﻿120 Messrs. H. T. Tizard and D. R. Pye on the 



owing to the sudden compression, but such an increase 

 cannot persist during the period of delay, and in any case is 

 negligible compared with the increase in radiation density 

 which would occur if the temperature of the walls of the 

 vessel were raised to the compression temperature of the 

 gas. Again, the emission and absorption of radiation by 

 the gas itself at the compression temperature of 500-600 

 absolute is negligible compared with that of the solid walls. 

 Hence we arrive at the conclusion that, although the density 

 of radiation in the system is not appreciably changed, the 

 gases react at a high rate. This fact appears to us to prove 

 conclusively that the radiation theory cannot be accepted 

 either in its original form, or as modified to meet its failure 

 to account quantitatively for the temperature coefficients of 

 chemical reactions in liquids under steady conditions of 

 temperature. 



It must be pointed out, however, that in spite of this con- 

 clusion, there does seem to be an indirect connexion between 

 the effect of temperature on the rate of combustion of many 

 substances and their absorption of infra-red radiation. For 

 example, Ooblentz has shown that all paraffin hydrocarbons 

 have very similar absorption spectra, with a weak band at 

 \ = about 2*4 //,, and strong bands at X = 3*43, 6*86 yu,, etc. 

 Now, Bicardo has shown that the tendency of hydrocarbon 

 fuels to detonate in an internal combustion engine depends 

 consistently on their ignition temperatures as determined in 

 the manner described above. According to our views this 

 is strong evidence that the temperature coefficients are 

 practically the same throughout. Again, it has been shown 

 that ethyl ether has approximately the same temperature 

 coefficient of combustion as that of heptane; while Coblentz 

 has found that it's absorption spectrum is also nearly 

 identical, with bands at 2*4 p and 3'45 /x. Carbon bisulphide, 

 on the other hand, has a much smaller tendency to detonate 

 in an internal combustion engine than heptane, although it 

 has a lower ignition temperature; corresponding to this we 

 find that the temperature coefficient is low, and that the first 

 strong absorption band in the infra-red occurs at A, = 4* 6 //,. 

 Finally, hydrogen " detonates " easily in spite of its high 

 ignition temperature ; its temperature coefficient must 

 therefore also be high, a deduction which is confirmed by 

 some preliminary experiments we have made on the delay 

 before the ignition of a non-turbulent mixture of hydrogen 

 and air. We should expect from this point of view to find 

 an absorption band in the short infra-red region (say about 

 1*0 yu,); actually no absorption is observed, but that the 



