540 FRAGMENTS OF SCIENCE. 



Not only did they discover the alcoholic ferment of yeast, 

 hut they had to exercise a wise selection in picking it out 

 from others, and giving it special prominence. Place an 

 old boot in a moist place, or expose common paste or a pot 

 of jam to the air; it soon becomes coated with a blue-green 

 mold, which is nothing else than the fructification of a 

 little plant called Penicillium glaucum. Do not imagine 

 that the mold has sprung spontaneously from boot, or 

 paste, or jam; its germs, which are abundant in tho 

 air, have been sown, and have germinated, in as legal and 

 legitimate a way as thistle-seeds wafted by the wind to a 

 proper soil. Let the minute spores of Penicillium be sown 

 in a fermentable liquid, which has been previously so 

 boiled as to kill all other spores or seeds which it may con- 

 tain; let pure air have free access to the mixture; the 

 Penicillium will grow rapidly, striking long filaments into 

 the liquid, and fructifying at its surface. 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, producing alcohol exactly as the real 

 alcoholic leaven produces 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 



