MILK FERMENTATIONS 69 



gas in the closed arm should be noted, and the amount of gas 

 determined by measuring the same with Frost's gasometer. 



When the culture ceases to form gas, remove from the in- 

 cubator, cool to room temperature, and measure. Test the 

 reaction of the liquid in the open arm by the use of litmus 

 paper, or titrate a definite amount as in the case of milk. 

 The culture should not be allowed to stand too long after 

 gas evolution ceases, as the carbon dioxide formed is rapidly 

 absorbed by the liquid. 



To determine the relative proportion of the component 

 gases formed, fill the bulb of the fermentation tube with a 

 2 per cent solution of sodium hydroxide, close the opening 

 of the tube with the thumb, tip the tube so as to allow the 

 gas to fill the bulb and come in contact with the alkaline solu- 

 tion ; return the gas to the closed arm, and repeat the process 

 several times, keeping the open arm tightly closed. With the 

 gas in the closed arm, remove the thumb. The liquid will rise 

 in the closed arm on account of the absorption of the C0 2 by 

 the alkali. Measure the residue, which is usually considered 

 as hydrogen, close the tube again with the thumb, and bring 

 the gas into the bulb ; remove the thumb and introduce a 

 lighted match. The air mixed with the hydrogen forms an 

 explosive mixture. The composition of the gas is expressed 

 by the ratio of hydrogen to carbon dioxide (H/C0 2 ). 



With bacteria that do not grow at 37 C. the test should 

 be made at 20 C., the time of incubation being extended at 

 least four days. 



A qualitative test for gas production from the various 

 sugars can be made by the use of "shake. cultures." A tube 

 of melted gelatin or agar containing 2 per cent of the sugar 

 to be tested is inoculated with the culture, mixed well, and 

 solidified by placing in ice water. If the sugar is fermented 



