STUDIES ON FERMENTATION. 37 



he no longer perceives the foreign ferment he will be inclined 

 to believe that it has vanished, and that he may regard the 

 alcoholic ferment as being free from all impurity, without testing 

 for the purity by a direct experiment. 



An example of this mistake is to be found in a recent work 

 by M. Jules Duval. This writer has published a theory accord- 

 ing to which yeast becomes transformed into lactic ferment, 

 and likewise into other ferments — that of urea, for example — 

 the only condition of change, according to M. Jules Duval, 

 being that we should cultivate it in suitable mediums. The 

 proofs by which he supports his conclusion are altogether 

 inadmissible, and a simple glance at his experiments enables 

 us to detect innumerable causes of error. M. Duval believes 

 that yeast becomes transformed into lactic ferment from 

 the fact that he obtained a fermentation which furnished 

 lactate of lime and lactic ferment from some sour milk to 

 which he had added some glucose, chalk, and phosphate 

 of ammonia before impregnating it with yeast ; but he took 

 no steps to secure himself against introducing into his medium 

 — which was, as a matter of fact, well adapted to lactic fermen- 

 tation, inasmuch as it was a little alkaline — an alcoholic 

 ferment containing impurities. This was the crucial point in 

 his experiments. M. Duval recognizes this, but he deceives 

 himself when he says, without proof: — "My alcoholic ferment 

 is pure, for I have grown it over and over again in must, pre- 

 served in flasks prepared after the manner of those which M. 

 Pasteur uses in his experiments." Here we have merely a 

 simple assertion ; a direct experimental test might have proved 

 that the yeast was impure. 



Yeast cannot transform itself into lactic ferment. No matter 

 what the medium may be in which it is sown, if it is actually 

 pure it will never present the least trace of lactic ferment or of any 

 other ferments. By certain changes in the nature of the medium, 

 the temperature, and other conditions, the cells of the ferment 

 may become oval, elongated, spherical, and larger or smaller in 

 size, but they will never produce the most minute quantity of 



