CENTRIFUOALIZA TION 



953 



the realization of the full potentialities of this instrument, it is said to be possible 

 now, under o})timal conditions, to resolve particles as small as 1 m/x and to use 

 magnifications up to 200,000 times (see Marton 1941, Stanley and Anderson 1941, 

 Sharp et al. 1942, 1943, 1944, Luria et al. 1943, Taylor el al. 1943). 



Centrifugalization. — There are considerable mechanical difficulties in con- 

 structing a machine that is sufficiently powerful to throw down very fine suspended 

 particles ; this difficulty is increased if the suspending fluid, as is usually the 

 case, has a specific gravity greater than water. The centrifugal force of a machine 

 varies with the square of the rate of rotation, and directly with the distance of 

 the centrifuged material from the centre of the plate. But neither of these factors 

 can be increased indefinitely, because with increasing rate of rotation, and with 

 increasing diameter of the plate, a vibration develops that very largely counter- 

 acts the centrifugal force. Numerous other mechanical factors, such as the air- 

 resistance and the heat generated in the machine, come into play when high speeds 

 are developed, and limit the rate and time during which the machine may be run. 

 Nevertheless, serious attempts have been made in recent years to overcome these 



a h c 



Fig. 232. — On left (a), elementary bodies of vaccinia from rabbit testis, and on right 



(c) ELEMENTARY BODIES OF CANARY POX ; IN THE MIDDLE (6) ChromO. prodigioSUtn FOR 



COMPARISON. Photographed in ultra-violet light ( x 3200). (After Barnard.) 



difficulties, and very real progress has been registered. The introduction of higher 

 speed electric motors, of the Lundgren angle centrifuge, and of the spinning-top 

 centrifuge ofHenriot and Huguenard (1925, 1927) (see also Mcintosh 1935, Mcintosh 

 and Selbie 1937) have each contributed to this end. Indeed with the spinning- 

 top centrifuge, in which friction is diminished to a minimum, sj^eeds of 80,000 r.p.m. 

 have been reported. The greatest advance, however, has been made by Svedberg 

 and his colleagues (1934) (see also Svedberg 1937), who have devised a centrifuge 

 capable of revolving at 160,000 r.p.m. In this machine the rotor carries a cell which 

 contains a column, 8 mm. in height, of the fluid to be centrifuged, situated 36 mm. 

 from the centre. The cell has windows of crystralline quartz to allow of serial 

 photographs being taken to register the progress of sedimentation. The rotor 

 is driven by two twin turbines fed with oil at a pressure of 15 kgm. per sq. cm. 

 Rotation takes place in an atmosphere of hydrogen at 25 mm. pressure, so as 

 to limit air friction and convection currents. In the original design (Svedberg 

 and Nichols 1927) 240 litres of oil were required per minute to drive the 

 turbines, and 7 litres of oil per minute to lubricate and cool the bearings. The cost 

 of this machine has so far prohibited most laboratories from testing it, but in 



