BIOCHEMICAL TESTS 9/ 



filled, and the open arm partly filled, with broth containing 

 the sugar to be tested. The gas found after inoculation col- 

 lects in the closed arm and may be conveniently measured 

 by means of a Frost gasometer. The approximate composition 

 of the gas may be determined by filling the open arm with normal 

 sodium hydrate and securely closing the opening with the thumb, 

 mixing the gas with the alkaline solution by passing it several 

 times from one arm to the other, finally returning it to the closed 

 arm and removing the thumb. The liquid will then rise in the 



Fig. 53. Fermentation tube and Frost gasometer (Heinemann). 



closed arm to replace the carbon dioxid absorbed. The remaining 

 gas may be transferred to the open arm and tested by the flame. 

 Hydrogen is indicated by a slight explosion. The relative pro- 

 portion of carbon dioxid and hydrogen is sometimes of importance 

 in the differentiation of species. More important still is the 

 ability of a species to ferment different kinds of carbohydrates. 

 Some ferment dextrose, but not lactose or saccharose some fer- 

 ment two only, and some all three. 



Reduction Processes. Some bacteria, when living in the absence 



7 



