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



Hence, from the values of =^ we get finally for the true 

 temperature coefficient B : ° 



Temperature coefficient B 

 (equation 5). 



Heptane-air... 13,200+ 9 per cent. 

 Ether- „ ... 12,600 ± 9 „ 

 0S 2 - „... 8,500 + 14 „ 



The significance of these figures will perhaps be better 

 appreciated by the statement, that the percentage rise in 

 absolute temperature necessary to treble the reaction velocity 

 is 4 per cent, in the case of heptane and ether and 7 per 

 cent, in the case of CS 2 . 



XX. Of recent years, considerable attention has been 

 directed to the "radiation" theory of chemical reactions. 

 According to this theory, the ultimate cause of any chemical 

 reaction is to be found in the absorption of radiation of a 

 frequency which depends upon the nature of the reactants. 

 In the case of the majority of chemical reactions, namely 

 those which are not " photochemical " in nature, this radiation 

 will belong either to the visible, or more usually in the short 

 infra-red part of the spectrum. The supporters of the theory 

 hold the view that it is only through the absorption of such 

 radiation that a molecule is able to acquire that excess of 

 energy, over the average at any temperature, which enables 

 it to decompose or to react with another molecule. The 

 frequency of the radiation is therefore known as the 

 "activating" frequency. 



This reasoning leads to the conclusion that the temperature 

 coefficient B of a mono-molecular reaction is determined by 

 the relation 



B-£, d2) 



where "»>" is the activating frequency, which should 

 correspond to an absorption band in the reacting species. 

 No reliable experimental evidence has yet been brought 

 forward in support of this theory, but in view of the scanti- 

 ness of the data existing on homogeneous gas reactions, it is 

 of particular interest to apply it to the results of the 

 experiments described above. 



In attempting to apply the theory, a difficulty at once 

 arises. Evidence has been brought forward to show that 

 the ignition temperature of substances with oxygen is 



