FERMENTATION. 261 



lar 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 observation among the ancients to which we 

 owe so vast a debt. Not only did they discover the 

 alcoholic ferment of yeast, but 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 mould, 

 which is nothing else than the fructification of a little 

 plant called Penicillium glaucum. Do not imagine 

 that the mould has sprung spontaneously from boot, or 

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

 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 contain; let pure air have free 

 access to the mixture; the Penicillium will grow rapid- 

 ly, striking long filaments into the liquid, and fructify- 

 ing 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, sup- 

 plying 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 flour- 

 ish 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 



