I20 



MANUAL OF BACTERIOLOGY. 



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 develop- 

 ment 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 accomplish the latter fill the bulb 

 with 2 per cent, solution of sodium hydroxide, close the outlet, 



and tilt the tube to allow the mix- 

 ture J;o come in contact with the gas. 

 After shaking, this causes the ab- 

 sorption 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 Detec- 

 tion of Bacillus coli communis in 

 Water, Part lY.) 



The development of gas may 

 readily be tested by inoculating the 

 bacteria by a deep puncture into 

 agar containing i per cent, of dex- 

 trose or other sugars. The devel- 

 opment of gas causes bubbles to 

 form in the agar, often to the extent of splitting it, and some- 

 times 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 



Fig. 46. — Fermentation-tube. 



