FERMENTATION. 173 



They are then to be sterilized. During sterilization 

 they are to be maintained in the upright position. 

 Under the influence of heat the 

 tension of the aqueous vapors in the ^'°" ^' 



closed arm forces most of the fluid 

 into the bulb. As the tubes cool 

 the fluid 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 origi- 

 nally 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 carbo- Einhom's ^fermentation- 

 hydrate, as glucose, lactose, or sac- 

 charose. After inoculation the flasks are placed in the 

 incubator and the amount of gas that collects in the 

 closed arm is, from day to day, noted. 



From studies that have been made this gas consists 

 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 nature and 

 quantitative relations of these gases Smith ' recommends 

 the following procedure : " The bulb is completely 

 filled with a 2 per cent, solution of sodium hydroxide 

 (NaOH) and closed tightly with the thumb. The fluid 



1 Loc. cit., p. 196. 



