298 LOUIS PASTEUR 



that this fixation of oxygen in yeast, as well as the oxida- 

 tions resulting from it, have the most marked effect on the 

 life of yeast, on the multiplication of its cells, and on their 

 activity as ferments acting upon sugar, whether immediately 

 or afterwards, apart from supplies of oxygen or air. 



In the preceding experiment, conducted without the pres- 

 ence of air, there is one circumstance particularly worthy 

 of notice. This experiment succeeds, that is to say, the 

 yeast sown in the medium deprived of oxygen develops, 

 only when this yeast is in a state of great vigour. We have 

 already explained the meaning of this last expression. But 

 we wish now to call attention to a very evident fact in 

 connection with this point. We impregnate a fermentable 

 liquid; yeast develops and fermentation appears. This lasts 

 for several days and then ceases. Let us suppose that, from 

 the day when fermentation first appears in the production of 

 a minute froth, which gradually increases until it whitens 

 the surface of the liquid, we take, every twenty-four hours, 

 or at longer intervals, a trace of the yeast deposited on 

 the bottom of the vessel and use it for starting fresh fer- 

 mentations. Conducting these fermentations all under pre- 

 cisely the same conditions of temperature, character and 

 volume of liquid, let us continue this for a prolonged time, 

 even after the original fermentation is finished. We shall 

 have no difficulty in seeing that the first signs of action in 

 each of our series of second fermentations appear always 

 later and later in proportion to the length of time that has 

 elapsed from the commencement of the original fermentation. 

 In other words, the time necessary for the development of 

 the germs and the production of that amount of yeast suf- 

 ficient to cause the first appearance of fermentation varies 

 with the state of the impregnating cells, and is longer in 

 proportion as the cells are further removed from the period 

 of their formation. It is essential, in experiments of this 

 kind, that the quantities of yeast successively taken should 

 be as nearly as possible equal in weight or volume, since, 

 ceteris paribus, fermentations manifest themselves more 

 quickly the larger the quantity of yeast employed in im- 

 pregnation. 



It we compare under the microscope the appearance 







