66 LOUIS PASTEUR 



considerations that determined Pasteur's point of 

 attack upon the subject of fermentation at Lille. 

 His observations on the relation of the molecular 

 asymmetry of the tartrates to fermentation and 

 the fact that this fermentation was accompanied 

 by the appearance of multitudes of minute organ- 

 isms impressed him profoundly. The thought that 

 fermentation is essentially a vital phenomenon and 

 not a mere chemical transformation due to decom- 

 posing substances became more deeply impressed 

 upon his mind as he continued to investigate the 

 subject. It was one of those "preconceived ideas" 

 which he frequently alluded to as guides to the 

 investigator in the discovery of truth. In his case 

 it was an idea that was ever-present during the 

 years which he devoted to the study of fermenta- 

 tion. 



When Pasteur began his studies on fermentation 

 the prevalent ideas on the subject were very ob- 

 scure. One peculiar feature of the chemical trans- 

 formations occurring in a fermenting body is that 

 they may be set up in the same kind of material 

 by transferring to it a small part of the fermenting 

 substance. The Swedish chemist Stahl held that 

 fermentation is the result of a peculiar "internal 

 movement" which may be communicated from one 



