;>8 THE PHYSICS OF VIRUSES 



iiiil)()rtance for protein studies but, as the purity of virus prepa- 

 rations increases, will play a part in virus research. 



Diffusion-Constant Measurement During Sedimentation 



A rough value for the diffusion constant of a virus can be 

 obtained while a sedimentation run is in progress. The measure- 

 ment is in terms of the gradual diffusion of the boundary. If 

 Cz is the concentration at a distance z from the boundary, C'o 

 the concentration in the solution where diffusion has not yet 

 developed, then Cz follows the form of an error integral, and 



By taking experimental values of C^, deduced from pictures 

 taken during centrifugation, the value of y at any point z can 

 be found. The diffusion constant, D, is then 



z~ 



1) = ^ (9 QC}) 



41J -t 

 where t is time. 



The above was pointed out by Svedberg, and the method is 

 described by Svedberg and Pedersen (1940). Such a method of 

 measuring the diffusion constant, although useful since one 

 experiment gives two pieces of data, is not too accurate because 

 of the short times of diffusion, the lack of temperature stability, 

 and the wedge-shaped cell used. Nevertheless, as Lauffer (1942) 

 has pointed out, diffusion measurements made in this way can 

 give very good additional evidence regarding the homogeneity 

 of the preparation. 



Hydrated Partial Specific Volume 



The hydrated partial specific volume can be found by sedi- 

 mentation measurements in solvents of different densities. The 

 value of p, the solvent density, can be varied by introducing an 

 additional solvent until the medium reaches the condition 

 where pFo = 1, when there is no sedimentation. Actually it is 



