FERMENTATION AND DISEASE. 135 



it sooner or later ferments ; but the chances are, that the produce of 

 that fermentation, instead of being agreeable, would be disgusting to 

 the taste. By a rare accident we might get the true alcoholic fer- 

 mentation, but the odds against obtaining it would be enormous. 

 Pure air acting upon a lifeless liquid will never provoke fermentation; 

 but our ordinary air is the vehicle of numberless germs which act as 

 ferments when they fall into appropriate infusions. Some of them pro- 

 duce acidity, some putrefaction. The germs of our yeast-plant are 

 also in the air ; but so sparingly distributed that an infusion like beer- 

 wort, exposed to the air, is almost sure to be taken j>ossession of by 

 foreign organisms. In fact, the maladies of beer are wholly due to 

 the admixture of these objectionable ferments, whose forms and modes 

 of nutrition differ materially from those of the true leaven of beer. 



Working in an atmosphere charged with the germs of these organ- 

 isms, 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 accom- 

 plished experimenter, who, moreover, avails himself of every means 

 of checking his conclusions, that can walk without tripping through 

 this land of pitfalls. Such a man is the French chemist Pasteur. He 

 has taught us how to separate the commingled 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 ferment- 

 able liquid, which is supplied with plenty of pure air. The plant 

 will nourish 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 vigor 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 sur- 

 rounding 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? Reflec- 

 tion on the facts already referred to suggests a reply, and rigid ex- 

 periment confirms the suggestion. Consider the Alpine cherries in 

 their closed vessels. Consider the beer in its barrels, 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 over- 

 lying it w r ould be totally incompetent to supply the necessary oxygen. 



