Radiation emitted in Gaseous Explosions. 95 



(v.) The proportion of: the heat of combustion of hydrogen 

 Tind air mixtures between the limits of mixture strength 

 10 per cent, to 25*4 per cent, decreases very rapidly as the 

 mixture strength decreases. The variation in this proportion 

 in coal-gas and air mixtures between the limits of mixture 

 strength 9'8 per cent, to 15 per cent, is small. 



20. Theoretical Conclusions. 



(vi.) The radiation emitted during the cooling of coal-gas 

 and air mixtures and of hydrogen and air mixtures after 

 explosion is largely due to thermal causes. 



(vii.) The transfer of energy between the vibratory degrees 

 of freedom corresponding to radiation of wave-length 2'8 /j, 

 and the other degrees of freedom of the molecules is com- 

 paratively slow at 1200° C. abs. and practically ceases at 

 1000° C. abs. Transfer of energy to or from the vibratory 

 degrees of freedom corresponding to longer wave-length 

 goes on at least until the mean temperature of the gas falls 

 to 900° C. abs. 



(viii.) The energy acquired on combustion by the various 

 vibratory degrees of freedom of the freshly formed C0 2 and 

 H 2 molecules depends upon the degree of violence with 

 which the combustion proceeds. The greater the violence 

 of combustion the greater the ratio of the energy in those 

 degrees of freedom corresponding to radiation of wave- 

 length 2'8 [jl to that in the degrees of freedom corresponding 

 to radiation of greater wave-length. 



It is thought that there are grounds for promoting this 

 into a general law which may be briefly stared thus : — The 

 intra-molecular energy acquired on combustion by the 

 freshly formed molecules of G0 2 and H 2 is not equally 

 partitioned over the various internal degrees of freedom 

 possessed by these molecules. When the combustion is 

 gentle the intra-molecular energy is concentrated in the 

 rotational degrees of freedom, and in such very low frequency 

 vibrations as the molecules may be capable of executing. As 

 the combustion becomes more violent the higher frequency 

 vibrations share in this energy, < r md it is possible that during 

 combustion of extreme violence equi-partitioning of energy 

 amongst all the internal degrees of freedom of the molecules 

 may be approached momentarily. 



Such a theory as this would serve to explain many of the 

 phenomena of the explosion period — including the pre- 

 pressure period during which a considerable volume of in- 

 flammation takes place. This will be attempted in another 

 paper. 



