FERMENTATION. 539 
What, then, are the conditions under which the yeast- 
plant must be placed so that it may display its character- 
istic quality? Reflection on the facts already referred to 
suggests a reply, and rigid experiment confirms the sug- 
gestion. Consider the Alpine cherries in their closed 
vessel. Consider the beer in its barrel, with a single small 
aperture open to the air, through which it is observed not 
to imbibe oxygen, but to pour forth carbonic acid. Whence 
come the volumes of oxygen necessary to the production 
of this latter gas? The small quantity of atmospheric air 
dissolved in the wort and overlying it would be totally 
incompetent to supply the necessary oxygen. In no other 
Avay can the yeast-plant obtain the gas necessary for its 
respiration than by wrenching it from surrounding sub- 
stances in which the oxygen exists, not free, but in a state 
of combination. It decomposes the sugar of the solution 
in which it grows, produces heat, breathes forth carbonic 
acid gas, and one of the liquid products of the decomposi- 
tion is on r familiar alcohol. The act of fermentation, then, 
is a result of the effort of the little plant to maintain its 
respiration by means of combined oxygen, when its supply 
of free oxygen is cutoff. As defined by Pasteur, fermenta- 
tion 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 yeast-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 trans- 
planted 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 life, 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 pro- 
voking alcoholic fermentation? It would be singular if 
amid the multitude of low vegetable forms no other could 
be found capable of ac.ting in a similar way. And here 
again we have occasion to marvel at that sagacity of obser- 
vation among the ancients to which we owe so vast a debt. 
