26 THE PHYSICS OF VIRUSES 



solute. If, as a result, the solute molecules move with a velocity v, 

 then, by Stoke's law, the force due to viscous drag for a spherical 

 particle of radius a is Q-Kiqav, where 77 is the coefficient of viscosity. 

 For A^i molecules per unit volume, we then have a total force 

 on unit area of QTrr]avNi, and this balances the force due to 

 osmotic pressure. Thus 



NkTdC/dx = QirrjavNi (2.4) 



Now, C is expressed in moles per unit volume, and if we sub- 

 stitute Ci grams per unit volume we have C = Ci/Mw, where 

 My, is the mass of a mole of the solute. Then 



But Mw/N is the mass, w, of one "molecule," and we then 

 have 



kTdCi/dx = QirrjavNim 



The mass, in grams, of A^i molecules is N\m, or M. The product 

 of V and Nim is the mass of solute flowing across unit area per 

 second, or vN\m = dM/dt. Then we find 



dM _ kT_ dC, 

 dt Girrja dx 



Now, the diffusion constant, D, is defined by Pick's law equation 

 as 



where A is area. Since we have considered unit area above, we 

 derive from Eq. 2.5 that 



1) = P- (2.7) 



birr] a 



This important result was derived by Sutherland in 1905 and 

 applied by him to determine the molecular weight of egg albumin 

 from Graham's diffusion data, he obtained the value 33,000. 



