90 THE PHYSICS OF VIRUSES 



The second feature of protective action is seen in the work 

 of Luria and Exner (1941). When wet T-1 phage is irradiated by 

 X-rays, a semilogarithmic inactivation is found. If the inactiva- 

 tion volume is measured as a function of broth concentration, 

 it can be seen that, as the broth concentration is diminished, the 

 inactivation vokime remains constant until about 0.5% broth is 

 used, whereupon it rises sharply. The strong broth inactivation 

 volume, and that found in irradiating dry virus (and dry broth), 

 are very nearly the same. This is usually taken to mean that the 

 whole of the indirect action is "protected" for 3% broth in solu- 

 tion. This is actually rather surprising since the physical state of 

 a virus would certainly be expected to influence its sensitivity 

 to radiation. It is quite possible that two factors are compensat- 

 ing to give an apparent identity of inactivation volume. Prob- 

 ably, dry virus has a rather larger inactivation volume than 

 does virus in solution. Around the virus in solution there will be a 

 thin layer of water which is probably not truly part of the solu- 

 tion and so does not contain the protective agents of broth . This 

 may accumulate radiation energy and so give an inactivation 

 volume that is larger than expected. Watson (1952) quotes 

 experiments of Doermann which show that adding a strong 

 protective agent (cysteine) cuts the inactivation volume. We are, 

 therefore, led to the idea that in strong-broth radiation action 

 is mostly ( > 50%) due to primary ionization, but that in dilute 

 solution it is largely due to the indirect effect of secondary prod- 

 ucts produced in water. 



Summary of Utility of Radiation Studies 



We can now summarize the value of radiation work. 



Infectivity. (a). Bombardment of dry virus at high specific 

 ionization should give the virus cross sectional area. 



(6). Variation of specific ionization should give evidence of 

 the presence of internal structure. 



(c). Bombardment of dry virus by electrons should give an 

 inactivation volume to be correlated with the results of (b) . 



Detailed Virus Properties, (a). Dry bombardment at high 

 specific ionization should give the total area involved for such 

 factors as adsorption, killing power, and serological affinity. 



