90 E. G. P I C K E L S 



the results to zero sedimentation. Usually salts, sucrose, urea and 

 other low molecular substances have been used to increase the density 

 of the media but it has been found that these introduce an uncertainty 

 because of their apparent ability to alter the partial specific volume 

 and the frictional ratio by osmotic extraction of water or through par- 

 tial imbibition of the denser medium (53). Through the use of larger 

 molecules, such as proteins, with a sedimentation rate that is low in 

 comparison to that of the material under study these disadvantages 

 have been largely overcome (61) and the method now appears very 

 promising, particularly for the investigation of viruses. 



In applying the equations for molecular weight (equations 1, 14, 

 and 15) it is generally not necessary to know the amount of hydration, 

 as will be shown below. The density of dried or crystalhzed material 

 can be determined by measuring with a pycnometer the volume dis- 

 placed by a known weight of substance. If it can be assumed, as is 

 generally the case, that the density of the material being bound to the 

 particle is approximately equal to that of the medium, then the partial 

 specific volume obtained with dried material may be applied to the 

 above equations. In such an instance the value computed for M 

 represents the molecular weight of the particles in such a state that 

 their density is the same as that which characterized the material 

 when the weight of a sample was determined. The reason the density 

 of the unsolvated particle may be used is that the frictional ratio in 

 both sedimentation and diffusion is changed to the same degree in 

 accordance with equation (9) and also that the net centrifugal force 

 acting on the particle has not been appreciably altered in spite of its 

 increased size. 



It is important to note that in using equations (2) and (3) one 

 must know the solvated density of the particle or must assume some 

 degree of solvation. Fortunately, correction to the differential den- 

 sity term is seldom very appreciable and probably can be neglected in 

 most cases. 



4. Selection of Equipment and Methods 



The first requirement of an ultracentrifuge is that its "resolving 

 power" be adequately high. This expresses the ability of an ultra- 

 centrifuge to resolve separate components and is also directly related 

 to the possible precision of measurement for a monodisperse system. 

 Svedberg {1, p. 44) has shown that the resolving power is proportional 



