216 BACTERIOLOGY. 



prefers peptone to meat-infusion bouillon for the reason 

 that the latter often contains traces of muscle-sugar, 

 and thereby is 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 a 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 differ- 

 ences are observed, and how may they be explained? 



Cultivation without Oxygen. — As we have 

 already learned, there is a group of bacteria to which 

 the designation "anaerobic" has been given, which 

 are characterized by inability to grow in the presence 

 of free oxygen. For the cultivation of the members of 

 this group, a number of devices are employed for the 

 exclusion of free oxygen from the cultures. 



Koch's method. Koch covered the surface of a gela- 

 tin plate, wliich had been previously inoculated, with 

 a thin sheet of sterilized isinglass. The organisms 



