298 H. K. SCHACHMAN AND R. C. WILLIAMS 



Suspensions of particles of a macromolecular size can be investigated for 

 homogeneity in at least a half-dozen ways: (1) Size, shape, particle weight — 

 these are physical characteristics appropriate to examination by the electron 

 microscope, and by ultracentrifugal methods; (2) density — a parameter 

 observable by centrifugation; (3) infectivity — discoverable only biologically; 

 (4) antigenicity — subject to examination by serological methods (5) surface 

 potential — a physicochemical parameter measured by electrophoretic 

 methods; (6) internal structure— amenable to some degree of determination 

 by X-ray crystallography. It is entirely possible for a virus-containing pre- 

 paration to be homogeneous in one or more respects and inhomogeneous in 

 others. 



Some samples of preparations containing virus particles may be offered 

 in order to show more clearly the ways in which the word "homogeneous" 

 may be interpreted. These will be suspensions of: 



1. A T-even bacterial virus. 



2. Bushy stunt virus. 



3. Tobacco mosaic virus (TMV). 



4. A hypothetical mixture of two plant viruses which are identical in 

 shape, size, and particle weight, 



1. T-even bacteriophage particles are probably uniform with respect to 

 shape, size, density, and electrophoretic mobility, though these properties 

 are only inexactly known for these viruses. They are antigentically uniform, 

 while little is known of their internal structure. They are uniform in infec- 

 tivity, in the sense that there is a one-to-one ratio between particle numbers 

 and infective miits. 



2. Bushy stunt virus particles are found to be highly uniform with respect 

 to all physical parameters, including some X-ray evidence on internal 

 structure. They are probably serologically uniform. But a suspension of 

 bushy stunt virus may be quite inhomogeneous with respect to infectivity, 

 since it is found that over 10^ particles must be contained in an inoculum 

 that will produce only one leaf lesion. 



3. Most, but not all, preparations of TMV are inhomogeneous with respect 

 to the lengths of the particles, although they are uniform in width. They are 

 apparently homogeneous with respect to particle density. A preparation of 

 a single strain of virus is serologically and electrophoretically homogeneous. 

 The specific infectivity is low, as with bushy stunt virus, and so the uni- 

 formity of particles with respect to infectivity is unknown. X-ray analysis 

 of paracrystalline arrays indicates a high degree of uniformity of internal 

 structure from particle to particle. If the TMV suspension is of particles of 

 different strains, it will have demonstrable inhomogeneity with respect to 

 antigenicity, electrophoretic mobility, and infectivity. 



