SIZE, SHAPE, AND HYDRATION OF VIRUSES 29 



which is not spherical at all. The same workers report an 

 increase in diffusion rate at low concentrations. This is a sur- 

 prising result, although not out of line with other protein 

 diffusion data. In the case of protein diffusion, the increase 

 is ascribed to dissociation, which seems to be rather difficult 

 to apply to phage particles. More work needs to be done on this 

 effect. Since the survival of a bacteriophage always requires 

 the presence of a certain amount of synthetic medium, it would 

 be worth while to repeat these experiments using specific sero- 

 logical affinity as the indicator. 



Diffusion coefficients can also be measured through porous 

 membranes. The difficulty of using these lies in the fact that 

 the absolute diffusion coefficient is necessary, and therefore a 

 preliminary calibration of the membrane is needed. This is not 

 easy to do. In addition, charge effects, as described in Chapter 5, 

 may introduce a complication. 



Sedimentation 



In the ultracentrifuge, the virus solution is placed in a cell 

 which forms part of a rotor which may be driven in various 

 ways. Leaving the technique aside for a moment we can con- 

 sider what happens in the solution in the cell. The action of the 

 motor and the suspension forces the whole rotor into circular 

 motion. For this to occur, every point in the rotor has a varying 

 inward acceleration co-.r, where co is the angular velocity in 

 radians per second and x is the radius. In a solid, the force 

 necessary to produce this acceleration is transmitted through 

 the intermolecular forces as an internal stress which is adapted 

 to provide the proper force. If adaptation cannot occur, the 

 rotor bursts and a tremendous kinetic energy of the order of 

 5 ft-tons is released. Such explosions are very dangerous and, 

 at the least, very frightening experiences. 



When a liquid is in the cell the same force has to be applied, 

 and it also takes place via the molecules of the liquid. However, 

 if the liquid contains some molecules of greater density, the 

 force necessary to hold these in place may be lacking, in which 

 case the denser particles drift outwards. This outward drift 



