FERMENTATION. 587 



into a constant property, and these organisms excite 

 fermentation even when the presence of a plentiful 

 supply of oxygen does away with the necessity for the 

 act, the fermentation occurring whenever they are pro- 

 vided with fermentescible materials. 



A sine qua non for the occurrence of fermentative Classification 

 action is the presence of fermentescible material. Rations ac 

 Only a limited number of chemical bodies form suit- J^J 

 able materials, and it is not every one of the substances bie material. 

 which are capable of undergoing fermentation which 

 can be broken up by all of the fermentative agents; 

 on the contrary, each substance is only decomposed by 

 one or a few organisms, and every species of bacteria is 

 limited to a few suitable substances. In the case of 

 many micro-organisms no body is as yet known which 

 they are able to split up by fermentation. It is possible 

 that the functions of these organisms are always limited 

 to the ordinary respiratory tissue changes ; but it is 

 possible that, in the case of one or other of these 

 bacteria, the necessary fermentescible substance will yet 

 be found. 



It is only by the consideration of individual cases that 

 we can recognise and understand the nature and pro- 

 perties of the material suitable for fermentation, as well 

 as the mode in which it is split up by the fermentative 

 action. We differentiate a large number of specific 

 fermentations, which receive their names either from 

 one or a few characteristic products, or from the nature 

 of the fermentescible material, or, finally, from the fer- 

 menting agent. In what follows we shall describe, in 

 the first place, fermentation by the yeast fungi ; then 

 the various fermentations caused by bacteria, which may 

 be divided into five groups, viz., a. Fermentation of 

 carbo-hydrates ; /3. Fermentation of the higher alcohols 

 (glycerine, erythrite, mannite) ; y. Fermentation of the 

 fatty acids ; . Putrefaction ; * . The formation of acetic 

 acid from alcohol. 



