VIRUSES — STANLEY 265 



be responsible for changes in the virulence of a virus may also be 

 elucidated. 



Tobacco mosaic virus contains 50 percent carbon, 7 percent hy- 

 drogen, 16 percent nitrogen, 0.6 percent phosphorus, and 0.2 percent 

 cysteine sulfur. It has an isoelectric point at pH 3.5, a density of 

 1.37, and at a concentration of about 2 mg. per cc. a sedimentation 

 constant of 174 X 10"" cm. in unit centrifugal field and a diffusion 

 constant of 3 X 10"® sq. cm. per sec. It has been estimated by indirect 

 methods that the particles of the virus are remarkably anisometrical 

 and are about 400 m/x in length and about 12 niju, in diameter. Re- 

 cently, by direct observation by means of the electron microscope, Dr. 

 Anderson and I found that most of the particles in a dilute solution 

 of the virus are about 280 m/x in length and about 15 m/x in diameter 

 (pi. 1, fig. 3). Several kinds of evidence indicate that the molecular 

 weight of tobacco mosaic virus is about 50 millions. The value of 

 17 millions, which was estimated several years ago when the asym- 

 metry was unknown and which was based on an assumed asymmetry 

 constant of 1.3, is incorrect. However, it is possible that different 

 strains of tobacco mosaic virus may have different molecular weights, 

 for the sedimentation constant of the aucuba mosaic strain is meas- 

 urably larger than that of tobacco mosaic virus and the X-ray data 

 indicate that the molecules of the former have the same diameter 

 as that of the molecules of tobacco mosaic virus. Furthermore, Melch- 

 ers and coworkers, in a study by means of the electron microscope, 

 found the molecules of the two strains of tobacco mosaic virus with 

 which they worked to have particle lengths of about 190 m/x. and 

 140 m/x. 



Tobacco mosaic virus gives a sharp boundary and migrates at a 

 uniform rate in the Tiselius electrophoresis apparatus (pi. 2, fig. 1). 

 When carefully prepared, the virus gives a sharp boundary in the 

 ultracentrifuge, but on treatment with salt at room temperature 

 some of the particles appear to aggregate end-to-end to give a prep- 

 aration which shows two boundaries in the ultracentrifuge. The 

 second more rapidly sedimenting boundary is due apparently to a 

 component formed by the end-to-end aggregation of pairs of mole- 

 cules (pi. 2, fig. 2). Further aggregation yields a very inhomoge- 

 neous product which shows a very broad boundary in the ultracen- 

 trifuge. The sedimentation constant of tobacco mosaic virus has 

 been found to vary with the concentration, owing apparently to inter- 

 particle forces which become of considerable magnitude in concen- 

 trated solutions. Solutions of tobacco mosaic virus exhibit strong 

 double refraction of flow and electrical double refraction, the former 

 being due to the rodlike shape of the particle and the latter to the 

 particle being asymmetrically charged, either permanently or as a 

 result of the electrical field (pi. 2, fig. 3) . The fact that tobacco mosaic 



