218 BACTERIOLOGY 



which has been driven out by the heat. The air-bubble 

 should be tilted out after each sterilization; and finally, 

 after the third exposure to steam, this arm of the tube will 

 be free from air. The medium employed is bouillon con- 

 taining some fermentable carbohydrate, as glucose, lactose, 

 or saccharose. After inoculation the flasks are placed in the 

 incubator, and the amount of gas that collects in the closed 



FIG. 36 



Fermentation tube. 



arm is noted from day to day. From studies that have been 

 made this gas is found to consist usually of about one part 

 by volume of carbonic acid and two parts by volume of an 

 explosive gas consisting largely of hydrogen. For deter- 

 mining the nature and quantitative relations of these gases 

 Smith 1 recommends the following procedure: "The bulb 



1 Loc. cit., p. 196. 



