46 THE PHYSICS OF VIRUSES 



microscope. The field contained particles of rather irregular 

 circular shape, with estimated volumes spreading from 5.0 X 

 10^ A^ to 1.3 X 10^ A^. The mean volume was close to 8.5 X 

 108 A^ 



If the electron micrographs are taken as justification for 

 treating the virus as approximately spherical, the authors 

 deduce, from Eq. 2.13 for sedimentation, that the virus diameter 

 is 1,100 A, the volume is 6.9 X 10^ A^ and the "molecular 

 weight" is 490,000,000. The hydrated density of 1.17 is rela- 

 tively low and implies a high degree of hydration, which may 

 partly account for the virus instability. 



The value of the above studies lies mostly in the fact that the 

 correlation between the objects seen in the electron micrographs 

 and the rate of sedimentation of infectivity is very good, so that 

 it can be concluded that the electron microscope observations 

 are not merely of plant debris. The electron microscopy thus 

 acquires added strength, and a further proposal that possibly 

 the virus particles are more ellipsoidal in character can be made. 



It is of interest that the visible boundary sedimented at the 

 same rate, within experimental error, as the infectivity. This 

 offers some more evidence that the virus particles are indeed the 

 infectious bodies, but only circumstantially so. 



The above study is chosen because it may well be representa- 

 tive of virus work. Viruses, in general, are not exceptionally 

 stable, can not be highly purified, and are hydrated so much 

 that simple physical observations on them are difficult. The 

 conclusions that can be drawn are nevertheless not trivial, they 

 serve to characterize the virus particle as a definite object and 

 are helpful in further studies. 



It is to be regretted that very few studies of the rate of sedi- 

 mentation of plant viruses as measured by serological affinity 

 have been made. This virus assay is rather easier than the local- 

 lesion technique, and the results would add a further piece of 

 information regarding virus size and shape. 



As a second example we can choose southern bean mosaic 

 virus, studied by Miller and Price (1946). This is a spherical 

 plant virus capable of being highly purified, and so is fully 



