BACTERIOLOGICAL ANALYSIS OF AIR. 643 



apparatus is to be sterilized in the hot-air sterilizer. 

 When cool the cotton plug is removed from the large 

 end (c), and thoroughly dried and sterilized No. 50 

 granulated sugar is poured in until it just fills the 

 10 cm. (d) of the narrow tube above the wire gauze. 

 This column of sugar is the filtering-material em- 

 ployed to engage and retain the bacteria. After 

 pouring in the sugar the cotton- wool plug is replaced, 

 and the tube is again sterilized at 120 C. for several 

 hours. 



Taking the air sample. In order to measure the 

 amount of air used the value of each degree on the 



FIG. 98. 



i ; ; 



d a o 



The Sedgwick-Tucker aerobioscope. 



vacuum-gauge is determined in terms of air by means 

 of an air-meter, or by calculation from the known ca- 

 pacity of the cylinder. This fact ascertained, the nega- 

 tive pressure indicated by the needle on exhausting the 

 cylinder shows the volume of air which must pass into 

 it in order to fill the vacuum. By means of the air- 

 pump one exhausts the cylinder until the needle 

 reaches the mark corresponding to the amount of air 

 required. 1 



1 Such a cylinder and air-pump are not necessary. A pair of ordinary 

 aspirating-bottles of known capacity graduated into litres and fractions 

 thereof answer perfectly well. Or one can determine by the weight 

 of water that has flowed from the aspirator the volume of air that has 

 passed in to take its place i. e., the volume of air that has passed 

 through the aerobioscope. 



