330 LOUIS PASTEUR 



Brefeld, without knowing it, operated on yeast in one of the 

 states in which it requires gaseous oxygen to enable it to 

 germinate again. A perusal of what we have previously 

 written on the subject of the revival of yeast according to 

 its age will show how widely the time required for such 

 revival may vary in different cases. What may be per- 

 fectly true of the state of a yeast to-day may not be so 

 to-morrow, since yeast is continually undergoing modifica- 

 tions. We have already shown the energy and activity 

 with which a ferment can vegetate in the presence of free 

 oxygen, and we have pointed out the great extent to which 

 a very small quantity of oxygen held in solution in fer- 

 menting liquids can operate at the beginning of fermenta- 

 tion. It is this oxygen that produces revival in the cells 

 of the ferment and enables them to resume the faculty of 

 germinating and continuing their life, and of multiplying 

 when deprived of air. 



In our opinion, a simple reflection should have guarded 

 Dr. Brefeld against the interpretation which he has at- 

 tached to his observations. If a cell of ferment cannot bud 

 or increase without absorbing oxygen, either free or held 

 in solution in the liquid, the ratio between the weight of 

 the ferment formed during fermentation and that of oxy- 

 gen used up must be constant. We had, however, clearly 

 established, as far back as 1861, the fact that this ratio is 

 extremely variable, a fact, moreover, which is placed beyond 

 doubt by the experiments described in the preceding section. 

 Though but small quantities of oxygen are absorbed, a con- 

 siderable weight of ferment may be generated; whilst if the 

 ferment has abundance of oxygen at its disposal, it will 

 absorb much, and the weight of yeast formed will be still 

 greater. The ratio between the weight of ferment formed 

 and that of sugar decomposed may pass through all stages 

 within certain very wide limits, the variations depending on 

 the greater or less absorption of free oxygen. And in this 

 fact, we believe, lies one of the most essential supports of 

 the theory which we advocate. In denouncing the im- 

 possibility, as he considered it, of a ferment living without 

 air or oxygen, and so acting in defiance of that law which 

 governs all living beings, animal or vegetable, Dr. Brefeld 



