CHANGES IN THE REACTION OF MEDIA 203 



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, solution 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 neutralization 

 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 the 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 only 0.3 per cent, 

 of glucose has been added. Sterilize and inoculate with 

 bacillus coli communis. How do the two tubes differ from 

 one another after eighteen to twenty-four hours in the 

 incubator? First, as regards the reaction of the fluid in the 

 open arms of the tubes. Second, as to accumulation of gas 

 in closed arms of the tubes. Third, as to the capacity of 

 each solution for reducing copper in Fehling's solution. 

 What differences are observed, and how may they be ex- 

 plained? 



Indol Production. — ^The detection of products other than 

 those that give rise to alterations in the reaction of the 

 media, and whose presence may be denionstrated by chemical 



