THEORIES OF FERMENTATION. 243 



yeast on the surface of the gelatine, the fermentation on 

 gelatine ceased much more quickly than in wort, where the 

 yeast collected on the bottom of the flask. It was further 

 proved that considerably more carbon dioxide was formed in 

 the surface cultures than in the parallel experiments with 

 wort. This carbon dioxide must be due to the respiration of 

 the yeast. Nevertheless, only about one-seventh of the sugar 

 was decomposed by oxidation, whilst more than six-sevenths 

 were fermented. Although the yeast had been submitted, 

 according to Pasteur's view, to the most favourable condi- 

 tions for life without bringing about fermentation, this did 

 not prove to be the case. As is now universally known, a 

 free supply of oxygen exercises a favourable influence on the 

 propagation of cells, but these experiments served to establish 

 the fact that oxygen has scarcely any influence on the process 

 of fermentation, and that the absence of oxygen must not 

 be regarded as being of immense moment for the fermentative 

 activity, for even in presence of a full quantity of oxygen 

 the fermentative power of yeast still exceeds the respiratory 

 power. 



From Nageli's many-sided work on the lower organisms, 

 we can only refer, in connection with the preceding, to his 

 " molecular-physical " theory of fermentation, which may be 

 regarded as a modification of Liebig's theory. Whilst Pasteur 

 regarded fermentation as the result of an activity taking place 

 in the cell, Nageli defined fermentation as a transference of 

 states of motion of the molecules, groups of atoms or atoms 

 of different compounds, constituting living protoplasm (which 

 otherwise undergo no change) to the fermenting material, 

 whereby the stability in these molecules is destroyed and 

 disruption is brought about. During fermentation the vibra- 

 tions of the molecules of protoplasm are transferred to the 

 fermenting material. The cause of fermentation is to be 

 nought in the living protoplasm in the interior of the cells, 

 but its activity extends for some distance outside the cells. 

 The decomposition of sugar takes place to a slight extent 

 inside the yeast cells, but principally outside them. This 

 theory is opposed to that of Pasteur, and is related to that 

 of Stahl and Liebig. 



