214 BACTERIOLOGY. 



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 em2:)loyed 

 is bouillon containing 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 A^olume 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 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 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 re- 

 moved. 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 meas- 

 ured before shaking with the sodium hydroxide solution 

 gives the proportion of CO2 absorbed. The explosive 

 character of the residue is determined as fo^o^^•s : the 

 thumb is placed again over the mouth of the bulb and 

 the gas from the closed branch is allo^vtd to flow into 

 ' Loc. cit., p. 190. 



