PERMENTATION. 251 



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 

 ferments 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 putrid or sour, separating like blood into clot 

 and serum. Place a drop of such milk under a powerful 

 microscope and watch it closely. You see the minute 

 butter-globules animated by that curious quivering 

 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. 

 Part of the change wrought in the milk is due to this 

 organism, which from its motions receives the name of 

 vibrio. In curdled milk you find other organisms, 

 small, motionless, and usually linked together like 

 beads on a string. It is these which cause the milk to 

 separate into clot and serum. They constitute the lactic 

 ferment of milk, as the yeast-plant does the alcoholic 

 ferment of sugar. But milk may become putrid without 

 becoming sour. Examine putrid milk microscopically, 

 and you find it swarming with shorter organisms, some- 

 times associated with the vibrios, sometimes alone, and 

 often manifesting a wonderful alacrity of motion. Keep 

 these organisms and their germs out of your milk and it 



