BACTERIOLOGICAL AIR ANALYSIS. 461 



stop for the filtering material which is to be placed over 

 it. Beneath the gauze (at 6), and also at the large end 

 (c), the apparatus is plugged with cotton. When thor- 

 oughly cleaned, dried, and plugged, the apparatus is to 

 be sterilized in the hot-air sterilizer. When cool, the 

 cotton plug is removed from the large end (c), and thor- 

 oughly 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 employed to engage and retain the bac- 

 teria. 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 

 vacuum-gauge is determined in terms of air by means 

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

 capacity of the cylinder. This fact ascertained, the 

 negative 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 



A sterilized aerobioscope is now to be fixed in the 

 upright position and its small end connected by a rubber 

 tube with a stopcock on the cylinder, or to a glass tube 

 tightly fixed in the neck of an aspirating bottle by 

 means of a perforated rubber stopper. The cotton plug 



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

 pirating 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. 



