80 FERMENTS AND MICRO-PARASITES. 



development' of the micro-organisms in the fermenting 

 mixture, that the fermentations go on best at that tem- 

 perature which corresponds with the optimum of tem- 

 perature for the growth and the other vital functions of 

 the micro-organisms, and that the typical physiological 

 poisons, such as chloroform, ether, and hydrocyanic acid, 

 are able even in small doses to prevent the ferment- 

 ation. It has also been demonstrated by accurate 

 chemical analysis of the products of fermentation, that 

 the breaking up of the fermenting material during 

 fermentation implies such a profound transformation 

 of the molecules, and such an intense displacement of the 

 atoms, that an approximately similar alteration could 

 only be obtained by our strongest chemical agents. And 

 as such chemical means do not come into play, we are 

 thrown back on the physiological actions, the profound 

 effects of which are everywhere evident. 



Specific excit- Of greater importance for the further develop - 

 fermentation, ment of the vitalistic theory of fermentation was 

 the demonstration of the fact that different sorts of 

 micro-organisms gave rise to diverse and specific 

 actions. At the time when the germ theory was 

 founded, one only spoke of organised ferments in 

 general. The course and the products of fermentation 

 and putrefaction were studied under varying conditions, 

 without paying special attention to the species of fer- 

 mentative agents present, and without ascertaining 

 whether one definite species alone, or a mixture of 

 different fungi, were concerned in the decomposition of 

 the fermentescible materials. And yet rigid observation 

 of this kind was absolutely necessary in order to learn 

 accurately the conditions of the life of the organisms, 

 and the relation between their life and nutritive pro- 

 cesses and the phenomena of fermentation. In this 

 " direction also Pasteur's researches formed the real 

 foundation. He distinguished with striking perspicuity 

 a definite form of micro-organism which set up lactic 

 acid fermentation, another which furnished butyric 

 acid, &c., and he laid stress on the necessity for further 

 differentiation. It was thus that the advantages of 



