FERMENTATION. 265 



nay it was a striking illustration of Liebig*s power to 

 penetrate and unveil molecular actions; but it was an 

 error, and as such has proved an ignis fatuus instead 

 of a pharos to some of his followers. 



I have said that our air is full of the germs of fer- 

 ments differing from the alcoholic leaven, and some- 

 times seriously interfering with the latter. They are 

 the weeds of this microscopic garden which often over- 

 shadow and choke the flowers. Let us take an illus- 

 trative case. Expose milk to the air. It will, after 

 a time, turn sour, separating like blood into clot and 

 serum. Place a drop of this sour milk under a power- 

 ful microscope and watch it closely. You see the 

 minute butter-globules animated by that curious quiv- 

 ering motion called the Brownian motion. But let not 

 this attract your attention too much, for it is another 

 motion that we have now to seek. Here and there you 

 observe a greater disturbance than ordinary among the 

 globules; keep your eye upon the place of tumult, and 

 you will probably see emerging from it a long eel-like 

 organism, tossing the globules aside and wriggling 

 more or less rapidly across the field of the microscope. 

 Familiar with one sample of this organism, which from 

 its motions receives the name of vibrio, you soon detect 

 numbers of them. It is these organisms, and other 

 analogous though apparently motionless ones, which by 

 decomposing the milk render it sour and putrid. They 

 are the lactic and putrid ferments, as the yeast-plant 

 is the alcoholic ferment of sugar. Keep them and 

 their germs out of your milk and it will continue sweet. 

 But milk may become putrid without becoming sour. 

 Examine such putrid milk microscopically, and you 

 will find it swarming with shorter organisms, some- 

 times associated with the vibrios, sometimes alone, and 



