PRODUCTS OF THE GROWTH OF BACTERIA. 13! 



ward 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 con- 

 sideration, 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 FlG 



quality tested. To accomplish 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 absorp- 

 tion 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 com- 

 pounds will be indicated by an 

 explosion. (See The Detection 

 of Bacillus coli communis in 



, T . Fermentation-tube. 



\Vater, Part II., Chapter III.) 



The development of gas may readily be tested by inocu- 

 lating 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. 68). 



The activities of bacteria which have just been enumer- 

 ated 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 dif- 

 ferent writers, but in general both are used as names for 



12 



