84 THE PHYSICS OF VIRUSES 



which lead to r = 98 A, and I = 1,200 A. The radiation data 

 therefore fit a long, thin virus, rather fatter and shorter than 

 that fitted by the electron microscope data. The measurements 

 are not accurate enough to do more than claim a fair check and 

 to conclude that not quite all (about 80 %) of the virus is highly 

 sensitive. 



Unfortunately, TMV is the only virus so far studied for which 

 a consistent analysis can be made. To show the kind of trouble 

 encountered, the figures for SBMV are here analyzed. The 

 deuteron cross section is 6.2 X 10~^^ cm^. The inactivation 

 volume from electron data is 3.4 X 10~^^ cm^ Now we have 

 already stressed that SBMV is spherical (though it may not be 

 so in the dried state) . So if we assume a radius r, we have 



7rr2 = 6.2 X lO^^^ ^^2.^ r = 141 A 

 Httt^ = 3.4 X 10-i« cm^; r = 93 A 



Although these are not way out of line, from a crude point of 

 view, the "electron" radius is too small. Also, if the critical 

 volume is as thick as even 186 A (twice the "electron" radius), 

 then deuterons of low ion density should certainly produce one 

 primary ionization in it. Yet if they do, they seemingly don't al- 

 ways inactivate, because S is not the full value for fast deuterons. 



Notice that one figure seems to be very reasonable. The 

 radius for maximum deuteron action is a little less than that of 

 the virus in solution, and corresponds rather closely to the radius 

 of the virus less hydration as observed in the electron microscope. 



To weasel out of the dilemma, it can be supposed that as 

 the virus dries it flattens. The actual sensitive volume is then 

 less than that for the spherical virus. Assuming it to be a flat 

 cylinder, of height h and radius 141 A, the value of h turns out 

 to be 76 A. This effective thickness fits the variable-energy 

 deuteron data of Fig. 3.6 quite well, for in such a thickness, the 

 chance of a deuteron passing through without producing a 

 primary ionization is 22%, so that the observed smaller cross 

 section is quite plausible. The loss in volume from the X-ray 

 scattering volume is 10.4 X 10"^*^ cm^, which is rather great to 

 correspond to the known hydration of the virus. There is, there- 



