PRODUCTS OF THE GROWTH OF BACTERIA. 133 



by dry heat, afterward filled with the bouillon, and sterilized 

 by steam in the usual manner. After the last sterilization it 

 ; should be tilted until the closed end is completely filled with the 

 medium. After it has been inoculated with the species under 

 consideration, any development of gas will be indicated by the 

 collection of the gas at the closed end. The amount of gas 

 formed may be estimated and its quality tested. To accomp- 

 lish the latter fill the bulb with 2 per cent, solution of sodium 

 hydroxide, close the outlet, and tilt the tube to allow the 

 mixture to come in contact with the gas. After shaking, this 

 causes the absorption of the carbon dioxide and diminution in 

 the quantity of gas. The portions which remain may be mixed 

 with air and ignited, when the presence of hydrogen and some 

 of its compounds will be indicated by an explosion. (See The 

 Detection of Bacillus coli communis in Water, Part IV.) 



The development of gas may readily be tested by inoculating 

 the bacteria by a deep puncture into agar containing i per cent, 

 of dextrose or other sugars. The development of gas causes 

 bubbles to form in the agar, often to the extent of splitting it, 

 and- sometimes forcing out the cotton plug (see Fig. 73). 



The activities of bacteria which have just been enumerated 

 are fundamental to the phenomena which go by the names of 

 fermentation and putrefaction. These words have been defined 

 differently at different times and by different writers, but in 

 general both are used as names for the breaking up of complex 

 organic compounds by micro-organisms with the formation of 

 simpler compounds. Fermentation refers especially to the 

 formation of useful products like alcohol. The term putre- 

 faction is employed chiefly for the breaking up of nitrogenous 

 compounds with the development of foul-smelling gases. The 

 term fermentation is also applied to the decomposition of com- 

 plex substances through the influence of unorganized ferments 

 or enzymes. The work of bacteria in decomposition is indis- 

 pensable to the existence of the organic world as 'we find it. 



