FERMENTATION. 267 



proved an ignis fatuus instead of a pharos to some 

 >f 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 sour, separating like blood into clot and 

 serum. Place a drop of this sour 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. 

 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 find it 

 swarming with shorter organisms, sometimes 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 will never 



