24 



THE PHYSICS OF VIRUSES 



and compared it with results from electron microscopy and 

 sedimentation. The filtrate of "Texas 1" Coxsackie virus is 

 markedly more infective when the average pore diameter exceeds 

 500 A, and, from the actual point of onset of infectivity and the 

 correction factor mentioned above, they conclude that the 

 particle diameter is between 150 and 230 A. The same virus 

 was measured by sedimentation methods, as described in the 

 next section, with the result that a value of 330 A was obtained. 

 An electron microscope figure for the same virus gave 350 A. 

 The values are included in the above table. 



It can be seen that ultrafiltration can give valuable approxi- 

 mate values with initial preparations of new viruses. It would 

 appear as though too much should not be expected of the 

 method. 



Observations of the Motion of Viruses: Diffusion, 

 Sedimentation, and Viscosity 



The use of the motion of an object in a force field and in a 

 viscous medium formed the basis for early measurements of the 

 electronic charge exemplified by Millikan's oil-drop experiment. 

 The mobility of gaseous ions formed one part of the first dis- 

 covery of structure in the atom. The same idea is used in study- 

 ing macromolecules. The rate of diffusion, the viscosity, and 

 the rate of drift under the action of a rotational acceleration 

 are concerned with the size, shape, and hydration of a virus 

 particle and so afford a method of studying these features. 



Since all three of these observations of virus motion are 

 important, and, if possible, all three should be made on any one 

 virus preparation, a word about the interrelation of the three 

 methods of study is first in order. In all three cases, a force is 



