250 The Bacteriology of the Air 



on a plate or Petri dish to the air for a while, cover, and 

 observe whether or not bacteria grow upon it. 



To make a quantitative estimation is, however, more diffi- 

 cult. Several methods have been suggested, of which the 

 most important may be briefly mentioned : 



Hesse's method is simple and good. It consists in mak- 

 ing a measured quantity of the air to be examined pass 

 through a horizontal sterile glass tube about 70 cm. long and 

 3.5 cm. wide (Fig. 69), the interior of which is coated with a 

 film of gelatin in the same manner as an Bsmarch tube. The 



Fig. 69. Hesse's apparatus for collecting bacteria from the air. 



tube is closed at both ends with sterile corks carrying small 

 glass tubes plugged with cotton. When ready for use the 

 tube at one end is attached to a hand-pump, the cotton re- 

 moved from the other end, and the air slowly passed through, 

 the bacteria having time to sediment upon the gelatin as 

 they pass. When the required amount has passed, the 

 tubes are again plugged, the apparatus stood away for a 

 time, and subsequently, when they have grown, the colonies 

 are counted. The number of colonies in the tube will repre- 

 sent pretty accurately the number of bacteria in volume of 

 air that passed through the tube. 



