CHAPTER III 



BACTERIA IN THE AIR 

 METHODS OF EXAMINING AIR FOR BACTERIA 



THE basis of the usual methods in practice is to pass air 

 over or through some nutrient medium. By this 

 means the contained organisms are waylaid, and rinding 

 themselves under favourable conditions of pabulum, tem- 

 perature, and moisture, commence active growth, and thus 

 reveal themselves in characteristic colonies. These are 

 examined, as directed on page 43, by the microscope and 

 sub-culture. Quantitative estimation is not generally 

 made, as a fixed standard is even less a possibility than in 

 milk and soil. Returns of the number of bacteria in the 

 sample taken may be made for the sake of information, but 

 little or no conclusion of value can be drawn from such 

 data. The standard recognised in Europe is the cubic metre, 

 and one may speak, for example, of the air of a room con- 

 taining 500, 1000, or 3000 germs per cubic metre. 



The following are the chief methods : 



i. Pouchet's Aeroscope. This apparatus was in use some 

 time ago in France, and by its means all the solid matter of 

 a given quantity of air was drawn through an air-tight glass 

 tube by aspiration and made to impinge upon a small plate 

 of glycerine. The air escaped to the aspirator at the sides, 

 leaving upon the glycerine plate only its particulate matter. 

 This remnant could then be examined. 



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