IONIZING RADIATION AND VIRUSES 91 



(6). Varying the specific ionization should show the approxi- 

 mate depth of the units responsible for these factors. 



(c). Dry electron bombardment should give the total volume 

 of each factor. 



(d). Wet bombardment should enable some idea to be found 

 of the degree to which these factors are involved with the surface. 



Results of Infectivity Studies 



An excellent account of some of the earlier studies on viruses 

 is given by Lea (1947). Among the first of these were studies by 

 Gowen and Lucas (1939) on vaccinia and by Gowen (1940) 

 on tobacco mosaic virus. The probable complication from the 

 indirect effect of radiation was not properly realized at first, 

 although Gowen's work on TMV includes dry inactivation. 

 The best summary of data up to 1946 is undoubtedly that of 

 Lea, who concludes from the data then available that the inacti- 

 vation volume can be used as a measure of the virus size for the 

 smaller viruses. It has been pointed out that this cannot be 

 maintained in view of more recent work under w^hat should be 

 better conditions. Nevertheless, the use of ionizing radiation 

 as a means of determining some critical volume of importance 

 to infectivity seems to have been suggested by Gowen (1940), 

 Wollman, Holweck, and Luria (1940), Lea (1940), and Luria and 

 Exner (1941) at very nearly the same time. 



Of particular interest among work done at this time is Lea's 

 analysis of the available data on vaccinia. Remarkably advanced 

 studies on this virus were made by Lea and Salaman (1942). The 

 virus was irradiated dry, by gamma rays, X-rays, and alpha 

 particles. The inactivation dose for vaccinia is 80,000 r. At this 

 figure there is 37%, or e~\ survival. Using the conversion factor 

 of 6.13 X 10^^ primary ions cm^ for 1 rep, and assuming roent- 

 gens and rep to be equivalent, the inactivation volume is found 

 to be 2.04 X 10~^^ cm^. The actual volume, judged from electron 

 micrographs, is roughly 9 X 10~^^ cm^ so that the volume 

 sensitive to one primary ionization is only one four-hundredth 

 of the whole virus, in round numbers. The inactivation dose for 

 alpha particles was 211,000 r, which can be reduced to number of 

 particles per square centimeter by using the fact that 6.5 X 10^ 



