538 VRA GMENTS 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. Jf 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. 
