INTRODUCTION 57 



The thermal reactions of these radicals and molecules will then de- 

 termine the course of the overall reaction. Ions similarly produce 

 radicals or molecular products, either directly or as a result of 

 recombination. 



At the present time it is generally believed that the results of all 

 radiochemical reactions (at least for non-vital systems) can be explained 

 as the consequence of a set of reaction steps, similar in nature to those 

 observed in ordinary photochemical and thermal reactions. Since 1936 

 (2), when this view was first clearly stated, there has been such rapid 

 progress (3) in the field of radiation chemistry, especially during and 

 subsequent to World War II, that we are sometimes inclined to over- 

 look the important contributions which Lind and Mund made before 

 1936. It should be worth while, therefore, to review briefly some of 

 these earlier results and the hypotheses which were suggested to interpret 

 them. Bragg (4) in 1907 noticed that the number of molecules of liquid 

 water decomposed by radon was equal to the number of ions which the 

 same amount of radon would produce in air and referred to this relation 

 as "a curious parallelism in numbers." Three years later LeBlanc (5) 

 interpreted this parallelism as an analog to Faraday's law, and referred 

 to the radiochemical decomposition of water as "electrolysis without 

 electrodes." These ideas were extended and systematized in 1918 by 

 Lind (6), who presented the formal or "stoichiometric" cluster hypothesis 

 in an attempt to explain the constancy of the ion-pair yield observed for 

 the formation of w^ater from its elements. As experimental evidence 

 accumulated, it was found that the measured values of the ion-pair yields 

 of the oxidation of carbon monoxide and of methane (including the 

 sensitizing action of inert gases) , of cracking reactions of simple saturated 

 hydrocarbons, and (possibly) of the decomposition of nitrous oxide were 

 all consistent with the simple or formal cluster hypothesis. However, 

 the large ion-pair yields observed in the polymerization of unsaturates 

 (especially acetylene) demanded the assumption of large clusters, and 

 accordingly a modified "physical" or "dynamic" cluster hypothesis was 

 suggested by Mund (7). The results of the measurements of the decom- 

 position of ammonia required the addition of an arbitrary (but not 

 unreasonable) assumption to render these data compatible with the 

 cluster theory. Finally, the experimental results for the formation of 

 hydrogen chloride and of hydrogen bromide from their elements, as well 

 as results for the or^/io-para-hydrogen conversion, were completely 

 divergent from predictions based upon the simple cluster theory. Eyring, 

 Hirschfelder, and Taylor (2) published in 1936 the first attempt to inter- 

 pret the observed rates of radiochemical reactions in terms of ordinary 

 reaction kinetics. Their analysis of the o/1/io-para-hydrogen conversion 



