THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 239 



a solution of uniform particles. In addition, information can be obtained 

 about the existence of a permanent dipole moment in the particles as con- 

 trasted to one that is induced under the influence of the field. The production 

 of birefringence in an electric field is known as the Kerr effect, and its 

 demonstration with tobacco mosaic virus was first made by Lauffer (1939). 



a. Streaming Birefringence. Birefrmgence with rodlike particles can also 

 be established by imposing a velocity gradient on a solution of such particles. 

 As already indicated in the discussion on viscometry, the particles become 

 oriented in a flowmg stream as a consequence of the differing velocities of 

 the streamlines. The resulting birefringence is known as streaming bire- 

 fringence or double refraction of flow. It is of considerable interest historically 

 to note that streammg birefringence was demonstrated with tobacco mosaic 

 virus before the virus was isolated in pure form (Takahashi and Rawlins, 

 1932, 1933). When the juice extracted from mosaic-diseased tobacco plants 

 was passed through a fine capillary placed at 45° between crossed Nicol 

 prisms, the field of view corresponding to the flowing solution was bright 

 against a dark background. The extract from normal plants was not bire- 

 fringent. Although this birefringence in the juice extracted from diseased 

 plants could result from various types of particles in the solution, it was 

 shown even at that early date that rodlike particles were involved and that 

 the solution of oriented macromolecules resembled a crystal. Further in- 

 vestigations (Lauffer, 1938b) after the virus was obtained in purified form 

 revealed that the birefringence was not an intrinsic property of the particles; 

 i.e., the virus is optically isotropic and the birefringence produced in the 

 flowing stream is the result of alignment of nonbirefringent particles. 

 Although this simple apparatus is satisfactory for demonstrating double 

 refraction of flow, the measurement of rotational diffusion coefficients 

 tlirough the exploitation of a velocity gradient requires elaborate apparatus 

 similar to a Couette viscometer. In effect, the average orientation of the 

 macromolecules relative to the streamlines is determined by optical means. 

 Measurements are made at a variety of shear gradients, and rotational 

 diffusion coefficients are calculated from each measurement with the aid of 

 an elaborate hydrodynamic theory (for review see Cerf and Scheraga, 1952). 

 Here, too, information of value is provided about the homogeneity of the 

 particles. This technique has been applied frequently by different workers 

 to preparations of tobacco mosaic virus. 



Hi. Anomalous Viscosity. Recently a third method has become available 

 as a result of theoretical investigations of the complex hydrodynamic prob- 

 lem relating the intrinsic viscosity to the shear gradient (Saito, 1951). It has 

 been pointed out earlier that the contribution of anisometric particles to the 

 viscosity is a fmiction of the orientation of the particles relative to the 

 direction of flow. Opposing the orientation caused by the velocity gradient 

 VOL. I — 17 



