III. CENTRIFUGATION 77 



ance are of the same general character as those used witli the oil- 

 driven ultracentrifuges. Speeds up to about 20,000 r.p.m. can be em- 

 ployed. Machines of this type are now commercially available {90). 



3. Optical Ultracentrifuges Exposed to Air Friction 



Small optical ultracentrifuges, usually less than 5 cm. in diameter, 

 have been constructed without mechanical bearings according to 

 principles originally described by Henriot and Huguenard {15). The 

 rotor is cone-shaped at the base and rides on a whirling layer of air 

 issuing under pressure from properly directed jets in a cone-shaped 

 stator. Beams, Pickels, and Weed {16) have pointed out the dif- 

 ficulties of obtaining normal sedimentation in such centrifuges be- 

 cause of the disturbing action of convection currents set up by tem- 

 perature gradients through the rotor. They were able to obtain 

 sedimentation photographs of hemoglobin, but found that for this 

 the thickness of the fluid column had to be kept below 1 mm. if 

 reasonably dilute preparations were used. McBain and Lewis {19h) 

 and Beams {8) have made various improvements for minimizing the 

 convection currents, and the former have described novel methods for 

 photographically recording boundary positions. These "spinning 

 tops" are relatively inexpensive and could be used in many cases 

 where more elaborate equipment is not justified. However, from 

 the standpoint of completely convection-free sedimentation at low 

 concentrations, they do not approach the ideal of a thermally isolated 

 system (as does the vacuum centrifuge) and because of practical 

 limitations regarding the length, axial thickness, and radial displace- 

 ment of the cell, their resolving power, precision of measurement, and 

 range of application with respect to solute concentration do not com- 

 pare very favorably with the ultracentrifuges already described {1, 

 p. 47). 



A small air-driven ultracentrifuge {92) of plastic material spinning 

 about a horizontal axis on mechanical bearings has been described 

 by Stern {18). While probably sufficient for some studies, it is not 

 suited to work with most proteins or with very dilute solutions be- 

 cause of its limited speed range and its susceptibility to convective 

 disturbances. 



4. Optical Methods 



In the first analytical ultracentrifuges, measurements were based 

 on the light absorption method of Svedberg and Rinde {20). Filtered 



