M. A. LAUFFER 



lion held a much liiglicr intrinsic viscosity and a biniodal distribution of 

 lengths, with one maximum at the same particle length as in the case 

 of the first preparation and the other maximum at a greater value of 

 particle length. The ratios of the average length to thickness of the 

 particles of the two preparations were calculated from the intrinsic 

 viscosities by means of the Simha equation for elongated ellipsoids; 

 and the weight averages of the squared axial ratios were computed 

 for the two preparations from the lengths of the particles determined 

 by the electron microscope and from the thickness obtained by x-ray 

 diffraction measurements. The weight averages of the squared axial 

 ratios were calculated because, according to the Simha equation, 

 intrinsic viscosity is approximately a function of the amount of ma- 

 terial and of the square of the axial ratio. The results are shown in 

 Table IV. 



Table IV 

 Axial Ratio of Tobacco Mosaic Virus Particles 



The determination of particle lengths with the electron micro- 

 scope is a direct measurement subject only to the error of the empirical 

 evaluation of the magnification factor of the microscope. There is 

 evidence to indicate that the tobacco mosaic virus particles in solution 

 are not appreciably hydrated. However, even if they are hydrated 

 to a considerable extent, the error in the intrinsic viscosity would be 

 small for such extremely elongated particles. Therefore, the data 

 shown in Table IV constitute a direct experimental verification of the 

 essential correctness of the Simha equation for the intrinsic viscosity 

 of rodlike ellipsoids of revolution. 



The verification of the Simha equation is important because 

 it removes the most serious objection to the use of viscosity data in the 

 determination of particle shapes. It is quite important that one be 

 able to estimate the shape of a biological particle, for, from the shape, 

 the friction ratio can be computed by means of the Perrin equation. 

 The evaluation of the friction ratio is necessary for the interpretation of 



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