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By Ernest C. Pollard (Auth.)

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5 X 108 À3. If the electron micrographs are taken as justification for treating the virus as approximately spherical, the authors deduce, from Eq. 9 X 108 A3, and the "molecular weight" is 490,000,000. 17 is relatively low and implies a high degree of hydration, which may partly account for the virus instability. The value of the above studies lies mostly in the fact that the correlation between the objects seen in the electron micrographs and the rate of sedimentation of infectivity is very good, so that it can be concluded that the electron microscope observations are not merely of plant debris.

V*)\» , e r n These relations are all set out in Svedberg and Pedersen (1940, Eqs. 68-70b). Actual values for the constants applicable to 20° C are given there. I t can be seen that, by employing Eq. 17, the results of sedimentation, diffusion, and partial-specific-volume measurement can, in the case of nonspherical viruses, yield a value of ///o, the ratio of the frictional drag coefficient to the equivalentsphere frictional drag coefficient. One obvious reason why / / / 0 should not be unity is the fact that the virus may have an asymmetrical shape.

A second case of interest is the Rothamsted variety of tobacco necrosis virus for which Ogston (1942) finds two sedimentation constants of 240 and 51 S. Bawden and Nixon (1951) report that these are two spherical components of diameter 370 and 180 A. In comment on this work it should be pointed out that the basic physical methods of size and shape measurement by motion studies are vulnerable to errors of interpretation. I t has already been seen that hydration and axial ratio can be confused. I t is quite likely that a reconsideration of the actual physical factors involved may change detailed points of the measurement analysis.

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