118 MICRO-ORGANISMS AND FERMENTATION. 



generally still, it is the necessary result of chemical work 

 carried out on a fermentable substance, which by its decom- 

 position is capable of evolving heat ; the heat necessary to 

 effect this work being borrowed from a part of that which is 

 liberated by the decomposition of the fermentable substance. 

 The class of fermentations properly so-called is limited by 

 the small number of substances which are capable of evolving 

 heat on decomposition, and which will serve as nourishment 

 for the lower organisms when the admission of air is excluded " 

 " (fitudes sur la biere," page 261). This is briefly Pasteur's 

 famous theory of fermentation. 



Fermentations dependent upon oxidation, such as the 

 acetic acid fermentation, which, as Pasteur himself had 

 observed, requires an abundant supply of air, were con- 

 sequently not regarded by him as true fermentations. It is 

 seen, moreover, that he does not strictly adhere to his 

 definition, in that he emphasises the fact that yeast also pos- 

 sesses fermentative properties when air is present, although 

 to a less degree than when oxygen is excluded. The correct- 

 ness of this under certain conditions has been confirmed in 

 the case of bottom yeast by Pedersen (1878), and by Hansen 

 (1879), who came to the conclusion that the amount of 

 substance in a wort which a definite quantity of yeast can 

 convert into alcohol and carbonic acid is smaller when the 

 liquid is aerated during fermentation than when no aeration 

 takes place. Ed. Buchner (1885) obtained a similar result in 

 his experiments with bacteria. 



Hansen arranged his experiments in such a way that a 

 rotatory motion was imparted to the liquid which was 

 being aerated, and the cells thus brought into continual 

 contact with the vigorous current of air which was blown 

 through the fluid. Nevertheless, there was a distinct alcoholic 

 fermentation, and it certainly follows that this was not 

 induced by life without air. 



In NagelVs " Theorie der Gariing " (1879) it is shown 

 that the admission of oxygen is highly favourable to alcoholic 



