FERMENTATION. 193 



in the upright position. Under the influence of heat the 

 tension of water-vapor in the closed arm forces most 

 of the fluid into the bulb. As the tube cools 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 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, as glucose, lactose, or saccha- 

 rose. 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 is found 

 to consist usually of about one part by volume of car- 

 bonic acid and two parts by volimie 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 (XaOH) and closed tightly with the thumb. 

 The fluid is shaken thoroughly with the gas and allowed 

 to flow back and forth from bulb to closed branch and 

 the reverse several times to insure intimate contact of 

 the CO2 with the alkali. Lastly, before removing the 

 thumb aU the gaa is alloiced to collect in the dosed branch, 

 so that none may escape when the thumb is removed. 

 If CO2 be present, a partial vacuum in the closed branch 

 causes the fluid to rise suddenly when the thumb is re- 



» Loc. cit., p. 196. 



