FERMENTATION. 539 



What, then, are the conditions under which the yeast- 

 plant must be placed so that it may display its character- 

 istic quality? Reflection on the facts alread} 7 referred to 

 suggests a reply, and rigid experiment confirms the sug- 

 gestion. Consider the Alpine cherries in their closed 

 vessel. Consider the beer in its barrel, with a single small 

 aperture open to the air, through which it is observed not 

 to imbibe oxygen, but to pour forth carbonic acid. Whence 

 come the volumes of oxygen necessary to the production 

 of this latter gas? The small quantity of atmospheric air 

 dissolved in the wort and overlying it would be totally 

 incompetent to supply the necessary oxygen. In no other 

 way can the yeast-plant obtain the gas necessary for its 

 respiration than by wrenching it from surrounding sub- 

 stances in which the oxygen exists, not free, but in a state 

 of combination. It decomposes the sugar of the solution 

 in which it grows, produces heat, breathes forth carbonic 

 acid gas, and one of the liquid products of the decomposi- 

 tion is ou r familiar alcohol. The act of fermentation, then, 

 is a result of the effort of the little plant to maintain its 

 respiration by means of combined oxygen, when its supply 

 of free oxygen is cutoff. As defined by Pasteur, fermenta- 

 tion is life ivitlwut ai^. 



But here the knowledge of that thorough investigator 

 comes to our aid to warn us against errors which have been 

 committed over ai>d over again. It is not all yeast-cells 

 that can thus live without air and provoke fermentation. 

 They must be y.nmg cells which have caught their 

 vegetative vigor from contact with free oxygen. But 

 once possessed of this vigor the yeast may be trans- 

 planted into a saccharine infusion absolutely purged of 

 air, where it wiJl continue to live at the expense of the 

 oxygen, carbon, and other constituents of the infusion. 

 Under these new conditions its life, as a plant, will be 

 by no means so vigorous as when it had a supply of free 

 oxygen, but its action as a ferment will be indefinitely 

 greater. 



Does the yeast-plant stand alone in its power of pro- 

 voking alcoholic fermentation? It would be singular if 

 amid the multitude of low vegetable forms no other could 

 be found capable of acting in a similar way. And here 

 again we have occasion to marvel at that sagacity of obser- 

 vation among the ancients to which we owe so vast a debt. 



