FERMENTATION. 195 



returns to its place in the closed arm and fills it again, 

 with the exception of a small space at the top, which is 

 occupied by the air originally dissolved in the liquid 

 and 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 containing some fermentable carbohydrate, 



. 86. 



Fermentation-tube. 



as glucose, lactose, or saccharose. After inoculation the 

 flasks are placed in the incubator, and the amount of 

 gas that collects in the closed 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 determining the 



