III. CENTRIFUGATION 99 



from a solid piece of metal (Duralumin) for reasons of strength. For 

 well designed rotors capable of accommodating between 100 and 400 

 ml. of fluid, the maximum rotational speed and the maximum force 

 on the fluid are approximately proportional to the reciprocal of the 

 rotor diameter {62). With a properly constructed rotor 6 in. in 

 diameter, one can attain routine operating speeds up to about 60,000 

 r.p.m. and forces above 200,000 times gravity {62). In such fields 

 of force all viruses can be separated readily and even particles in the 

 size range of the serum proteins can be sedimentcd, though not with 

 so sharp a differentiation. Tubes for containing the fluid are usually 

 thin-walled and of either Celluloid or metal. In the inclined tubes, 

 particles (except very large ones of high sedimentation rate) do not 

 pack against the walls and slide to the bottom of the tube as a 

 precipitate b}^ reason of their diffusibility. Instead, there is a con- 

 tinual circulation of fluid caused by the downward movement of the 

 more concentrated layer of fluid continuallj^ being formed by sedi- 

 mentation against the outer wall, and the actual precipitation of par- 

 ticles occurs near the bottom of the tube {76). Most rapid separa- 

 tion and the greatest capacity of fluid are possible when the inclina- 

 tion of the tubes is made small (10 to 20°). In Figure 8 are ten 

 Celluloid tubes inclined at a 26° angle to the axis and having a total 

 capacity of 130 ml. Tubes are individually sealed by expansion of 

 rubber plugs in caps and the rotor itself is hermetically sealed with 

 the aid of two rubber gaskets. A knurled rod, internally threaded at 

 one end, is used for extracting capped tubes. 



High speed angle centrifuges are commercially available {89,93, 

 94) in a variety of forms. For further discussion of the general field 

 of preparative centrifugation, the reader is referred to a previous pub- 

 lication {2). Also, a list of pertinent references (with parenthetical 

 notes) has been included in the bibliography at the end of the present 

 article. These deal with design, typical applications, and the prob- 

 lems connected with preparative centrifugation. Commercially avail- 

 able centrifuges {91) of the continuous flow type that operate in the 

 open air have been found practical for the large scale concentration of 

 some of the larger viruses, and the reader is referred to the original 

 papers by Mcintosh and Selbie {73), Stanley {74), and Taylor {75) 

 for discussions of the efficiency of such centrifuges. These machines 

 are operated by compressed air at speeds up to about 50,000 r.p.m. 

 and the rotating member through which the fluid passes is essentially 

 a hollow tube a few inches in diameter. 



