xlix 



the optically active compounds known at the time was the amyl 

 alcohol, which is obtained as a by-product in a number of fermenta- 

 tions. In 1857 he brought before the scientific world the result of 

 his researches on the lactic fermentation, the first of that series of 

 masterly investigations which he was to pursue during the next 

 twenty years. In 1860 this was followed by a paper on the alcoholic 

 fermentation. 



In the lactic fermentation, Pasteur noticed that a greyish solid 

 material was deposited, and that the quantity of this increased during 

 the process. On examining some of this substance under the micro- 

 scope, he found that it consisted of very minute corpuscles, quite 

 different from the yeast cells observed in the alcoholic fermentation, 

 but which he felt convinced must be of analogous nature. Taking a 

 trace of this grey material, he introduced it into an artificial solu- 

 tion of sugar, to which he had added some decoction of yeast and 

 chalk, and soon he had the intense satisfaction of witnessing the lactic 

 fermentation in full activity in this liquid. From this fermenting 

 liquid he transferred again a minute trace into another similar solu- 

 tion of sugar, and so on, invariably obtaining the same fermentation, 

 invariably finding .also the same corpuscles in the deposit. 



In order to meet the objection which he conceived might be raised 

 by Liebig and his supporters that the fermentative change in the 

 sugar was due to the decomposition of the albuminoids present in the 

 decoction of yeast employed, Pasteur replaced the albuminoids in his 

 fermentations by ammonium salts. In these solutions of pure sugar, 

 with nothing but mineral additions, he demonstrated that the yeast 

 grew and multiplied, and that its growth was accompanied by the 

 conversion of the sugar into alcohol and carbonic anhydride, whilst 

 similarly those totally distinct living corpuscles, to which he gave the 

 name of levure lactique, proliferated in solutions of the same com- 

 position, and their multiplication was accompanied by the trans- 

 formation of the sugar into lactic acid. 



The amount of new experimental material collected by Pasteur in 

 support of this vitalistic as opposed to the time-honoured chemical 

 theory of fermentation is enormous, whilst his extraordinary power 

 of seeing what others had failed to observe before him is again 

 exemplified in his discovery of succinic acid and glycerine as invari- 

 able products of the alcoholic fermentation of sugar. 



These researches, besides being of fundamental importance in 

 throwing light upon one of the oldest, but hitherto obscurest, depart- 

 ments of scientific investigation, opened up an entirely new field of 

 work, for with the inauguration by Pasteur of artificial culture solu- 

 tions, that path was first indicated which has gradually expanded 

 into the fascinating science of Bacteriology. 



Whilst busily immersed in his researches on fermentation at Lille, 



