FERMENTATION. 



197 



FIG. 87. 



The medium to be used is run into the larger tube 

 until there is present about 50 per cent, 

 more than the volume of the smaller tube. 

 The whole is then sterilized in streaming 

 steam by the fractional method. After 

 the first sterilization the small tube will 

 be found almost filled with fluid, over 

 which a small air-bubble lies. After the 

 second or third sterilization this air- 

 bubble is completely expelled, and the 

 small tube contains nothing but the liquid. 

 The medium that Durham employs for 

 the fermentation-test is a 1 per cent, solu- 

 tion of Witte's peptone in distilled water, 

 to which have been added known amounts 

 of some such fermentable sugar as glucose, 

 saccharose, lactose, mannite, etc., as the 

 case may demand. He prefers peptone 

 to meat-infusion bouillon for the reason 

 that the latter often contains traces of 

 muscle-sugar, and is thereby likely to 

 complicate the results. He prefers neutra- 

 lization with organic acids rather than 

 mineral acids, and uses citric acid by 

 preference, the reason for this being that 

 where sugars such as those mentioned are 

 acted upon by mineral acids under 

 influence of heat their composition is apt to be altered. 



NOTE. Prepare two fermentation-tubes as follows : 

 Fill one with 1 per cent, watery solution of peptone 

 to which 2 per cent, of glucose has been added ; fill 

 the other with a similar peptone solution, but to which 



