STUDIES ON FERMENTATION. 281 



may vary in different cases. What may be perfect!}^ true of 

 the state of a yeast to-day may not be so to-morrow, since 

 yeast is continually undergoing modifications. We have already 

 shown the energy and activity with which a ferment can vege- 

 tate 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 fermenting liquids can operate at the beginning 

 of fermentation. 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 attached 

 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 ferment formed 

 during fermentation and that of oxygen used up must be con- 

 stant. 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 paragraph. Though but small 

 quantities of oxygen are absorbed, a considerable weight of 

 ferment may be generated ; whilst if the ferment has abun- 

 dance 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 between 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 impossibility, 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 ought also to have borne in mind the fact which we 

 have pointed out, that alcoholic 3'east is not the only organized 

 ferment which lives in an anaerobian state. It is really a 



