280 FRAGMENTS OF SCIENCE 



ular motion, and the changes produced by the interac 

 tions of purely chemical forces, nothing could be mor^ 

 natural than to see in the process of fermentation a simple 

 illustration of molecular instability, the ferment propagat- 

 ing to surrounding molecular groups the overthrow of its 

 own tottering combinations. Broadly considered, indeed, 

 there is a certain amount of truth in this theory; but 

 Liebig, who propounded it, missed the very kernel of the 

 phenomena when be overlooked or contemned the part 

 played in fermentation by microscopic life. He looked 

 at the matter too little with the eye of the body, and too 

 much with the spiritual eye. He practically neglected the 

 microscope, and was unmoved by the knowledge which 

 its revelations would have poured in upon his mind. His 

 hypothesis, as I have said, was natural 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 sometimes 

 seriously interfering with the latter. They are the weeds 

 of this microscopic garden which often overshadow and 

 choke the flowers. Let us take an illustrative case. Ex- 

 pose milk to the air. It will, after a time, turn sour, sepa- 

 rating 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 



