174 GENERAL PHYSIOLOGY 



known that this explanation is not sufficient, since some 

 substances are reduced, as, for example, methylene blue, 

 which do not contain oxygen. In these cases it may be that 

 nascent hydrogen is added. Certain bacteria produce 

 oxidative changes. The acetic acid bacteria oxidize ethyl 

 alcohol to acetic acid. The nitrifying bacteria oxidize 

 ammonia and the nitrites to nitrates. The sulphur bacteria 

 oxidize sulphuretted hydrogen to sulphur, and the iron bac- 

 teria oxidize the ferrous to the ferric salts. Gases are fre- 

 quently produced as the result of the metabolism of bacteria. 

 Among the more common gases are carbon dioxide, hydro- 

 gen, nitrogen, methane, and ammonia. These gases are 

 frequently of a good deal of value in differentiating one or- 

 ganism from another. Closely allied bacteria differ greatly 

 in their power to ferment certain substances. One may pro- 

 duce an abundance of gas in a food medium in which a close 

 relative will produce none. The gaseous by-products of 

 bacteria have been carefully studied in a number of cases, 

 but the complete analysis of the gases is a matter of consid- 

 erable chemical difficulty. An approximate method, how- 

 ever, for determining the amount of gas produced and its 

 formula, is very commonly used in the bacteriological labora- 

 tory and is known as the fermentation tube which was intro- 

 duced for this purpose by Theobald Smith. 



Bacteria very frequently produce acids as a result of the 

 decomposition of sugar. The acids most commonly pro- 

 duced are lactic, acetic, and butyric. Here, again, the acid 



