THEORIES OF FERMENTATION. 235 



tion is brought about by living matter, the vitalistic theory of 

 fermentation, in contrast to the chemical theory which found 

 its best-known advocate in the distinguished Liebig, who- 

 built largely on the theories propounded by Willis and Stahl, 

 after Gay-Lussac's idea that oxygen was the direct cause had 

 been given up. 



Although Liebig's theory has been abandoned, it is neces- 

 sary, even in a brief historical description, to touch upon 

 it, because it held the field for a long time, on account of its- 

 author's great renown. 



Experimental chemistry had won great triumphs in the 

 last twenty years of the seventeenth century. Chemists had 

 succeeded in ascribing extremely complex organic processes, 

 previously ascribed to the mysterious vital energy, to a simple 

 action of chemical affinities. As an obvious consequence, the 

 attempt was made to explain fermentation phenomena in the 

 same way, without the help of living beings. Liebig, however, 

 regarded the yeast which appeared in the fermenting liquid 

 as a substance constantly undergoing decomposition, by which 

 the chemical motion incidental to these processes was trans- 

 mitted to the sugar, and brought about the decomposition 

 of the latter. It will be seen at once that this theory 

 could not be held when the presence of living and vigorous- 

 yeast cells was recognised. Liebig, however, did not regard 

 yeast as a plant ; it represented to the chemist a substance 

 without life, and microscopical investigation, according to- 

 Liebig, could contribute nothing of importance to the under- 

 standing of the process. 



We shall now proceed to discuss how the knowledge of 

 yeast developed and led to the vitalistic theory which 

 prevailed for such a long time. 



The Austrian Plenciz declared, as early as 1762, that decay 

 only takes place in a body when " germs of a wormy character 

 develop and begin to multiply." Probably we have here the 

 first definite announcement regarding the cause of such decom- 

 positions. 



A long time elapsed before Leeuwenhoek's observations on 

 yeast cells, in 1680, carried us a step further. As far as we 

 can judge from the known literature, it is believed that the 



