

300 LOUIS PASTEUR 



learn that at this point certain preconceived ideas suggest 

 themselves to the mind of an attentive reader on the subject 

 of the causes that may serve to account for such strange 

 phenomena in the life of these beings which our ignorance 

 hides under the expressions of youth and age; this, however, 

 is a subject which we cannot pause to consider here. 



At this point we must observe for it is a matter of great 

 importance that in the operations of the brewer there is 

 always a time when the yeasts are in this state of vigorous 

 youth of which we have been speaking, acquired under the 

 influence of free oxygen, since all the worts and the yeasts 

 of commerce are necessarily manipulated in contact with 

 air, and so impregnated more or less with oxygen. The 

 yeast immediately seizes upon this gas and acquires a state 

 of freshness and activity, which permits it to live afterwards 

 out of contact with air, and to act as a ferment. Thus, in 

 ordinary brewery practice, we find the yeast already formed 

 in abundance even before the earliest external signs of fer- 

 mentation have made their appearance. In this first phase 

 of its existence, yeast lives chiefly like an ordinary fungus. 



From the same circumstances it is clear that the brewer's 

 fermentations may, speaking quite strictly, last for an in- 

 definite time, in consequence of the unceasing supply of 

 fresh wort, and from the fact, moreover, that the exterior 

 air is constantly being introduced during the work, and that 

 the air contained in the fresh worts keeps up the vital ac- 

 tivity of the yeast, as the act of breathing keeps up the 

 vigour and life of cells in all living beings. If the air could 

 not renew itself in any way, the vital activity which the 

 cells originally received, under its influence, would become 

 more and more exhausted, and the fermentation eventually 

 come to an end. 



We may recount one of the results obtained in other ex- 

 periments similar to the last, in which, however, we em- 

 ployed yeast which was still older than that used for our 

 experiment with flask A (Fie. 2), and moreover took still 

 greater precautions to prevent the presence of air. Instead 

 of leaving the flask, as well as the dish, to cool slowly, after 

 having expelled all air by boiling, we permitted the liquid 

 in the dish to continue boiling whilst the flask was being 



*t 







