92 Major W. T. David : Analysis of the 



Theoretical Discussion, 



13. Radiation due to temperature. — These experiments 

 offer strong support to the view that the large amount 

 of radiation emitted during the cooling of gaseous mixtures 

 of coal-gas and air and of hydrogen and air after explosion 

 is mainly of thermal origin. The dependence of the ratio of 

 the energy in the 2'8 fi radiation to that in the 4*4 /uu radiation 

 upon temperature, the disappearance of the 2'8 p radiation 

 in the neighbourhood of 1000° 0. abs., and the small emission 

 in weak hydrogen mixtures in which the maximum tempe- 

 ratures developed do not greatly exceed this figure, all tend 

 to this conclusion. 



14. Theoretical considerations lead to the belief that the 

 •rate of partitioning of energy between the vibratory and 

 the translational and rotational degrees of freedom of the 

 molecules of a gas is dependent upon the relation between 

 the vibration periods and the duration of collisions between 

 pairs of molecules. When the duration of collisions is long 

 in comparison with the vibration-period of any particular 

 vibratory degree of freedom (in other words, when the 

 collisions are soft in respect to this type of vibration) 

 the rate of partitioning between it and the other degrees 

 of freedom is slow ; but when it is short (that is, when 

 the collisions are hard in respect to this particular type 

 of vibration) the rate of partitioning may be very rapid. 



15. Confine the attention for the moment to one of the 

 types of vibration possessed by C0 2 molecules — say that 

 corresponding to radiation of wave-length 4'4^. According 

 to the Kinetic Theory the molecules of a gas at any given 

 temperature are moving with widely different velocities 

 grouped about the mean according to the law of errors, 

 and, since the duration of collisions between pairs of 

 molecules depends upon the velocity of approach, a pro- 

 portion of the collisions at any moment will be soft as 

 regards this type of vibration, while the remainder will be 

 hard. These proportions will clearly vary with temperature. 

 The higher the temperature the larger the proportion of hard 

 collisions, and therefore the quicker the transfer of energy 

 to or from this type of vibration. 



16. As regards the other types of vibration possessed 

 by G0 2 molecules (viz. those corresponding to radiation 



