20 Illinois State Laboratory of Natural Hist 



or 



the settling method. In our machine we have utilized the 

 double arms, aluminum shields, and percentage tubes of the 

 Purdy Electric Centrifuge. The tubes contain 15 cubic cm., 

 are graduated to tenths of a cubic cm., and the conical tips 

 permit the measurement of small planktons with accuracy. 

 The arms are borne upon an upright shaft which is driven by 

 a system of gears turned by means of a crank handle, one 

 turn of the crank giving 24 revolutions of the vertical shaft. 

 The direction and the speed are thus easily controlled by the 

 operator. The machine is clamped firmly to a table when in 

 operation. All parts of the machine, except those from the 

 Purdy Centrifuge, were devised by Professor W. H. VanDer- 

 voort, of the College of Engineering, and were constructed in 

 the University shops. Planktons are subjected to 2,000 revo- 

 lutions in two minutes, the motion at first being slow and 

 frequently reversed. The practical limit of compression by 

 this machine is thus reached, and successive measurements of 

 the same planktons show that its action is quite uniform. 

 The average amount of compression in a wide range of 

 planktons is about 50%, the volume by the centrifuge 

 method ranging from 30° o to 70% of that obtained by the 

 settling method. No discussion is needed to prove that the 

 more perfect the compression the more accurate are the volu- 

 metric determinations of the plankton. In this lies the main 

 argument for the use of the centrifuge in quantitative plank- 

 ton work. It also permits rapid work and is easily manipu- 

 lated. Our machine was completed in January, 1896, and 

 this is, I believe, the first application of the centrifugal 

 machine to quantitative plankton work. Cori ('96) has de- 

 vised a simple centrifugal machine for precipitation purposes 

 in zoological work, but it does not seem to be fitted for quan- 

 titative determination of plankton. 



The machine employed by us was also in use for the precipi- 

 tation of living plankton from the water when Dolley's paper 

 ('96) was received describing a large and powerful centrifuge, 

 "the planktonokrit," devised for the same work. It is only by 

 means of some such machine as this that a complete exami- 

 nation of the contents of the water is possible. 



