ORIGIN OF THE YEASTS OF WINE 217 



But let us leave this dust, evidently living, in a thin 

 layer of sugar solution exposed to the air under the micro- 

 scope, and we shall see come forth in profusion from 

 certain groups of the brown corpuscles, cells (Ai, A 2 , 

 BI, B 2 , Fig. 19) or branching filaments which at once 

 bud and segment into cells. Now these cells are yeast 

 cells, for, once rejuvenated in contact with air and sown 

 in a sweet must, they produce in some hours an active 

 alcoholic fermentation. 



Pasteur must have felt a profound joy in discovering 



FIG. 19. Thick-walled brown cells that give rise to wine yeasts. 



these facts, for, with his usual perspicacity, he must 

 have seen immediately the solution of a problem which 

 had been in his mind for a long time, and of which 

 he had many times tried in vain to find the solution. 

 The origin of this difficulty was the experiment of Gay- 

 Lussac which we have described, and in which this scien- 

 tist had seen some bubbles of oxygen produce fermen- 

 tation in the juice of grapes crushed in a test-tube under 

 mercury and remaining inert up to that time. 



It is here that we can find a proof of the truth of that 



