SIZE, SHAPE, AND HYDRATION OF VIRUSES 35 



distance of the initial boundary from the axis of rotation, and 

 Xi is the radius of the boundary after a time t. 



The sampling technique described above must be aimed at 

 locating the position of the boundary. In place of finding the 

 boundary at a known time, the time to reach a definite radius 

 is measured. This is done by plotting the concentration on the 

 upper side of Xi at different times and by interpolating between 

 points determining the time at which the concentration has 

 dropped to zero. 



1 Xi 



Equation 2.20 is the product of two terms. The term — -^ In — 



tOi^ Xi 



is concerned with the description of the motion of the boundary, 

 and the term "" ^-^ is the term which reduces the observed 



m^KPv — p) 



sedimentation to standard conditions at 20° C. 



When such measurements are made, it is usually found that 

 the value of the sedimentation constant depends on the con- 

 centration of the virus used. This is interpreted as being due to 

 the presence of impurities which modify the rate of sedimenta- 

 tion by introducing local variations of density, and the normal 

 procedure is to plot the sedimentation constant as a function of 

 concentration and extrapolate the line back to zero concentra- 

 tion, where extraneous effects are assumed to be nil. It will be 

 seen later that correction for the viscosity of the solution re- 

 moves some of the dependence on concentration, a point made 

 by Lauffer. 



The availability on the market of a single wire suspension, 

 vacuum ultracentrifuge has widened the interest in this type of 

 research. The Spinco ultracentrifuge, manufactured by Special- 

 ized Instruments Corporation of Belmont, California, follows a 

 design by Bauer and Pickels (1937) and is now found in many 

 laboratories. It consists of a rotor of aluminum alloy, shaped for 

 dynamical balance, with a hole cut through it about three 

 inches from the axis of rotation. The hole accommodates a 

 wedge shaped cell, with accurately ground, flat glass plates top 

 and bottom, and within which the virus suspension can be 

 placed. As rotation proceeds, this cell is brought, once per 



