THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 



249 



Both diffusion and sedimentation theories are based, in large part, on ther- 

 modynamic and hydrodynamic considerations which relate the velocity of 

 movement of a particle to the driving force acting on it and the frictional re- 

 sistance experienced during the particle's migration tlirough a viscous medium. 

 The particles, which are considered to be at rest initially, accelerate rapidly to 

 a limiting velocity at which the frictional force is equal to the driving force. 



DIFFUSION OF BUSHY STUNT VIRUS 



BOUNDARY DURING SHARPENING 



P 



solvent 



0.3g./IOO ml. 



solution 



BOUNDARY AFTER DIFFUSION 

 FOR 14581 MIN. 



solvent solution 



0.6 gV 100 ml 



Fig. 2. Representative Rayleigh interference patterns obtained during the diffusion 

 of bushy stunt virus. (From Cheng and Schachman, unpublished.) 



The former is written as the product of the frictional coefficient, /, and the 

 velocity, dx[dt. For diffusion the driving force is the rate of change of the 

 chemical potential with distance. This relation is occasionally expressed in 

 terms of the osmotic pressure by statmg that the force of diffusion is equal 

 and opposite to the force of osmotic pressure tending to drive the solvent 

 that is layered above the solution into the region of the solution. The former 

 is the more rigorous treatment, and the relation between the diffusion co- 

 efficient and molecular parameters is derived from it. It is interesting to 

 note that the thermodynamic derivation gives the same result as that 

 obtained earlier by Einstein (1906) from a kmetic analysis of Brownian 

 motion. For solutions sufficiently dilute that concentration effects are 

 neghgible, the diffusion coefficient is related to the frictional coefficient as 

 follows: 



-7 



(U) 



