84 LOUIS PASTEUR 



' I pondered it for nearly ten years before producing 

 it,' he wrote. Pasteur, putting aside all subtleties of 

 argument, went straight to the two objections of the 

 German chemist which lay at the root of the discussion. 



It may be remembered that one of the most de- 

 cisive proofs by which Pasteur overthrew Liebig's 

 theory resulted from the experiments in which by 

 the aid of mineral bodies and fermentable matter he 

 produced a special living ferment for each definite fer- 

 mentation. By removing all nitrogenous organic 

 matter, which in Liebig's theory constitutes the fer- 

 ment, Pasteur established, at one and the same time, 

 the life of the ferment and the absence of all action of 

 albuminoid matter in process of alteration. Liebig 

 here formally contested the fact that Pasteur had been 

 able to produce yeast and alcoholic fermentation in a 

 sweetened mineral medium by sowing therein an in- 

 finitesimal quantity of yeast. It is certain that, ten 

 years previously, when Pasteur announced the pro- 

 duction of yeast life and alcoholic fermentation under 

 such conditions, his experiment was one so difficult to 

 perform that it sometimes happened to Pasteur him- 

 self to be unable to reproduce it. The cells of yeast 

 sown in the sweetened mineral medium found them- 

 selves often associated with other microscopic or- 

 ganisms, which were singularly hurtful to the life of 

 the yeast. Pasteur was at this period far from being 

 familiarised with the delicacy which such experiments 



