62 THE PHYSICS OF VIRUSES 



plant viruses, at least, is spherical, and so offers a simple geo- 

 metrical shape for later speculation. 



This class of study adds one other datum to virus knowledge. 

 Because any water attached externally will scatter like the 

 water solvent, it cannot contribute to the actual net scattering. 

 So any evidence for hydration cannot be due to water found on 

 the surface. Now Miller and Price (1946), from sedimentation 

 and diffusion measurements, found the value of the unhydrated 

 virus volume to be 7.66 X 10"^'' cin^, whereas these X-ray 

 experiments give 13.8 X 10^'^ cm^ for the volume of the inter- 

 nally hydrated virus. There is, therefore, 6.1 X 10^^- cm^ or 

 roughly 6.1 X 10~^^ gm, of water for each 1.10 X 10~^' gm of 

 unhydrated virus, which amounts to 0.55 gm water per gram 

 virus. \Mien this is compared with the total hydration of 0.67 gm 

 water per gram virus, there is a residue of 0.1*2 gm water per 

 gram virus, which is the external layer. Ihis amounts to a 

 thickness of about 4 A and corresponds very nearly to a mono- 

 molecular layer. The same considerations can be applied to 

 tomato bushy stunt virus with rather less accuracy. Ihe internal 

 hydration is found to be 0.50 gm water per gram of dry virus. 



Picture of a Virus 



One of the jiotential contributions of physical studies of viruses 

 is some sort of pictorial representation of a virus. To some extent 

 this can be begun as a result of the work described in this chap- 

 ter. Taking as an example two of the best studied plant viruses, 

 we deduce the appearances shown in Fig. "2. 13. These are the 

 bare canvases on which the real, detailed structure of the virus 

 must be filled in. As further physical studies are elaborated, it 

 will be seen that the ojjen circle and long rod develop surface 

 structures and internal constitutions. It should also be seen that 

 it is far from hopeless to add a great deal of detail by patience 

 and care. 



Identity of Physical Particle and Infectious Unit 



If the objects which are under study by these means are to be 

 described in as detailed a way as we hope, it is plain that we 



