264 H K. SCHACHMAN AND R. C. WILLIAMS 



For a comprehensive review of these studies the reader should consult the 

 discussion by Lauffer and Bendet (1954). If hydration is defined as the 

 selective solvation of the virus particle, then this method of determining 

 hydration is ideal. Unfortunately, the total amount of liquid associated with 

 the virus particle as a kinetic unit cannot be measured by this technique. 

 Many workers, however, desire the latter information. This can be obtained, 

 in principle, from measurement of the frictional coefficient, or the intrinsic 

 viscosity, and independent knowledge of the shape of the hydrodynamic 

 unit. 



vi. Sedimentation in a Partition Cell. Frequently the amomit of material 

 available is so small as to preclude sedimentation analyses employing optical 

 techniques, and recourse is made to the so-called analytical method based 

 on the determination of the quantity of substance sedimenting across a 

 fixed plane in a centrifuge cell. This method requires a special ultracentri- 

 fuge cell called the separation or partition cell which is so constructed as to 

 effect a sejDaration of the contents of the cell into two parts at the con- 

 clusion of the experiment. The amount sedimenting across the partition is 

 the difference between the total amount of material originally in the cell 

 and that which is left above (or centripetal to) the separating plate at the 

 conclusion of the experiment. A specific and sensitive analytical method for 

 the substance in question is required also. This may be a bioassay, a chemical 

 analysis, or a physical chemical measurement such as radioactivity. The 

 analysis need be only relative so that measurement, at the conclusion of the 

 run, of the number of units of activity in the upper solution relative to the 

 number in the original solution leads to a value of the sedimentation co- 

 efficient. 



The separation cell affords still another important advantage to virologists 

 interested m a specific biologically active substance. Frequently, in the iso- 

 lation and purification of a given substance two or more boundaries are 

 observed optically in the ultracentrifuge, and it is necessary to determine 

 which of these corresponds to the material of interest. Even when only one 

 component is observed optically and the material is thought to be purified, 

 it may be profitable to measure the sedimentation rate by infectivity 

 measurements to see if there is a correlation between the physical properties 

 of the bulk constituent and the component with the biological activity. 

 This method has been used in an impressive series of investigations by 

 Lauff"er and his colleagues (see Epstein and Lauffer, 1952). There are now 

 two types of partition cells available (Tiselius et al., 1937; Yphantis and 

 Waugh, 1956). These work in different ways, as illustrated in Fig. 4. Through 

 the use of this technique, in conjunction with the porous disk diffusion cell, 

 the molecular weight of a virus can be determined even prior to its purifica- 

 tion. 



