40 THE PHYSICS OF VIRUSES 



density of the medium of 1.104 gives the vakie O.OOO for the 

 partial specific vohime. 



The use of sucrose as a buoyancy variant substance has often 

 been employed. This is a small molecule and so introduces 

 osmotic effects. However, if the slope of the line at low concen- 

 trations is used to extrapolate to the zero-force point, the figure 

 obtained should be valid. 



For smaller, or thinner viruses, this method does not always 

 yield consistent results, and Schachman and Lauffer (1949) 

 have suggested that a layer of water, half the thickness of solute 

 molecules, forms on the virus. For larger viruses this is not a 

 very serious correction. 



Before describing some actual measurements on viruses, a 

 description of the place of studies of viscosity in virus research 

 is in place, and then a general discussion of this approach can 

 be given. 



Viscosity 



When very pure virus preparations are available, so that the 

 virus solution can be treated as containing only virus particles 

 and solvent, the measurement of the viscosity of the vii-us 

 solution can give information regarding size and shape. The 

 viscosity of a liquid is measured in terms of a rate of shear of 

 the liquid, and, since a force is required to push one layer of 

 liquid over another at a certain rate, viscosity is essentially a 

 process in which mechanical work is continually being dissi- 

 pated. The rate of such dissipation is measured by the quan- 

 tity 'r}A{dv/dx)v, where v is the velocity of the liquid, A is the 

 area of a plane surface of the liquid, dv/dx is the rate of change 

 of velocity with distance at the area A, and r] is the coefficient 

 of viscosity. Einstein (1906, 1911) investigated the flow of 

 liquid around a small sphere — small, yet large compared to 

 molecular separations — and derived an expression for the 

 energy dissipation due to viscous flow around this sphere. He 

 then examined the energy dissipation for a very large number 

 of such spheres and derived the approximate exj)ression 



r//^o = 1 + 'i-^cf) (2.24) 



