234 STUDIES ON FERMENTATION. 



To what may we attribute the peculiarity of the beer as just 

 described ? It is probable that during our processes of purifi- 

 cation some ferment had taken the place of the principal yeast. 

 Commercial yeasts, even those with which the brewer is 

 thoroughly satisfied, generally contain various ferments, which 

 are maintained in their relative proportions, or very nearly so, 

 by the uniform conditions under which work is carried on in a 

 brewery; but these proportions, it is obvious, might be very 

 seriously afiected by any radical change in the conditions of 

 growth. 



