370 S. GAED AND O. MAAL0E 



When protected against indirect effects, viruses are, as a rule, inactivated 

 exponentially by X-rays. This has been observed by Levin and Lominski 

 (1936) for fowl plague virus, by Gowen and Lucas (1939) for vaccinia virus, 

 by Friedewald and Anderson (1940) and by Syverton et al. (1941) for Shope's 

 rabbit papilloma virus, by Wollman and Lacassagne (1940) for several 

 phages, and Wollman et al. (1940) for phage C16. 



Throughout this section, we deal with experiments carried out with 

 "hard" X-rays (wavelengths < about 1 A), The electrons produced by such 

 radiation are fast, and the primary ionizations they give rise to may be con- 

 sidered to be randomly distributed in the specimen. This is the reason why 

 the volume of a radiation-sensitive target determmes the chance of its being 

 hit and, therefore, the rate at which it is inactivated. In Section B, 2, 6, we 

 shall discuss experiments with other types of radiation that produce ion- 

 izations that are closely spaced along the track of the ionizing particles. The 

 use of such radiation permits estimation of target area. 



It was noted already by Wollman and Lacassagne (1940) that big phage 

 particles are more sensitive than smaU ones, and the possibility of estimating 

 virus size by means of X-ray inactivation has been thoroughly explored 

 by Lea and co-workers. Lea (1947) states: "If the inactivating dose. Do, were 

 used to calculate the target volume (assuming spherical target) the diameter 

 obtained would be withm a factor two of the virus diameter for 20 out of 23 

 measurements (on 18 different viruses)." Some of these estimates of virus 

 size and of D^, value are quite rough; Lea's analysis only goes to show that the 

 ratio between the calculated target volume and the particle volume is 

 usually between 1 : 1 and 1 : 10. 



With the improved methods now available for measuring virus size 

 (Williams, 1954), the rough estimates obtained by equating target and virus 

 volumes, as suggested by Lea, are of little interest except, perhaps, in cases 

 where purification and concentration of a small virus present great difficulties.^ 



An interesting series of studies has been reported by Lauffer and co- 

 workers; these studies comprise inactivation experiments with phage T5, 

 influenza viruses, and TMV: 



Buzzell and Lauffer (1952) found that in " 4 % broth " but not in " 0-8 % 

 broth," phage T5 was inactivated exponentially. In the first case, surviving 

 particles and nonirradiated phage showed the same resistance to high tem- 

 perature; whereas, in the second case, where indirect effects presumably 

 played a role, the survivors were more sensitive to heat than normal phage. 

 This apparently is another example of "sensitization" (cf. p. 3G9), and it con- 

 stitutes an easUy testable difference between phage particles inactivated by 

 direct and by indirect X-ray effects, respectively. 



^ In such a case a better estimate would probably be obtained using a-rays^ estimate 

 the target area (Lea, 1947; Bonet-Maury, 1948). See also Section II, B, 2, h. 



