BACTERIOLOGICAL ANALYSIS OF AIR. 607 



Upon this narrow tube, 5 cm. from the lower end, a 

 mark is made with a file, and up to this mark a small 

 roll of brass-wire gauze (a) is inserted ; this serves as 

 a stop for the filtering-material which is to be placed 

 over it. Beneath the gauze (at b), and also at the 

 large end (c), the apparatus is plugged with cotton. 



Fig. 109. 



c e " 



The Sedgwick-Tucker aerobioscope. 



When thoroughly 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 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 

 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- 



