192 STUDIES ON FERMENTATION. 



at the bottom of the vessels. When " high " fermentation 

 takes place in vessels that are not filled, the ferment forms a 

 thick layer, a kind of cap on the surface of the beer. This 

 characteristic may be witnessed even in the fermentation of 

 very small quantities of liquid. In our flasks, in which the 

 volume of fermenting wort does not exceed 100 c.c. or 150 c.c. 

 (about 4 or 5 fluid ounces), we may perceive, as the violence 

 of fermentation subsides, and the head falls, the sides of the 

 vessel covered to a height of from 1 cm. to 2 cm. (about 

 |-in.) above the surface of the liquid, with particles of yeasty 

 matter, in little masses, or in a thin film, raised to that height 

 by the head, and left behind when that fell. 



"Low" Ferment. — Whilst high yeast performs its func- 

 tions in the breweries in which it is used at somewhat high 

 temperatures — namely, between 16° C. and 20° C. (60° F. to 

 68° F.) — " low " yeast is never employed at a higher tem- 

 perature than 10° C. (50° F.), and it is even thought preferable 

 that it should not be subjected to more than 6° C, 7° C, or 

 8° C. (43° F. to 46° F.). At these comparatively low tem- 

 peratures "high" yeast would have no perceptible action, 

 whereas it is at such temperatures that " low " yeast best 

 performs its functions. 



In our Memoir on alcoholic fermentation, published in 1860, 

 in the Annales de Chiviie et de Physique, the idea of the identity 

 of the two yeasts was accepted ; but we had at that time 

 made no special observations of our own on the subject. 



Upon closer investigation we are inclined to believe that 

 the two yeasts are quite distinct. We might keep our " high " 

 yeast at the lowest temperatures that it can bear, and repeat 

 our growths under these conditions ; or, on the other hand, 

 we might subject the "low" yeast to temperatures higher 

 than those at which it ordinarily grows, without ever suc- 

 ceeding in changing the first into the second or the second 

 into the first, supposing, of course, that each of our yeasts 

 was pure to begin with. If they were intermixed the change 

 in the conditions of development would cause one or the 



