32 THE PHYSICS OF VIRUSES 



In the case of spherical viruses, which are quite commonly 

 found, the reasoning becomes relatively simple. For in this case, 

 as for diffusion, the frictional drag is precisely described by 

 Stoke's law and is Girrja, where a is the radius of the particle, 

 and r) the viscosity of the medium. We then have 



■m(l — Vop) 



S = — S. -— (2.13) 



bTTTja 



Since HI, the jmrticle mass, is a~r^^ where I o is the partial 



specific volume of the particle, the problem of finding a reduces 

 to finding the partial specific volume, T"o- 



In this expression, the partial specific volume is that which 

 applies to the unhydrated virus. This will not be valid if the 

 density of the bound water is changed because of added forces 

 on it, and so there is a little uncertainty here. However, the 

 best procedure is to use the unhydrated value of T o- 



Doing this, we have 



_ 4 7ra=>(l - Top) ^ ^Za\l - Vop) 

 * 3 r^67r77« QVoV ^ ^ ^ 



from which a can be determined. 



The Frictional Drag Coefficient for Nonspherical or 

 Hydrated Viruses 



The sim})le case considered above is nearly always obscured 

 by either asymmetry of the particles or by the presence of 

 hydration. We can therefore look on the sedimentation process a 

 little differently and solve the sedimentation relation for /, the 

 frictional drag coefficient. Then 



f = !^~XoA) (02.15) 



and, if we write /o for the case where the })article is spherical, 

 we have 



/o = i^TTTja — ()7n7 



■Itt 



