70 LOUIS PASTEUR. 



it is a plant, and of all plants one of the simplest and 

 most minute, which has been known from time imme- 

 morial under the name of flower of vinegar. This 

 little fungus is invariably present on the surface of 

 a wine which is being transformed into vinegar. 

 Liebig was not ignorant of this, but he regarded it as 

 a simple coincidence. Do we not know, said he, that 

 whenever an infusion of organic matter is exposed to 

 the air it becomes covered with a cryptogamic vege- 

 tation, or is invaded by a crowd of animalcule ? Is 

 not vinegar a vegetable infusion ? Vinegar affords a 

 refuge to the flower of vinegar, just as it gives refuge 

 to what are called the little eels of vinegar. 



We can appreciate here the uncertainties of pure 

 observation. The great art and no one practised it 

 better than Pasteur consists in instituting decisive 

 experiments which leave no room for an inexact inter- 

 pretation of facts. These decisive proofs of the true 

 part played by the little microscopic fungus, by this 

 flower of vinegar, this mycoderma aceti, are thus 

 formulated by Pasteur. It is but another example 

 of the method which he used in alcoholic, lactic, and 

 tartaric fermentations. The theories of Berzelius, of 

 Mitscherlich, and of Liebig were destined again to 

 receive the rudest shocks by the demonstration of these 

 rigorous facts. 



Let us place a little wine in a bottle, then her- 

 metically seal it, and leave it to itself. In these 



