BACTERIOLOGY OF AIR 603 



Sedgwick-Rafter method. 1 A 6-inch 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 60 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 super- 

 natant 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 One cubic centimetre of this is withdrawn by a 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 1000 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 



The Bacteriology of Air 



Just as in water, the bacteria in the air vary considerably 

 at different times and seasons, under different conditions, 

 and in various localities. The species met with are mostly 

 saprophytes, consisting largely of chromogenic forms. A 

 number of moulds occur (as spores), and, in fact, ordinarily 

 are in large excess, together with yeasts and torula?. 



It is not easy for micro-organisms to become diffused 

 through the atmosphere ; they are incapable of a volun- 

 tary rising, and cannot be torn from a fluid or moist solid 



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

 chusetts, 1891. 



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

 Water, 



