THE BACTERIA IN DIFFERENT MEDIA. 137 



1. ROLE OF BACTERIA IN FERMENTATIONS. 



We say that a liquid is fermenting whenever 

 modifications occur in its chemical constitution, as 

 a result of the nutrition of organized beings. 



Two kinds of fermentation are commonly distin- 

 guished. In the first group (false fermentations) 

 are arranged those which are produced by soluble 

 quarternary substances (diastase, soluble ferments) 

 secreted by living cells, from which they may be 

 separated in order to study their action upon fer- 

 mentable liquids. This action is comparable to that 

 of certain mineral acids, which operate in the same 

 manner, either by the breaking up of molecules 

 with addition of water or by the phenomena of 

 hydratlon. Veritable chemical reagents, when 

 these substances are once precipitated from their 

 solutions, purified and dried, they preserve their 

 properties indefinitely. A sufficient elevation of tem- 

 perature seems to destroy the edifice of their mol- 

 ecule ; for they lose all their specific power after 

 having been subjected to a temperature more or less 

 elevated, but always inferior to 100 (212 Fah.). 



In the second group (true fermentations) are 

 joined all the phenomena of chemical modifica- 

 tion which appear intimately united to the devel- 

 opment of inferior organisms, algse or fungi 

 (figured ferments). Compressed oxygen by kill- 

 ing these ferments, and chloroform by suspending 

 their vital functions, arrest the progress of these 

 fermentations, while the same agents do not mod- 



