BACTERIOLOGICAL EXAMINATION OF THE AIR. 255 



relatively larger numbers. Two sets of plates should be made 

 as described in the quantitative test, one to be kept at the 

 room temperature and the other to be incubated, and after 

 these have grown for twenty-four to forty-eight hours the 

 colonies should be examined under a low power of the micro- 

 scope, and the colonies picked up with a platinum needle and 

 planted in fresh agar and gelatin tubes, and these plated 

 again until pure cultures are obtained. 



Eisner's method, described in the chapter on typhoid fever, 

 is a very good method for the separation of water bacteria 

 from typhoid bacteria, and the reader is referred to that 

 chapter for its description. 



The addition of antiseptics, in small amounts, helps in the 

 recognition of pathogenic germs, such as typhoid, in water, 

 as these antiseptics interfere more materially with the growth 

 of saprophytes than they do with that of the pathogenic germs. 



The reader is again reminded that the isolation of typhoid 

 fever germs from water is difficult. 



It is easier to examine water for the Bacillus coli communis, 

 which when present shows contamination by animal or 

 human excreta, and makes the water unfit for consumption. 



By inoculating glucose- or lactose-bouillon in fermentation- 

 tubes, the presence of this bacillus is easily made out on ac- 

 count of the rapid development of gases it produces ; and 

 plates made from the bouillon in the closed arm of the 

 fermentation-tube will yield in such cases almost pure cult- 

 ures of this bacillus. 



BACTERIOLOGICAL EXAMINATION OF THE AIR. 



A number of methods have been suggested for this pur- 

 pose ; some consist in exposing plates of nutrient media, such 

 as gelatin and agar, in the air, the bacteria falling on these 

 plates and developing ; or, again, causing by slow aspiration 

 a measured volume of air to pass through substances, such as 

 sterilized sand or granulated sugar, afterward making agar 

 and gelatin plate cultures with them. Quiet air contains but 

 few bacteria, but air in motion, as in blowing wind, carries 



