28 THE PHYSICS OF VIRUSES 



TABLE 2.2 

 Diffusion Constants and Radii for Three Spherical Viruses 



Virus Diffusion constant (cm^/sec) Radius (A) 



Bushy stunt 

 Southern bean mo.saic 

 Rabbit papilloma 



Many classes of virus cannot l)e prepared in sufficiently high 

 concentration for optical observation. This is notably true of 

 bacteriophages. Poison (1944) has designed a simple and effec- 

 tive method for diffusion study which consists of a series of four 

 superposed cylinders with holes drilled through them. The 

 cylinders can be rotated so that the holes are all aligned, or they 

 can be turned to separate the material in each cylinder. In this 

 way the holes, which form the diffusion cell, can be filled, turned 

 into place for diffusion, and then separated for sampling of the 

 average concentration in the two layers. Poison has shown that 

 if Co is the original concentration of the solution, C the mean 

 concentration in the originally virus-free cell after time t, and // 

 the height of the upper section, then D, the diffusion coefficient, 

 is given by 



irm (C\ 



The measurement of the diffusion constant is thus a simple 

 matter of comparing two concentrations. The ratio has to be 

 accurately measured, as it occurs as its square. 



Poison (1944) has applied this to equine encephalomyelitis 

 virus, with the result that the diffusion constant is 8.0 X 10^^ 

 cmVsec. 



Poison and Shepard (1949) have studied T-3 and T-4 coli 

 bacteriophages, with the result that at high concentrations T-3 

 has a diffusion constant of 1.19 X 10~' cmVsec and T-4 a value 

 of 8.0 X 10~^ cm^/sec. T-3 is roughly spherical, with the pos- 

 sibility of a short tail. Assuming the spherical shape, the diam- 

 eter deduced is 362 A. No real estimate can be made for T-2, 



