100 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



organism. Carbon dioxide, hydrogen, nitrogen, and meth- 

 ane are the most common gases formed. 



As a test for the ability of an organism to produce gas 

 we may inoculate a liquid culture of agar or gelatin contain- 

 ing a suitable carbohydrate. The development of gas will 

 cause the formation of numerous bubbles. A qualitative 

 determination of the kinds of gas developed may be made 

 by use of a fermentation tube containing a suitable liquid 

 carbohydrate medium. The closed arm is entirely filled 

 with the medium. Organisms grown in the closed arm 

 will produce gas which will accumulate in the upper part 

 of this closed arm. The amount produced may be 

 readily estimated by noting the proportionate part of the 

 length of the tube displaced by gas. A qualitative test may 

 be made by filling the open arm with normal sodium hy- 

 drate, closing the open arm, shaking the gas into the open 

 arm, returning the gas to the closed arm, and removing the 

 cork. The liquid will rise to replace any carbon dioxide 

 which has been produced. Yeasts and some bacteria pro- 

 duce carbon dioxide only. With these the liquid will rise 

 practically to the top of the tube after the absorption of 

 the gas. Many bacteria produce both hydrogen and carbon 

 dioxide. The hydrogen may be measured after the absorp- 

 tion of the carbon dioxide by the alkali. When drawn into 

 the open arm, mixed with air, and brought near to a flame, 

 it will explode. A few microorganisms growing in the 

 presence of nitrates produce considerable amounts of free 

 nitrogen gas. 



Methane is produced by a few bacteria, but not by organ- 

 isms most commonly grown in the laboratory. 



Accurate studies of the comparative amounts of carbon 

 dioxide and hydrogen produced by bacteria have proved 

 useful in some cases in differentiating them. Such studies 

 can be made only by the use of an apparatus filled with 

 medium and containing no gas, sealed to prevent the escape 



