538 FRAGMENTS OF SCIENCE. 



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 fermenta- 

 tion, 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 produce 

 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 possession of by foreign organisms. 

 In fact, the maladies of beer are wholly due to the ad- 

 mixture of these objectionable ferments, whose forms and 

 modes of nutrition differ materially from those of the true 

 leaven. 



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 without 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 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 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 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 surrounding 

 liquid, it is certain that it would cease to act as a ferment 

 altogether. 



