306 LOUIS PASTEUR 



isting between the weight of fermentable substance de- 

 composed and that of ferment produced, there is no occasion 

 to speak of fermentations or of ferments. The phenomena 

 of fermentation and of ferments have been placed apart 

 from others, precisely because, in certain chemical actions, 

 that ratio has been out of proportion; but the time that 

 these phenomena require for their accomplishment has 

 nothing to do with either their existence proper, or with 

 their power. The cells of a ferment may, under some 

 circumstances, require eight days for revival and propa- 

 gation, whilst, under other conditions, only a few hours 

 are necessary; so that, if we introduce the notion of time 

 into our estimate of their power of decomposition, we 

 may be led to conclude that in the first case that power 

 was entirely wanting, and that in the second case it was 

 considerable, although all the time we are dealing with the 

 same organism the identical ferment. 



M. Schiitzenberger is astonished that fermentation can 

 take place in the presence of free oxygen, if, as we sup- 

 pose, the decomposition of the sugar is the consequence 

 of the nutrition of the yeast, at the expense of the com- 

 bined oxygen, which yields itself to the ferment. At all 

 events, he argues, fermentation ought to be slower in the 

 presence of free oxygen. But why should it be slower? 

 We have proved that in the presence of oxygen the vital 

 activity of the cells increases, so that, as far as rapidity 

 of action is concerned, its power cannot be diminished. 

 It might, nevertheless, be weakened as a ferment, and this 

 is precisely what happens. Free oxygen imparts to the 

 yeast a vital activity, but at the same time impairs its 

 power as yeast qua yeast, inasmuch as under this condi- 

 tion it approaches the state in which it can carry on its 

 vital processes after the manner of an ordinary fungus; 

 the mode of life, that is, in which the ratio between the 

 weight of sugar decomposed and the weight of the new 

 cells produced will be the same as holds generally among 

 organisms which are not ferments. In short, varying our 

 form of expression a little, we may conclude with perfect 

 truth, from the sum total of observed facts, that the yeast 

 which lives in the presence of oxygen and can assimilate 



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