CHANGES IN THE REACTION OF MEDIA 201 



influence of heat the tension of the 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 flUs 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 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 

 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 natiu'e 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 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 remomng the thumb all the 

 gas is allowed to collect in the closed 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 removed. After allowing 

 the layer of foam to subside somewhat the space occupied 

 by gas is again measured, and the difference between this 

 amount and that measured before shaking with the sodium 



I Loc. cit., p. 196. 



