TAKING THE AIR SAMPLE. 189 



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 up- 

 right position and its small end connected by a rubber 

 tube with a stop-cock on the cylinder. The cotton plug 

 is then removed from the upper end of the aerobioscope, 

 and the desired amount of air is aspirated through the 

 sugar. The organisms will be held back by the sugar. 

 During manipulation the cotton plug is to be protected 

 from contamination with germs from without. 



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. 

 When all the sugar is thus shaken down into this part 

 of the apparatus, about 20 c.c. of liquefied sterilized 

 gelatin is poured in through the opening at the end 

 c, the sugar dissolves, and the whole is then rolled on 

 ice, just as is done in the preparation of an ordinary 

 Es march tube. 



The gelatin is most easily poured into the aerobioscope 

 by the use of a small, sterilized cylindrical funnel (Fig. 



9* 



