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



tion and the backward diffusion. In this steady state condition the 

 concentration grades from a value below the original at the meniscus 

 to values higher than the original in the outer zone of the cell. It has 

 been shown by Svedberg and associates (1) that this concentration 

 distribution is a function of particle weight and can be employed 

 directly (see Sect. C) for its determination in the case of monodisperse 

 preparations. It has been demonstrated (27) that even the simpler 

 molecules with molecular weights below 100 can be studied by this 

 equilibrium method. However, its principal application has been as 

 an independent method for investigating proteins and other macro- 

 molecules, for which purpose it offers certain advantages by reason of 

 the simultaneous balancing of the sedimentation and diffusion proc- 

 esses under identical experimental conditions. 



B. ULTRACENTRIFUGES OF OPTICAL TYPE 



1. Ultra centrifuges of Vacuum Type 



Although the pioneering oil-driven ultracentrifuges of Svedberg 

 and collaborators (1,13,69) are still in active and fruitful use at Upsala 

 and several other research institutions, most present day machines 

 are of the somewhat simpler and more versatile vacuum type. A 

 commercial ultracentrifuge (89) of this kind suitable for both sedi- 

 mentation velocity and sedimentation equilibrium measurements, 

 as well as for preparative work, is available. The complete instru- 

 ment is slightly more than 6 ft. long, weighs approximately 2400 lb., 

 and requires no special foundation by virtue of its own system of vibra- 

 tion isolation. Size and other pertinent features of the analytical 

 rotor and its transparent cell have been patterned after those found 

 to represent the optimum by Svedberg (1). The rotor (Figs. 2 and 

 3), with a maximum diameter of 7.25 in., is made from a solid piece of 

 forged Duralumin and is provided with two 1-in. holes located 65 

 mm. from the axis of rotation and accommodating the solution cell 

 and its counterbalance. Metal is cut away to give the oval shape, 

 relieving the stresses in the plane of the cell holes and permittng 

 higher speeds (6) . The rotor may be operated routinely at speeds up 

 to at least 60,000 r.p.m., with average centrifugal forces in the cell 

 up to 260,000 times gravity. 



The cell (7) (see Fig. 2) is composed primarily of a Duralumin center- 

 piece, two quartz discs, two light-limiting apertures of Duralumin (which 



