I I 8 PHYSIOLOGICAL CHEMISTRY. 



what was described as a catalytic action. No real attempt, 

 however, was made to define more closely what was meant 

 by this catalytic or contact action. 



The Theory of Pasteur. The real nature of yeast as a 

 vegetable growth had finally become established. With this 

 admitted Pasteur advanced the proposition that alcoholic fer- 

 mentation is a consequence of the life of the organism in con- 

 tact with sugar and away from the air. Alcohol and carbon 

 dioxide are products of the yeast cell metabolism under these 

 conditions. The cell, according to the Pasteur view, must be 

 furnished with a proper supply of oxygen and this, under the 

 fermenting conditions, it takes from the sugar, giving off car- 

 bon dioxide as an oxidation product and producing alcohol at 

 the same time, as a result of the breaking down of the sugar 

 molecule. Fermentation is then to be considered from a 

 purely biological standpoint with alcohol and carbon dioxide 

 as excretory and respiratory products respectively. This 

 Pasteur theory soon found favor with the majority of scien- 

 tific men and gradually supplanted the mechanical notion of 

 Liebig which could not be brought into accord with experience 

 in other lines. Although the Pasteur view that the yeast pro- 

 duces alcohol only in absence of free oxygen was shown to be 

 incorrect the theory commended itself as otherwise satisfac- 

 tory and tangible. 



Following this a similar explanation was offered for the 

 action taking place in the formation of acetic acid, lactic acid 

 and butyric acid. Here microorganisms are also concerned. 

 These live on certain substances and produce others as meta- 

 bolic excreta. As to the mechanism of this metabolism we 

 know, of course, nothing; to describe the products formed as 

 excreta is perhaps not really warranted by what is actually 

 known. It must be remembered then that the term is used 

 in a broad and general sense only to indicate some kind of a 

 metabolic product. 



The work of Pasteur gave an enormous impetus to the study 

 of the common fermentations, but it was' evident that this bio- 



