N 

 40 AGRICULTURAL BACTERIOLOGY. 



changes is varied but they commonly, though not always, re- 

 sult in the production of simpler out of more complex com- 

 pounds, frequently with the absorption of water into the chem- 

 ical combination. As an example may be given the best 

 known fermentation, that of sugar, under the influence of yeast, 

 into alcohol and carbon dioxide. The following formula is com- 

 monly given as expressing the process : 



(sugar) (alcohol) (carbon dioxide) 



As a second type of fermentation may be given that effected 

 by the saliva when acting upon starch and commonly repre- 

 sented by the equation : 



(starch) (sugar) 



It must be stated that neither of these equations is believed 

 to-day to express the whole of the change that goes on, for the 

 phenomena are doubtless much more complicated than here 

 indicated. 



The kinds of fermentations are numerous but they are 

 readily grouped under comparatively few classes. The most 

 important ones which concern agriculture are the following : 

 The alcoholic fermentation ; the butyric fermentation which 

 produces butyric acid in butter ; the lactic fermentation which 

 causes the souring of milk and various other products ; the 

 acetic fermentation which produces acetic acid and forms vine- 

 gar ; the protcolytic fermentation which renders soluble certain 

 insoluble proteids, for example, in the ripening of cheese ; the 

 t>.\ii/i.::i>ig fermentation which causes the oxidation of or- 

 ganic matter, as in the fermentation of tobacco ; the nitrifying 

 fermentation which converts ammonia into nitrites or nitrates ; 

 the denitrifying" fermentation which converts nitrates into 

 nitrites or simpler compounds by depriving them of oxygen. 

 These different types of fermentation will be considered at 



