HELMHOLTZ IN BERLIN 



organisms may simply produce a substance, the mole- 

 cular vibrations of which may cause a re-arrangement 

 of the atoms of the substance undergoing fermenta- 

 tion. According to this view fermentation is essenti- 

 ally a chemical process. A substance of unstable 

 chemical composition is formed by the yeast cells, 

 and by vibrations, due to chemical changes in this 

 substance, movements are communicated to the atoms 

 of sugar, so that these are re-arranged to form alcohol, 

 carbonic acid, and small quantities of a few other 

 bodies. The growth of the yeast cell is only in- 

 directly connected with fermentation. 



For a considerable time this theory held its ground, 

 owing largely to the prestige of its illustrious author ; 

 but facts came to light that again concentrated the 

 attention on the biological aspect of fermentation. 

 Thus Gay-Lussac showed that clean grapes or boiled 

 grape juice passed into the Torricellian vacuum of 

 a barometer tube kept free from fermentation for any 

 length of time, but that if a single bubble of air 

 were admitted fermentation soon appeared. About 

 1838 Schwann repeated Gay-Lussac's experiment, and 

 showed that if the air were admitted to the vacuum 

 through a red hot tube then fermentation did not 

 occur. Clearly it was something in the air that 

 caused fermentation, and that something was destroyed 

 by heat. Further, it was shown that active fermentation 

 was always accompanied by increased growth of the 

 yeast, and that conditions of temperature affected the 

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