THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 231 



randomly oriented. Actually the very process of making a viscosity measure- 

 ment tends to orient anisometric particles because tlie streamlines of the 

 flowing hquid move at different velocities relative to one another. A particle 

 havmg its long axis oriented in the direction of flow creates less disturbance 

 to flow than does a similar particle oriented perpendicular to the direction 

 of movement of the liquid. Anisometric particles, such as rodlike or plate- 

 like objects, are subjected to rotary Brownian motion in solution; because 

 of this rotation the effective volume occupied by the particles is much larger 

 than their geometric volume. As a consequence, anisometric particles 

 which are randomly oriented in solution cause a greater mcrease in viscosity 

 than do spherical particles of the same volume. Thus differentiation of 

 particles of spherical shape from those which are elongated is readily 

 achieved by viscosity measurements. 



Flexible synthetic macromolecules resulting from the polymerization of 

 small units have also been treated theoretically in terms of their contribution 

 to the viscosity of solutions. For these substances both free draining and 

 impermeable models have been considered. Since virus particles are generally 

 thought to be rigid, only models for such undeformable, impermeable 

 structures are considered here; but it should be emphasized that the equa- 

 tions for these models are likely to be inapplicable to certain constituents of 

 viruses, such as ribonucleic acid, and perhaps even to some unusual viruses. 

 For a critical and thorough discussion of the viscosity of solutions of flexible 

 macromolecules, the reader should consult Flory (1953). 



All of the theories outlined above require knowledge of the volume con- 

 centration of the solute m terms of the kinetic or hydrodynamic unit. 

 Seldom is the concentration known in that form. Instead, only the dry weight 

 concentration can be determined satisfactorily. This causes a dilemma which 

 can be handled in different ways, depending upon the availability of auxiliary 

 information. If, for example, the electron microscope shows that the particles 

 are spherical and the material is pure, the viscosity data are used to compute 

 the volume concentration. This in turn is combined with the dry weight 

 concentration to calculate the amount of water associated with the dry 

 material in the kinetic unit. Some workers refer to this value as the hydra- 

 tion. In the absence of evidence that the particles are spherical, both the 

 shape and effective volume must be considered as unknowns each of which 

 can be evaluated from the appropriate combination of different hydro- 

 dynamic measurements. 



c. Measurement of Viscosity. Of the various techniques employed in the 

 measurement of the viscosity of solutions, that method based on the capillary 

 viscometer has achieved the widest popularity. It combines simplicity with 

 accuracy in a manner seldom experienced in physical chemical practice. 

 Various designs have been proposed but the operating principle in aU is the 



