160 BACTERIOLOGY OF WATER, AIR, MILK, ETC. 



allow the water to run out. The same quantity of air will be drawn through the 

 sugar of the aerobioscope as the amount of water passing out of the aspirating bottle. 

 The bacteria and moulds are caught by the sugar. 



Example. Passed 10 liters of air through the aerobioscope. The bacteria in this 

 quantity of air showed 75 colonies when incubated at 2OC. The unit being i 

 cu. m. or 1000 liters, we have only obtained the bacteria of one hundredth of the 

 unit. Hence multiplying 75 by 100 gives 7500 bacteria as present in i cu. m. of 

 the air examined. 



A very satisfactory method is to take a test-tube containing 5 c.c. 

 of sterile water and having a rubber stopper with two perforations, 

 one for a long piece of glass tubing which dips down into the water 

 and a short piece of glass tubing which is connected with the aspirating 

 bottle by rubber tubing. 



The air to be examined is drawn through the long tube and its bacterial or mould 

 content is caught in the water. By plating i c.c. which would represent one-fifth 

 of the total count for the amount of air aspirated we can easily calculate the con- 

 tent for a cubic meter. 



In comparing the results with the aerobiscope with those obtainec 

 by exposing a plate as in Petri's method for ten instead of five minutes, 

 it was found that the latter was sufficiently in accord to make it a satis- 

 factory approximate quantitative method. The simplicity and ease of 

 access to the colonies developing on it make it preferable when the air of 

 operating-rooms or hospital wards is to be examined. 



Of the fungi ordinarily obtained in examinations of the air the blue-green mould 

 and 'the red yeast are the most common. B. subtilis and sarcina types of cocci are 

 the most common bacterial colonies found upon exposed plates. Sewer air is as a 

 rule free from bacteria, due probably to the fact that bacteria tend to adhere to 

 moist surfaces. The importance of Fliigge's droplet method of contamination of 

 the air of a room is brought out in the discussion of infection with pneumonic plague. 

 This is an important method in the transmission of tuberculosis. 



