274 FRAGMENTS OF SCIENCE 



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 possession 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. 

 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. In- 

 deed 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 oxy- 

 gen. 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 



