THEORIES OF FERMENTATION. 229 



rence in the cell may be demonstrated by micro-chemical 

 methods e.g., by the action of pepsin (dissolved in 0-2 per 

 cent, hydrochloric acid) which, according to Zacharias, 

 attacks the other albuminoids, but not the nucleins ; they 

 are, however, dissolved by weak alkalies. 



The great importance of yeasts, both from the scientific 

 and practical standpoint, is due to their power of forming 

 alcohol from the sugars. From time to time numerous attempts 

 have been made to explain the actual processes which go on 

 during the course of fermentation. It is only quite recently 

 that a starting point has been found for the investigation, which 

 has made it possible to subject this physiological activity of 

 the yeast cell to experimental treatment. The labours of 

 previous workers in this field have, however, produced results 

 of the highest scientific and practical value, and not a few 

 of the investigations in this and adjacent fields of research 

 have laid the basis upon which modern views are built. Our 

 statement must, therefore, be based on a resume of the entire 

 development which has led, during the course of years, to the 

 various theories of fermentation. 



Theories of Fermentation. 



It was long ago observed that when a sugar solution or 

 fruit juice is exposed to the air, fermentation phenomena 

 occur after a certain lapse of time. The liquid becomes 

 turbid, an evolution of gas takes place, a precipitate is formed, 

 and the surface is covered with a layer of yeast. The liquid 

 gradually loses its sweet taste, clarifies at the same time, and 

 then proves to contain a new substance with a stimulating 

 action. 



What exactly the process might be was the object of 

 many speculations in olden times, which were not based upon 

 any true investigation of the processes. We will put all these 

 speculations on one side, and start with the end of the eighteenth 

 century, the time of the renowned Lavoisier, the founder of 

 modern chemistry, who gave the first explanation of the 

 phenomena based on facts, the first link of a theory of fer- 

 mentation. He proved that simultaneously with the dis- 



