64 THE PHYSICS OF VIRUSES 



talline bodies can be found in the different plants which, while 

 in the healthy state, have no common substances which are com- 

 parable. The infect ivity of purified preparations is very high 

 and, if allowance is made for the expected loss of activity in 

 ])urification, is high enough to account for all the infective agent 

 being in the purified prejiaration. Finally, the antibodies pro- 

 duced by clarified plant sap from infected plants combine 

 specifically and strongly with the jjurified virus preparations. 

 All these facts add up to make it most likely that the j^repara- 

 tions we call virus preparations are, in reality, strong concen- 

 trations of the infectious principle, as they should be. 



A little extra confidence in this belief can be obtained by sub- 

 jecting a virus preparation to ultracentrifugation and seeing 

 whether the optical boundary of the concentrated nucleoprotein 

 and the rate of transfer of infectivity coincide. By using a 

 separation cell, in which a barrier across a sedimentation cell 

 permits sedimentation through it under high acceleration, but 

 does not allow rapid back diffusion, the rate of transfer of in- 

 fectivity can be measured and comjjared with the rate of sedi- 

 mentation by conventional means. This method has been ex- 

 ploited by Lauffer in particular. For three viruses, tobacco 

 mosaic (Lauffer, 1942), influenza (Lauffer and Miller, 1944), and 

 southern bean mosaic (E})stein and Lauffer, 1952), the infectivity 

 follows the optical sedimentation. The identity of the two princi- 

 ples is therefore the more likely. 



The diffusion measurements of Poison and Shejjard (1949) on 

 T-3 and T-4 bacteriophages showed, at high concentration, that 

 the diffusion constant was in reasonable agreement with the 

 shape of the particles seen in electron micrographs. This argues 

 that the phage particle is indeed the infectious unit. The fact 

 that electron micrographs of bursting bacteria show about the 

 right population of sperm-like objects is further evidence. 

 However, it should be realized that, whereas the virus may have 

 one shape and structure between residences in a host, the whole 

 of this structure may not be needed for multiplication inside the 

 host. So there is still left a question as to the nature of the object 

 on which the biophysicist should concentrate his attention when 

 making theoretical speculations regarding virus multiplication. 



