METHODS OF RESEAECH : APPARATUS 



79 



their Type B, Size 2, Amperes 2.15, volts 220. It is 22 inches 

 high, the steel shell that encloses the whirling part having a 

 depth of 13 inches and an inside diameter of about 23 inches. 

 There are 8 carriers (two larger than the others), the total ca- 

 pacity being about 1100 cc, and the number of revolutions per 

 minute 3000 when accurately leveled, properly loaded and run- 

 ning at full speed. The levelling and equal loading are very 

 important. 



'iG. 52. — Electric centrifuge made by the International Instrument Company, 

 Cambridge, 



Filters. — The Berkefeld and Chamberland filters are often 

 necessary and in connection one must have some kind of device 

 for supplying compressed or exhaust air. In vol. I of "Bac- 

 teria in Relation to Plant Diseases," I figured (Plate 10) a 

 very good steam pump for furnishing compressed air and a high 

 vacuum. Having moved into another building we no longer use 

 this particular pump but obtain our exhaust and pressure from 

 the main engine room of the Department of Agriculture. Where 

 such steam pumps are not available small mercury pumps may 

 be used. Very perfect ones are now for sale, of which the Gaede, 

 the May-Nelson, and the Geryk are those commonly in use. Of 

 the three the Gaede is said to be the best. For removing the 



