CHAPTEE XV. 



FEEMENTATION. 



§1. INTRODUCTION. 



The word fermentation is probably derived from the Latin word fervere, 

 meaning "to boil," or "to seethe." This gives us the clue to the popular 

 conception of this term. When a lump of yeast was placed in a sugar 

 solution, the liquid appeared to boil or seethe, without the application 

 of heat. To the people of the pre-scientific age, yeast was just a lump 

 of clay-like material that possessed the wonderful power of changing 

 sugar into alcohol. Here was a process that clamoured, if not for an 

 explanation, at least for a name. The term "fermentation," therefore, 

 probably arose in connection with this property of yeast, the discovery 

 of which dates into the far past; Bacchus was worshipped by the 

 Greeks as the god of wine, and among the ancient Egyptians the 

 knowledge of the process of making wine was regarded as a gift from 

 Osiris himself. We do not find even an attempt at an explanation of 

 this process until Valentinus of Erfurt, in the fifteenth century, declared 

 that fermentation {i.e. yeast fermentation) was a process of purification. 

 When sugar is fermented with yeast, the latter falls to the bottom 

 after the process is over, leaving the upper portion of the liquid clear. 

 Valentinus thought that the alcohol was there all the time, and that 

 the function of the yeast was to separate it from the other constituents. 

 It is not necessary to follow the steps which led from this crude and 

 incorrect explanation to our present knowledge of the subject. Instead, 

 we will deal with one or two special cases of fermentation in the light 

 of modern research, and for this purpose we cannot take, as our first 

 example, a better instance than this yeast fermentation. 



