76 THE PHYSICS OF VIRUSES 



distributed in area, are fired at the virus, and it is supposed that 

 along the track there is dense enough ionization to produce a 

 detectable effect. For this purpose, slow deuterons or alpha 

 particles are suitable. The resulting measurement yields a 

 "cross section," or equivalent area. 



(6). Faster heavy particles, which ionize more sparsely, are 

 used for bombardment. These produce primary ionizations 

 spaced somewhat like the dimensions of the element of the virus 

 under study. By varying the particle speed, and hence the ion 

 spacing, the effective depth of the radiation-sensitive region can 

 be studied. 



(c). Fast-electron bombardment is used. This essentially 

 gives ionizations which are random in volume because the ion- 

 ization is sparse, is laterally spread, and the electrons scatter 

 readily. This kind of bombardment measures the volume of the 

 sensitive element. 



By this triple attack, it is possible to get an estimate of the 

 size and shape of a single, sensitive element, or the size and num- 

 ber of a multiple element. The method is relatively new in this 

 kind of application but has yielded quite valid information in 

 studies of enzymes, hormones, and antibiotics. These studies are 

 described in the review article quoted (Pollard, 1953), and, 

 to summarize the findings briefly, it can be said that the arrival 

 of a primary ionization in a dry enzyme or hormone, in 15 

 separate measured cases, results either in the removal of its 

 biological function, or in the removal of a definite fraction cor- 

 responding to a definite unit of substructure (as for catalase). 

 The premise that a primary ionization can be used to "feel 

 out" the sensitive shape is therefore justified, as far as a pre- 

 liminary study goes, at all events. 



In using the method, it must be borne in mind that in the 

 preceding description no explanation of the action of ionizing 

 radiation has been given. All that has been done is to suppose 

 it is drastic and disruptive. In actual fact, evidence is accumulat- 

 ing to the effect that only some of the observed consequences of 

 ionizing-radiation action in the dry state are due to such high- 

 energy disruption. A part is more gentle and possibly more 



