92 



THE PHYSICS OF VIRUSES 



particles cm^ correspond to 1 /' (no use of rep here), and thus the 

 sensitive area, »S, can be found. It is 7.3 X 10^^^ cm-. The whole 

 virus area is 5.5 X 10""'" cm'^ which is eight times larger. Lea 

 proposes to reconcile all these figures by supposing that there 

 is an internal genetic structure which is radiation sensitive. It 

 consists of n units each of radius r, as a first approximation. 

 Putting in the two relations ?z(4/3)7r?-^ = 2.04 X 10~" and 

 nirr- = 7.3 X 10~l^ we deduce 296 units of radius 21 A. This is 

 only a very first trial of such radiation analysis of internal 

 structure, but it is significant that electron micrographs show 

 clearly that there is a substructure; and 296 units in it is not a 

 hopelessly implausible figure. The danger of this type of reason- 

 ing lies in the fact that the nongenetic part is still somewhat 

 radiation sensitive, so that the alpha-particle cross section may 

 contain a part which is not genetic. This would modify that 

 conclusion to give fewer, bigger, units. When the various features 

 of radiation sensitivity have been sorted out and understood, the 

 deductions regarding internal structure will be of the utmost 

 value. 



Before passing on to consider some of the more recent work 

 on viruses and ionizing radiation, we give a table of some of the 

 measured inactivation cross sections and inactivation volumes, 



