FERMENTATION. 263 



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 Peni- 

 cillium 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 

 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, 



