FERMENTATION. 245 



Working in an atmosphere charged with the germs 

 of these organisms, you can understand how easy it is 

 to fall into error in studying the action of any one 

 of them. Indeed it is only the most accomplished 

 experimenter, who, moreover, avails himself of every 

 means of checking his conclusions, that can walk with- 

 out tripping through this land of pitfalls. Such a man 

 the French chemist Pasteur has hitherto proved himself 

 to be. He has taught us how to separate the com- 

 mingled ferments of our air, and to study their pure 

 individual action. Guided by him, let us fix our 

 attention more particularly upon the growth and action 

 of the true yeast-plant under different conditions. Let 

 it be sown in a fermentable liquid, which is supplied 

 with plenty of pure air. The plant will flourish in 

 the aerated infusion, and produce large quantities of 

 carbonic acid gas — a compound, as you know, of carbon 

 and oxygen. The oxygen thus consumed by the plant 

 is the free oxygen of the air, which we suppose to be 

 abundantly supplied to the liquid. The action is so 

 far similar to the respiration of animals, which inspire 

 oxygen and expire carbonic acid. If we examine the 

 liquid even when the vigour of the plant has reached 

 its maximum, we hardly find in it a trace of alcohol. 

 The yeast has grown and flourished, but it has almost 

 ceased to act as a ferment. And could every individual 

 yeast-cell seize, without any impediment, free oxygen 

 from the surrounding liquid, it is certain that it would 

 cease to act as a ferment altogether. 



What, then, are the conditions under which the 

 yeast-plant must be placed so that it may display its 

 characteristic quality ? Eeflection on the facts already 

 referred to suggests a reply, and rigid experiment 

 confirms the suggestion. Consider the Alpine cherries 

 in their closed vessel. Consider the beer in its barrel, 



