246 D. J. SCOURFIELD ON THE USE OF THE 



like myself, have to depend upon manual power to work their 

 centrifuge, the only way to obtain really high speeds is to cut 

 down the revolving head to the smallest practicable dimensions 

 so as to reduce as much as possible both the weight and the 

 air resistance. The means by which I have done this is b}^ 

 having the " haematocrit " head supplied with all two-speed 

 centrifuges, somewhat altered so as to take elongated vase-shaped 

 tubes holding no more than about 1 J c.c. With such a head,, 

 speeds up to 10,000 revolutions per minute can be obtained, but 

 the highest speed obtainable without too great an effort is about 

 7,000 levolutions per minute, and this is the rate I usually 

 employ when desiring to concentrate as nearly as may be all the 

 organisms present in the water. At this speed a run of from 

 one to two minutes seems to be sufficient for the purpose. Of 

 course very many, perhaps the majority of the organisms, even 

 of the smaller kinds, are deposited at lower speeds, and I have 

 found it useful sometimes, first of all to centrifuge samples of 

 water in the larger tubes usually supplied (holding about 15 c.c.) 

 at a comparatively low speed, and then to pipette ofi' some of the- 

 clearer water into the smaller tubes for more rapid rotation with 

 the high-speed gearing. This has the great advantage of 

 separating to a considerable extent the larger from the smaller 

 of these tiny organisms and so rendering the examination of the 

 latter more easy. It would probably be a good plan in every 

 case to subject a sample of water to a second centrifuging at as 

 high a speed as available. 



Two little practical points may be mentioned here in connection 

 with the centrifuging process. One is that a very cheap and 

 convenient protective cover for the centrifuge may be made from 

 a common round wooden cheese-box turned upside down. A 

 portion of the lower part of the side must of course be removed 

 to admit of the handle being turned, and a central hole should 

 ))e cut in the top, so that the box may be placed concentrically 

 with the spindle of the centrifuge. The other ]^)oint is that in 

 using the small tubes on the " haematocrit " head they should 

 be over-filled with water in the first instance, in order that,, 

 when placed in position with the open end resting against the- 

 rubber disc, they may be quite full without any air-bubbles. 



The best method of removing the centrifuged organisms from 

 either the large or small tubes is to pipette ofY all the water 



