136 THE POPULAR SCIENCE MONTHLY. 



In no other way can the yeast-plant obtain the gas necessary for its 

 respiration than by wrenching it from surrounding substances in which 

 the oxygen exists, not free, but in a state of combination. It decom- 

 poses the sugar of the solution in which it grows, produces heat, 

 breathes forth carbonic-acid gas, and one of the liquid products of 

 the decomposition is our familiar alcohol. The act of fermentation, 

 then, is a result of the effort of the little plant to maintain its respira- 

 tion by means of combined oxygen, when its supply of free oxygen is 

 cut off. As defined by Pasteur, fermentation is life without air. 



But here the knowledge of that thorough investigator comes to 

 our aid to warn us against errors which have been committed over 

 and over again. It is not all veast-cells that can thus live without air 

 and provoke fermentation. They must be young cells which have 

 caught their vegetative vigor from contact with free oxygen. But, 

 once possessed of this vigor, the yeast may be transplanted into a 

 saccharine infusion absolutely purged of air, where it will continue to 

 live at the expense of the oxygen, carbon, and other constituents of 

 the infusion. Under these new conditions its lite, as a plant, will be 

 by no means so vigorous as when it had a supply of free oxygen, but 

 its action as a ferment will be indefinitely greater. 



Does the yeast-plant stand alone in its power of provoking alco- 

 holic fermentation? It would be singular 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 sa- 

 gacity 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 fructi- 

 fication 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 boiled in 

 order to kill all other s]:>ores 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 decom- 

 position of the sugar, and producing alcohol as one of the results of 

 the decomposition. Many other low microscopic plants act in a similar 



