138 THE POPULAR SCIENCE MONTHLY, 



be what he had foreseen. He then extended the inquiry. Placing 

 under a bell-jar twenty-four plums, he filled the jar with carbonic- 

 acid gas ; beside it he placed twenty-four similar plums uncovered. 

 At the end of eight days he removed the plums from the jar, and 

 compared them with the others. The difference was extraordinary. 

 The uncovered fruits had become soft, watery, -and very sweet; the 

 others were firm and hard, their fleshy portions being not at all 

 watery. They had, moreover, lost a considerable quantity of their 

 sugar. They were afterward bruised, and the juice distilled. It 

 yielded six and a half grammes of alcohol, or one per cent, of the 

 total weight of the plums. Neither in these plums, nor in the grapes 

 first experimented on by Pasteur, could any trace of the ordinary 

 alcoholic leaven be found. The fermentation was the work of the 

 living cells of the fruit itself, after air had been denied to them. 

 When, moreover, the cells were destroyed by bruising, no fermenta- 

 tion ensued. The fermentation was the correlative of a vital act, 

 and it ceased when life was extinguished. 



Ltidersdorf was the first to show by this method that yeast acted, 

 not, as Liebig had assumed, in virtue of its organic, but in virtue of 

 its organized, character. He destroyed the cells of yeast by rubbing 

 them on a ground-glass plate, and found that with the destruction 

 of the organism, though its chemical constituents remained, the 

 power to act as a ferment totally disappeared. 



One word more in reference to Liebig may find a place here. To 

 the philosophic chemist thoughtfully pondering these phenomena, 

 familiar with the conception of molecular motion, and the changes 

 produced by the interactions of purely chemical forces, nothing 

 could be more natural than to see in the process of fermentation a 

 simple illustration of molecular instability, the ferment propagating to 

 surrounding molecular groups the overthrow of its own tottering com- 

 binations. Broadly considered, indeed, there is a certain amount of 

 truth in this theory ; but Liebig, Avho propounded it, missed the very 

 kernel of the phenomena when he 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 mo- 

 lecular actions ; but it w r as an error, and as such has proved an ignis 

 fatuus instead of a ^>/iros to some of his followers. 



I have said that our air is full of the germs of ferments 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. Expose boiled milk to the air. It will cool, and then turn 



