SIZE, SHAPE, AND HYDRATION OF VIRUSES 25 



applied to the virus: for the case of diffusion, the origin of this 

 force is the osmotic pressure due to a difference in concentra- 

 tion; for sedimentation, it is the internal stress set up by rota- 

 tional motion; and for viscosity, it is the force necessitated by a 

 certain rate of shear. 



Each of these has its own particular feature. Thus, the force 

 in the case of diffusion does not depend on the density of the 

 medium in which diffusion occurs, but it does depend on tem- 

 perature. In the case of sedimentation in an ultracentrifuge, 

 the applied force depends markedly on the density of the 

 medium but is otherwise not temperature dependent. Viscosity 

 is a matter of energy transfer through a medium and, therefore, 

 depends essentially, on how much of the medium is occupied by 

 the virus under study — this is not directly important in either 

 of the previous cases. Viscosity studies are made with respect to 

 the medium. For diffusion and sedimentation, the medium 

 enters as the agent which provides a retarding force proportional 

 to the velocity. This is in a different way from viscosity studies 

 themselves. 



Thus the three techniques, although really quite closely 

 related, are actually capable of giving different and supple- 

 mentary information about viruses. Rather oversimplifying, 

 we can say that diffusion is concerned with size and shape, 

 viscosity with number, size, and shape, and sedimentation with 

 mass, size, and shape. We can now treat these three forms of 

 study separately and later see how they can be combined. 



Diffusion 



The simplest way to visualize the diffusion of viruses is to 

 think in terms of the osmotic pressure produced by viruses con- 

 sidered as large molecules sharing in the thermal agitation. If 

 we consider a concentration, C, of virus per unit volume, the 

 osmotic pressure exerted is NkTC, where N is the number 

 of molecules per mole, k is Boltzmann's constant, and T is 

 absolute temperature. Now, if C is varying with distance, x, the 

 differential pressure over a distance dx is NkTdC/dx, and this 

 is the excess force on a unit area of the space containing the 



