THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 257 



In order to use theories relating sedimentation coefficients to the mole- 

 cular size and shape of the solute molecules it is necessary to employ the 

 sedimentation coefficient corresponding to infinitely dilute solutions. There- 

 fore, extrapolation procedures are employed. Sedimentation data at different 

 concentrations can be plotted as s or l/s versus c, where c is the concentra- 

 tion, and the best-fitting curve is extrapolated to provide a value of s at 

 infinite dilution. For systems showing a marked dependence of s upon c the 

 plot of 1/s versus c is preferred. Lauffer (1944b) has suggested a plot of 

 s{r}[rjg) versus c where rjjrjo is the relative viscosity of the solution. Such 

 plots along with a graph of s versus c on the same scale facilitate the extra- 

 polation to infinite dilution since {rjfrjo) approaches 1.0 as the concentration 

 decreases. This combination plot has been particularly useful for studies of 

 several animal viruses where impurities were still present in the preparations 

 and the viscosities of relatively concentration solutions were ascribed to the 

 contaminants (Lauffer and Stanley, 1944; Schachman, 1951a). With the 

 recently renewed application of fight absorption optical systems employing 

 ultraviolet light (Schumaker and Schachman, 1957), refiable data are ob- 

 tained with dilute solutions of viruses. Because of their content of nucleic 

 acids and the consequent absorption of ultraviolet fight, such solutions are 

 now examined at concentrations tenfold lower than those commonly exam- 

 ined with sclifieren optics. Difficulties in extrapolating the experimental data 

 are thereby circumvented. 



Corrections of the observed sedimentation coefficient, s^^,^, to the standard 

 state (a fiquid with the viscosity and density of water at 20°C.) are made 

 according to the equation 



= s^ 



u\h\fLilm^) ,21) 



where {r]tlrj2o) is the principal correction factor corresponding to the viscosity 

 of water at t relative to that at 20°C. (r^/r^o) is the relative viscosity of the 

 solvent to that of water, and p2o,w ^^^ Pt are the densities of water at 20°C. 

 and the solution at t°, respectively. ActuaUy F in the numerator should 

 correspond to water at 20°C. whereas the value of F in the denominator 

 refers to the solvent under the conditions of the experiment. 



a. Molecular Size and Shaj)e. The rate at which solute molecules migrate 

 through a fiquid is a fimction of their molecular weight, the difference in 

 density between the solute and the medium, and the frictional resistance 

 experienced by the molecules during their movement. There are various 

 theoretical treatments describing in quantitative terms the behavior of 

 molecules in a centrifugal field. Most popular of these is the so-caUed nficro- 

 scopic picture which considers the driving force on a suigle particle in terms 



