246 THE FLOATING-MATTER OF THE AIR. 



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 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 decomposes the sugar of the solution 

 in which it grows, produces heat, breathes forth car- 

 bonic acid gas, and one of the liquid products of the 

 decomposition is our familiar alcohol. The act of fer- 

 mentation, 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 cut off. 

 As defined by Pasteur, fermentation is life without 

 air. 



But here the knowledge of that thorough investi- 

 gator comes to our aid to warn us against possible error. 

 It is not, he says, all yeast-cells that can thus live without 

 air and provoke fermentation. They must be young cells 

 which have caught their vegetative vigour from contact 

 with free oxygen. But once possessed of this vigour the 

 yeast, he alleges, may be transplanted into a saccharine 

 infusion absolutely purged of air, where it will con- 

 tinue 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 ferynent will be indefinitely 

 greater. 



Does the yeast- plant stand alone in its power of 

 provoking alcoholic fermentation? It would be sin- 



