34 THE PHYSICS OF VIRUSES 



mentation constants is /s/fo, where 



^\ 1 ( f.\ 



(2.19) 



Both these relations are derived and discussed in the section 

 by Kraemer in Svedberg and Pedersen (1940). 



Sedimentation Technique 



The technique of sedimentation as applied to viruses is quite 

 varied. Because viruses are relatively large, their sedimentation 

 constants are high, and elaborate long centrifugation processes 

 are really not needed. One simple technique, used by Elford 

 (1936), is to sediment in a capillary tube, held horizontally, 

 and then to sample the virus at definite radii after exposure to 

 the centrifugal action for known lengths of time at a known 

 rate of rotation. He applied this to measure the sedimentation 

 of bacterial viruses. The same method was used by Mcintosh 

 and Selbie (1937) and more recently, for potato yellow dwarf 

 virus, by Brakke, Black, and Wyckoff (1951). The capillary 

 tubes used by these workers were 2 mm in diameter, and were 

 placed in a centrifuge cup filled with water and spun in an 

 International ISB centrifuge for 5 hr or so. At the end of the 

 centrifugation, a special fine capillary tube, turned to a hook 

 shape at the bottom, was inserted, and samples taken out at 

 known depths. The samples were then appropriately diluted 

 and tested for infectivity. When such a method is used, the 

 sedimentation constant is calculated from the formula derived 

 by Svedberg and Pedersen (1940), namely 



[toi^rjioiPv - P)| I '^'^} 



where S20 is the sedimentation constant at 20° C, 77 is the vis- 

 cosity of the solvent at experimental temperature, 7720 is the 

 viscosity at 20° C, po is the density of water at 20° C, Pv is the 

 anhydrous density of the virus, p is the density of the solvent 

 at experimental temperature, t is the time of centrifugation in 

 seconds, co is the angular velocity in radians per second, Xi is the 



