FERMENTATION. 47 



To isolate this substance and to prepare it in a 

 state of purity, Pasteur boiled a little yeast with from 

 fifteen to twenty times its weight of water. He then 

 carefully filtered the liquid, dissolved in it about fifty 

 grammes of sugar to the litre, and added to it some 

 chalk. Taking then, by means of a drawn-out tube, 

 from a good ordinary lactic fermentation a trace of 

 the grey matter of which we have just spoken, he 

 placed it as the seed of the ferment in the limpid 

 saccharine solution. By the next day a lively and 

 regular fermentation had set in, the liquid becoming 

 turbid and the chalk disappearing, and one could 

 distinguish a deposit which progressed continually 

 as the chalk dissolved. This deposit was the lactic 

 ferment. 



Pasteur reproduced this experiment by substituting 

 for the water of the yeast a clear decoction of nitro- 

 genous plastic substances. The ferment invariably 

 presented the same aspect and the same multiplica- 

 tion. These results, however, did not yet satisfy 

 Pasteur. He desired more rigour in a subject of such 

 theoretic importance. Might not the partisans of 

 Liebig's theory argue, if not without subtlety yet 

 with a semblance of justice, that the fermentation 

 was not due to the formation and progressive growth 

 of this feeble nitrogenous globular deposit, but rather 

 to the nitrogenous matter dissolved during the decoc- 

 tion of the yeast used in the composition of the 



