58 



THE AMERICAN MONTHLY 



[March, 



process too prolonged, and for ordinary samples I have fixed upon 500 

 c.c. as the proper amount. In the case of very pure waters a larger 

 amount will be desirable, and for such 1000 c.c. may be adopted as a 

 convenient unit. 



Experience indicates that however carefully the sand may be placed, 

 the filtration at the beginning will not be as complete as further on, 

 and in order to insure the certain removal of all the smaller organisms 

 the first 100 to 150 c.c. of the filtrate is returned to the funnel and 

 passed through the sand the second time. The fimnel is allowed to 

 stand until the completion of the filtration, when it is found on exam- 

 ination of the filtrate that nearly every organism has been removed and 

 we have the result that the organisms originally contained in the 500 



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c.c. of water are all in the sand at lower end of funnel stem. The 

 plug of wire cloth is now removed, and the sand and contained organ- 

 isms washed with 5 c.c. of freshly-filtered water, run from a 5 c.c. 

 pipette into a 5 or 6-inch test tube. The test tube is slightly shaken 

 in order to wash all the organisms clear from the sand. The sand by 

 reason of greater specific gravity sinks quickly to the bottom, leaving 

 the organisms distributed through the water. At the instant of the 

 completion of the settling of the sand the supernatant water is turned 

 into another smaller test tube, leaving the clean sand at the bottom of 

 the first tube. We now have the organisms from 500 c.c. of water 

 concentrated into 5 c.c. in the second tube, from which after slight 

 stirring, to insure uniform distribution, i c.c. is taken with a i c.c. 



