248 THE FLOATING-MATTEE OF THE AIR. 



only less copiously. For the discovery and apprehension 

 of these facts we are indebted to Pasteur. 



In the cases hitherto considered, the fermentation 

 is proved to be the invariable correlative of life, hemg 

 produced by organisms foreign to the fermentable sub- 

 stance. 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 fruits, exposed to the free atmosphere, absorb 

 the oxygen of the atmosphere and liberate an approxi- 

 mately equal volume of carbonic acid. He also found 

 that when ripe fruits were placed in a confined at- 

 mosphere, the oxygen of the atmosphere was first 

 absorbed, and an equal volume of carbonic acid given 

 out. But the process did not end here. After the 

 oxygen had vanished, carbonic acid, in considerable 

 quantities, continued to be exhaled by the fruits, which 

 at the same time lost a portion of their sugar, becoming 

 more acid to the taste, though the absolute quantity of 

 acid was not augmented. This was an observation of 

 capital importance, and Berard had the sagacity to 

 remark that the process might be regarded as a kind of 

 fermentation. 



Thus the living cells of fruits can absorb oxygen and 

 breathe out carbonic acid, exactly like the living cells 

 of the leaven of beer. Supposing the access of oxygen 

 suddenly cut off, will the living fruit-cells as suddenly 

 die, or will they continue to live as yeast lives, by 



