350 STUDIES OK FERMENTATION. 



transparent aspect, and the vacuoles disappear. The molecular 

 granulations, too, are less apparent. At a certain focus they 

 disappear ; at another they reappear, not as black spots, how- 

 ever, but as brilliant points so small as to be scarcely 

 perceptible. If germination has been suspended it is resumed ; 

 in short, everything tends to prove — and having the 3-east 

 actually under our eyes we cannot doubt the fact — that the life 

 of the cells is more decided, and the work of nutrition more 

 active after they have been brought into contact with the oxygen 

 of the air, and have absorbed a greater or less quantity of that 

 gas. 



Under the ordinary conditions of brewing, the atmospheric 

 air is present in very varying quantities, whether introduced 

 by the wort which holds more or less in solution, or b}^ diffusion 

 over the surface of the vessels, so that the same cells of yeast 

 live by turns without air and with air. At first they absorb 

 all the oxygen held in solution, and multiply under the 

 influence of this absorption. Afterwards, when the supply has 

 been exhausted, and various assimilations have resulted from it, 

 they are deprived of it. Their life continues apart from 

 oxygen, and if the vessel were closed, fermentation would be 

 accomplished under these conditions, although more slowly. 

 The vessel being open, a small quantity of air diflfuses continu- 

 ously through the layer of carbonic acid gas on the surface, 

 and supports the vitality of the cells. 



It is interesting to observe that, in the working of breweries 

 there are several empirical practices the explanation of which 

 is to be found wholly in the fact that the aeration of wort or 

 beer exercises a great influence on fermentation. In many 

 breweries we have seen the pitching performed in the following 

 manner : the brewer, having mixed his yeast in many times its 

 volume of wort, pours all the thick liquid from a height from 

 one bucket into another, and from that back again into the first, 

 and so on a great many times, until the two buckets are filled 

 with the froth enclosing air. In certain London breweries we 

 have seen a bucket suspended by a pulley over the fermenting 



