FERMENTATION 277 



face. Test the infusion at various stages of the plant's 

 growth, you will never find in it a trace of alcohol. But 

 forcibly submerge the little plant, push it down deep into 

 the liquid, where the quantity of free oxygen that can 

 reach it is insufficient for its needs, it immediately begins 

 to act as a ferment, supplying itself with oxygen by the 

 decomposition of the sugar, and producing alcohol as one 

 of the results of the decomposition. Many other low 

 microscopic plants act in a similar manner. In aerated 

 liquids they flourish without any production of alcohol, 

 but cut off from free oxygen they act as ferments, pro- 

 ducing alcohol exactly as the real alcoholic leaven pro- 

 duces it, only less copiously. For the right apprehension 

 of all these facts we are indebted to Pasteur. 



In the cases hitherto considered, the fermentation is 

 proved to be the invariable correlative of life, being pro- 

 duced by organisms foreign to the fermentable substance. 

 But the substance itself may also have within it, to some 

 extent, the motive power of fermentation. The yeast- 

 plant, as we have learned, is an assemblage of living cells ; 

 but so at bottom, as shown by Schleiden and Schwann, 

 are all living organisms. Cherries, apples, peaches, 

 pears, plums, and grapes, for example, are composed of 

 cells, each of which is a living unit. And here I have 

 to direct your attention to a point of extreme interest. In 

 1821, the celebrated French chemist, Berard, established 

 the important fact that all ripening fruit, exposed to the 

 free atmosphere, absorbed the oxygen of the atmosphere 

 and liberated an approximately equal volume of car- 

 bonic acid. He also found that when ripe fruits were 

 placed in a confined atmosphere, the oxygen of the at- 

 mosphere was first absorbed, and an equal volume of car- 



