BACTEEIA OF THE AIE 



49 



Two students working together should prepare the following 

 simple apparatus: Place 100 cc. of physiological salt solution 

 (6 g. XaCl in 1000 cc. water) in a 500-cc. Erlenmeyer flask. 

 Close the flask with a tight-fitting rubber stopper through which 

 two bent glass tubes are passed. One of the tubes ends about 

 2 mm. from the bottom of the flask. The ends of the tubes are 

 plugged with cotton and the flask is sterilized in the autoclave. 

 Place 5 liters of water in a large bottle and mark the level of 



Fig. 27. Aspirator and flask for determination of bacteria in air 



the water surface. Adjust a siphon with a shut-off and a suction 

 tube. After the Erlenmeyer flask and contents have cooled, the 

 end of the short tube is connected with the aspirator by means 

 of rubber tubing. The cotton plug is removed from the open end 

 of the long tube and the aspirator started (Fig. 27). 



When 5 liters of water have run out, a similar volume of air 

 has been drawn through the flask. The apparatus is then dis- 

 connected and the contents of the flask well shaken. Two 1-ec. 

 samples are drawn with a sterile pipette and plated — one in agar 

 and one in gelatin. The count multiplied by 100 represents the 

 number of organisms in 5 Hters of air. 



