440 BACTERIOLOGY. 



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.' 



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 

 is then removed from the upper end of the aerobioscope, 

 and the desired amount of air is aspirated through the 

 sugar. Dust particles and bacteria will be held back 

 by the sugar. During manipulation the cotton plug is 

 to be protected from contamination. 



When the required amount of air has been aspirated 

 through the sugar the cotton plug is replaced, and by 

 gently tapping the aerobioscope while held in an almost 

 horizontal position, the sugar, and with it the bacteria, 

 are brought into the large part (e) of the apparatus. 



' 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, ?'. e., the volume of air that has passed through the aerobioscope. 



