hernial 



to prove definitely that all the gas has not entered into com- 

 bination at the moment at which the gaseous mixture attains 

 -its maximum pressure and temperature ; but it seems hardly- 

 possible that the combustion proceeding after this time is 

 sufficient to account for this large emission. Neither would 

 it seem to be due to any large extent to dissociation and 

 recombination of the C0 2 and water-vapour, for it seems 

 very probable that the amount of dissociation of C0 3 and 

 steam at temperatures under 2000° C. is very small. 



The increase of the specific heats of 00 2 and steam with 

 'temperature seems to give fairly reliable evidence that some 

 of the intra-molecular vibrations of these gases are excited 

 by molecular collisions. When a gas absorbs heat the trans- 

 national, rotational, and vibrational energies of its molecules 

 are increased. According to the Maxweli-Boltzmann law 

 of equipartition of energy, the heat energy absorbed by a gas 

 is equally distributed amongst the various degrees of freedom 

 possessed by its molecules, or rather amongst those degrees 

 of freedom which share in the heat motion of the molecules, 

 and the specific heat of a gas is therefore proportional to the 

 number of those degrees of freedom possessed by its mole- 

 cules which share in the molecular heat motion. The number 

 of translational and rotational degrees of freedom possessed 

 by molecules must be independent of the temperature of the 

 gas (unless the nature of the molecules changes with the 

 temperature, in which case we should have a different gas 

 ■or mixture of gases), but the number of the vibratory degrees 

 of freedom of the molecules, which have energy given to 

 them during collision, may vary with the temperature with- 

 out affecting the composition of the gas. At any given 

 temperature the molecular collisions may be too soft to 

 excite any but the very low-frequency vibrations which the 

 molecules may possess ; but as the temperature is raised the 

 collisions become harder (i. e., the duration of collisions 

 becomes smaller), and consequently vibrations of higher 

 frequency may be excited*. The specific heat of a gas 

 would also increase with temperature, even though only one 

 type of vibration could be excited during collisions. The 



* It is not supposed here that the very high-frequency vibrations giving 

 rise to luminous radiation (which absorption spectra show to be very 

 numerous) are excited by molecular collisions as the gas-temperature is 

 raised. The vibrations referred to are those only which give rise to infra- 

 red variation. It seems probable that the vibrations of 00-2 molecules cor- 

 responding to radiation of wave-length 15 fx are excited by molecular 

 collisions at ordinary temperatures, and that as the temperature of the 

 CCj is raised the vibrations corresponding to 4'4 /x radiation, and later 

 those corresponding to 2*8 [x radiation," are excited during collisions. 



