STUDIES ON FERMENTATION. 219 



ferments. As regards the ordinary " high " yeast of breweries, 

 it may well be supposed, both from its power of rising to the 

 surface during fermentation and from the peculiar smell and 

 flavour of its beer, that we have in it the aerobian ferment 

 of some " low " yeast, as to the identity of which, however, 

 we can say nothing, having no knowledge as to where it is 

 to be found ; or, indeed, any certainty that such a yeast actually 

 exists. 



In writing these lines an idea suggests itself which might 

 be profitably made the subject of serious experimental study. 

 What would be the peculiar properties of the aerobious ferment- 

 form of an aerobian yeast ? Certain facts incline us to believe 

 that these forms differ from each other just as a "low" yeast 

 differs from its aerobian ferment. If this were actually the 

 case it would be very interesting to compare the peculiar pro- 

 perties of an indefinite series of aerobian ferments, all derived 

 from a common origin. We find recorded in our laboratory 

 notes that a certain aerobian ferment of the second generation 

 produced a beer different from that produced by the same 

 ferment of the first generation, being possessed of a fragrance 

 so marked that, on entering our laboratory, in which only a 

 few litres of this beer were fermenting, we were at once struck 

 by the powerful odour which it emitted. 



§ VI. — The Purification of Commercial Yeasts. 



We have already stated that the researches detailed in the 

 preceding chapter require for their successful prosecution that 

 the ferments on which we experiment should be absolutely free 

 from germs of other organisms, and we have shown how 

 impossible it would be, if this condition were not complied 

 with, to follow for weeks or months, sometimes even years, 

 the changes which occur in a yeast maintained in contact with 

 air, either in sweetened water or in a liquid which has fer- 

 mented under its influence. Equally necessary is it that the 



