76 LOUIS PASTEUR 



It occurred to Pasteur to see if yeast plants could 

 derive their nitrogen from simple inorganic salts 

 instead of albuminous matter of uncertain compo- 

 sition. Accordingly, he made up media consisting 

 of sugar, water, ammonium salts, phosphates, and 

 a few other inorganic ingredients to which he added 

 a minute amount of yeast. After experimenting 

 for some time he succeeded in making up solutions 

 in which yeast plants would grow and which would 

 undergo at the same time a typical alcoholic fer- 

 mentation. The yeast plants require several sub- 

 stances as food, but organic nitrogenous material 

 was proven unnecessary both for fermentation and 

 for the growth of the yeast cells. The theories of 

 Liebig and other chemists were dealt a severe blow. 

 Brought to the test of experiment they were found 

 wanting. On the other hand, it became more ap- 

 parent that it was owing to the growth and activity 

 of microscopic organisms that fermentations were 

 brought about. 1 



1 Although Liebig was in error in attributing ordinary 

 fermentation to decomposing nitrogenous substances, there is 

 an element of truth in his chemical theory. It has long been 

 known that enzymes of organic origin produce fermentation. 

 Micro-organisms may effect the decomposition of surround- 

 ing substances by giving rise to enzymes. In fact an enzyme 

 has been extracted from ordinary yeast that has the property 

 of converting sugar into alcohol and carbon dioxide. Fer- 



