48 THE PHYSICS OF VIRUSES 



diffusion measiiroment.s give f/fo, the frictioiial ratio, as l.'iB. 

 Third, viscosity ineasurements give (p, the apparent volume 

 fraction, as 1.7G. Fourth, the dry partial specific volume is 

 0.696 gm/ml, and the hydrated partial specific volume is 0.827. 

 These can be explained in only one way. If we suppose that 

 the f/fo value from diffusion is due to unhydrated ellipsoidal 

 particles, then the axial ratio deduced from Eq. 2.25 is a/b = 5. 

 This agrees with the value found from viscosity measurements. 

 However, the hydrated partial specific volume figure does not 

 fit this idea. Lansing and Kraemer (1936) have shown that if Fo 

 is the partial specific volume of a combination of two substances 

 with separate values Vu and Fi, and if /• grams of substance Vi 

 combine with 1 of substance T /„ then 



(, + i)]\^ = rl\ + V^ (2.27) 



Putting in the above figures, we find r = 0.76 gm of water per 

 gram of dry virus. 



Now this radically changes the explanation of both the diffu- 

 sion and the viscosity figures. In the former case, if we assume 

 spherical viruses and apply Eq. 2.18 for///o as due to hydration, 

 the value of r so found is 0.78 gm water per gram of virus. This 

 agrees beautifully with the direct measurements. Using the value 

 of V /m = ] .76, together with the dry partial specific volume of 

 0.696, the relation r = V /m — Fo gives r = F06. This is rather 

 high, but still in tolerable agreement. Miller and Price then con- 

 clude that an average value of 0.8.3 gm of water of hydration is 

 associated with each gram of virus; that the virus is spherical; 

 and that its mass, independent of hydration, is 1.1 X 10~^^ gm, 

 corresponding to a particle molecular weight of 6.63 X 10*^. 

 The hydrated particle has a diameter of 312 A, and the dry 

 particle a diameter of 244 A. 



Very recently, Lauffer, Taylor, and Wunder (1952) have made 

 quite extensive studies on the centrifugation of southern })ean 

 mosaic virus. They have examined very carefully the sedimenta- 

 tion in the presence of bovine serum albumin and various alkali 

 metal chlorides and also at different pH values and in sucrose. 

 Their studies show that buovancv methods can readilv be sub- 



