FERMENTATION. 55 



fermentation liberates pure carbonic acid, a gas much 

 heavier than atmospheric air, which rests on the 

 surface of the liquid in the vat in a layer thick enough 

 to protect the liquid underneath from any contact with 

 the external air. All this liquid mass, then, is inclosed 

 between the wooden sides of the vat and a deep layer 

 of heavy gas which contains no trace of free oxygen. 

 In this liquid, nevertheless, the life of the cells of the 

 ferment and the production of all its constituents go 

 on for several days with extraordinary activity. Here 

 certainly we have life without air, and the ferment 

 character expresses itself in the enormous difference 

 between the weight of the ferment formed and collected 

 from the vats under the name of yeast, at the end 

 of the operation, and the weight of the sugar which 

 has fermented, transforming itself into alcohol, car- 

 bonic acid, and various other products. 



Pasteur has studied experimentally that which 

 takes place when, without otherwise changing the 

 conditions of these phenomena, the arrangement 

 is so modified as to permit the introduction of the 

 free oxygen of the atmosphere. It sufficed for this 

 purpose to provoke a fermentation of the must of 

 beer, or the must of grapes, upon shallow glass dishes 

 presenting a large surface, or in a flat-bottomed 

 wooden trough with sides a few centimeters in height, 

 instead of in deep vats as before. In these new con- 

 ditions the fermentation manifests an activity even 

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