516 L. H. G R A Y 



radiation on the other, which undoubtedly reflect a dependence of the 

 yield on the linear ion density of the ionizing radiation. The twenty- 

 fold smaller yield in the inactivation of the enzyme carboxypeptidase 

 when irradiated in dilute aqueous solution bj' a radiation as com- 

 pared with X radiation has already been mentioned. The ionic yield 

 for the destruction of tyrosine observed by Nurnberger {S9) using a 

 radiation was nearly thirty times smaller than that observed by 

 Stenstrom and Lohman UO), who used X rays. Again, very early 

 in the history of the subject Duane and Scheuer (4i) made most care- 

 ful measurements of the decomposition of gas-free water by a radia- 

 tion and observed a large initial production of hydrogen peroxide 

 and hydrogen, which gave way at larger dose levels to decomposition 

 into oxygen and hydrogen, whereas it now seems clear from recent 

 French work, reviewed by Frilley (4^), confirming the much earlier 

 observations of Fricke (4-3), that the yield of hydrogen peroxide 

 obtained when air-free water is irradiated by X rays is very low. 



Evidence is steadily accumulating in support of the view ad- 

 vanced by Weiss (44) that the active agents in most radiation-in- 

 duced reactions in dilute aqueous solutions are radicals of hydroxyl 

 and atoms of hydrogen derived, respectively, from the positive and 

 negative water ions formed directly by radiation along the tracks 

 of the ionizing particles. Thus: 



H2O+ > OH + H+ H2O- » H + OH- 



We may presume that this transformation will take place rapidly 

 compared with processes of diffusion so that the initial distribution of 

 OH and H radicals will be sensibly that of positive and negative ions, 

 respectively, at the moment of formation. Thus radicals of both 

 types will be initially found within a cylinder of radius about 15 m/u 

 around the track of the ionizing particle. The subsequent movement 

 of radicals jnay be calculated by the application of the equations of 

 diffusion and of chemical kinetics, as has been done for certain simple 

 cases by Lea (i). In this connection it is important to distinguish 

 two ways in which the linear ion density of the radiation may influ- 

 ence the chemical changes brought about. In the first place, since 

 the radius of the cylindrical column of radicals is independent of 

 linear ion density the average initial concentration of radicals will in- 

 crease with increasing linear ion density, so that interaction with so- 

 lute that takes place in competition with the recombination of the 

 radicals to form water would, on this account, be expected to de- 



