Action of Radiations on Viruses 159 



From the experimental inactivation doses, one can calculate 

 the "target" diameter, i.e., an estimate of the diameter of the 

 virus based on the hypothesis that an ionization anywhere in 

 the virus particle will inactivate it. The agreement between the 

 three estimates of target diameter and their close approximation 

 to the size of the virus as determined by other methods (centrifu- 

 gation and filtration) satisfactorily confirms this hypothesis, and, 

 incidentally, establishes that this bacteriophage is one of the 

 macromolecular viruses. 



Organism-type Viruses 



If we attempt to apply the same type of reasoning to a large 

 virus, we find that the estimates of the target size deduced 

 from experiments with the three radiations do not agree, and 

 are all much smaller than the true size of the virus, as shown 

 in Table 11.^*^ It is evident that the hypothesis that an ionization 



TABLE II. 



Inactivation of Vaccinia Virus 



(Virus diameter 200 m|.i) 



Gamma X- Alpha 



rays rays rays 



Inactivation dose in millions of rontgens 0.080 0.104 0.211 



Inferred "target" diameter in mii 31 41 70 



anywhere in the virus particle leads to inactivation is incorrect. 

 It is believed that a single atom ionized can inactivate the virus, 

 but it must be an atom, not anywhere in the virus, but in certain 

 radiosensitive constituents of the virus, these constituents com- 

 prising only a small fraction of the total bulk of the virus par- 

 ticle. This differentiation between radiosensitive and radio- 

 insensitive constituents suggests a cell rather than a macromole- 

 cule, and it is probable that the radiosensitive material is to be 

 identified with the genes. The fuller analysis of the radiation 

 data enables an estimate of the number of genes to be made.^^ 

 We are thus led to regard vaccinia not as a naked gene, as 



