42 LOUIS PASTEUR. 



tervention of tKe hand of man. What is, then, the 

 cause of the processes of fermentation, of putrefaction, 

 and of slow combustion ? How is the disappearance 

 of the dead body or of the fallen plant to be accounted 

 for ? What is the explanation of the foaming of the 

 must in the vintage cask ? of dough, which, abandoned 

 to itself, rises and becomes sour ? of milk, which 

 curdles ? of blood, which putrefies ? of the heap of straw, 

 which becomes manure? of dead leaves and plants 

 embedded in the earth, which transform themselves 

 into soil ? 



Many different attempts were made to account for 

 this mystery before science was in a condition to ap- 

 proach it. In our age, and at the time when Pasteur 

 was led to the study of the question, one theory held 

 almost undisputed sway. It was a very ancient theory, 

 to which Liebig, in reviving it, had given the weight 

 of his name. ' The ferments,' said Liebig, ' are all 

 nitrogenous substances albumen, fibrine, caseine ; or 

 the liquids which embrace them, milk, blood, prine 

 in a state of alteration which they undergo in con- 

 tact with the air.' 



The oxygen of the air was, according to this system, 

 the first cause of the molecular breaking up of the 

 nitrogenous substances. The molecular motions are 

 gradually communicated from particle to particle in 

 the interior of the fermentable matter, which is thus 

 resolved into new products. 



