WATER FILTERS 727 



at intervals during the day, and again at the end of twenty-four 

 hours. If they are all sterile, the filter is capable of preventing 

 organisms from being directly washed through. In the case of 

 niters of very great density or depth of filtering medium, it may 

 be necessary to prolong the period of examination beyond the 

 first day ; but most ordinary filters which permit organisms to be 

 washed through do so within the first few hours. 



Protozoa and Algae in Water 



The examination of water for the minute forms of life other 

 than bacteria, and their enumeration, can be carried out by the 

 Sedgwick-Rafter method. 1 A 6-in. glass funnel is plugged at 

 the bottom of the stem with a perforated rubber cork, over 

 the upper end of which a disc of fine silk bolting cloth, cut by a 

 wad-cutter, is laid. Sharp, clean, dry quartz sand is then poured 

 into the stem of the funnel to the depth of half an inch above the 

 plug. The sand should be of such a size that the grains will 

 pass through a sieve of sixty meshes to the inch, but not through 

 one of 120 meshes. The sand is washed into place and well 

 moistened with a little distilled water free from organisms. 



The water to be examined is thoroughly shaken, and 500 c.c. 

 are poured into the funnel ; it runs through the sand, which 

 detains any organisms it may contain. After the water has all 

 passed through, the rubber plug is carefully removed and the sand 

 washed down into a test-tube with 5 c.c. of distilled water. The 

 contents of the test-tube are agitated and the tube is allowed to 

 rest until the sand has deposited. Immediately this is the case 

 the supernatant fluid is decanted into a second test-tube, carrying 

 with it the organisms. One cubic centimetre of this is withdrawn 

 by a pipette from midway between the top and bottom and 

 transferred to the counting plate. This consists of an ordinary 

 glass slide on which a rectangular brass cell (20 x 50 mm.) is 

 cemented, so enclosing exactly 1,000 square mm. The brass 

 cell is 1 mm. thick, so that the cell contains exactly 1 c.c. The 

 preparation is covered with a cover-glass and examined with a 

 low power. 2 



1 Calkin, Twenty third Ann. Rep. State Board of Health, Massachusetts, 

 1891. 



2 On the microscopy of water, see Whipple, Microscopy of Drinking Water. 



