SPONTANEOUS GENERATION. 11 



Intelligent people have now become so accus- 

 tomed to the thought that every living thing comes 

 from a pre-existing germ that it is difficult for us 

 to understand that any other belief was ever held. 

 When Pasteur, however, first began his demonstra- 

 tions that were to banish the last remnant of the 

 belief in spontaneous generation from the educated 

 world he was vehemently opposed by the leading 

 scientific men of the time. His chief immediate 

 opponents were Pouchet of France, who devoted a 

 volume to the advocacy of the spontaneous genera- 

 tion of microscopic organisms, and Liebig of Ger- 

 many, the eminent chemist. Liebig held that fer- 

 mentation is a change undergone by nitrogenous 

 substances under the influence of the oxygen of the 

 air. Pasteur proved that the alcoholic yeast plant, 

 which had long been known, but which had not 

 been regarded as of particular importance, was the 

 real cause of alcoholic fermentation. He discov- 

 ered also the germ which causes the fermentation 

 which takes place in the souring of milk, and made 

 many other discoveries of a similar nature. He 

 proved that when these germs were excluded no 

 fermentation took place. 



One of his experiments was to take a tube con- 

 taining a liquid liable to fermentation, destroy by 

 heat all the living germs it might contain, then 

 admit air which had passed through a red-hot tube 



