72 LOUIS PASTEUR 



made up a medium composed of a solution of sugar 

 to which was added a small amount of chalk and 

 the boiled and filtered extract of the yeast of beer 

 to furnish the albuminous material needed for car- 

 rying on fermentation. In this mixture he placed 

 some of the gray material composed mostly of the 

 small organisms just mentioned, and set the fluid in 

 a warm place. The next day revealed an active 

 fermentation. The liquid originally clear was now 

 turbid or clouded. Bubbles of gas, which proved 

 to be carbon dioxide together with variable amounts 

 of hydrogen, rose to the surface and escaped. The 

 chalk, which had settled on the bottom, disap- 

 peared. After fermentation had ceased, the fluid, 

 when evaporated down, gave a residue chiefly of 

 lactate of lime. The sugar had been transformed 

 into carbon dioxide and lactic acid, and the latter 

 had combined with the chalk (which is carbonate 

 of lime) and crystallized out. Here was revealed 

 a process very similar throughout to alcoholic fer- 

 mentation; with the same materials which would 

 form alcohol upon the addition of ordinary yeast; 

 we obtain quite different products solely by the 

 substitution of a different ferment. 



Lactic acid fermentation, like alcoholic fermen- 

 tation, was found by Pasteur to arise in suitable 



