ALCOHOLIC FERMENTATION 75 



On this first error, Dobereiner grafted another by 

 affirming that the nitrogen lost by the yeast was found 

 in the fermented liquid in the state of ammonia. As 

 organic matters in decomposition also produce ammonia, 

 this affirmation of Dobereiner was, clearly, very favorable 

 to the ideas of Liebig, and the latter, a great collector 

 of facts and abstractor of quintessence, did not fail to 

 seize on this and make it serve to prop his doctrine of 

 fermentation. 



For Pasteur, on the contrary, this fact was inexplicable, 

 since the ferment was not a dead substance in process of 

 destruction, but a living thing in process of organiza- 

 tion. In trying to discover whether the statement of 

 Dobereiner was true he found not only that the nitrogen 

 of the yeast did not leave it in the form of ammonia, 

 but, moreover, that the yeast in process of fermentation 

 caused the ammonia to disappear from ammoniacal 

 salts added to the liquid. 



But how could the yeast do this? He then was very 

 bold to reverse the reasoning of Liebig and of Dober- 

 einer, and to say: the albuminoid substance of the fer- 

 ment does not give up ammonia; it is, on the contrary, 

 the ammonia which produces the albuminoid substance. 



This way of looking at it was so new and the pre- 

 sumption seemed so ill-founded that Pasteur hesitated, 

 as he himself acknowledges. But it was in accordance 

 with facts and the logic of his ideas. In any case, the 

 only thing to do was to resort to experiment. After 

 some attempts, the latter succeeded, and it became the 

 critical experiment, the experimentum crucis, which 

 made it possible to judge the doctrines side by side. 



This experiment was entirely new. The problem was 

 to grow the yeast in a liquid deprived of all organic 

 nitrogenous matter one containing only perfectly pure 

 cane-sugar, various mineral salts to supply the yeast 



