124 BACTERIA. 



this condition yeast approaches the state in which it can carry on its vital 

 process after the manner of an ordinary fungus ; the mode of life, /.*-., in 

 which the ratio between the weight of sugar decomposed, and the weight of 

 the new cells produced, will be the same as holds generally among organ- 

 isms which are not ferments. In short, varying the form of expression a 

 little, it may be concluded, from the sum total of observed facts, that the yeast 

 which lives in the presence of oxygen, and can assimilate as much of that 

 gas as is necessary for its perfect nutrition, ceases to be a ferment at all. 

 Nevertheless, yeast formed under these conditions, and subsequently brought 

 into the presence of sugar, away from the influence of air, would decom- 

 pose more in a given time than in any other series of conditions under 

 which it could be placed. The reason is, that yeast which has formed in 

 contact with air, having the maximum of free oxygen that it can assimilate, 

 is fresher and possessed of greater vitality than that which has been formed 

 without air or with an insufficiency of air." 



In other words, when the ordinary respiratory power which 

 it has in common with the fungi is reduced to its lowest 

 point, after the protoplasm of the cell has been thoroughly 

 rejuvenated, its fermentive power in the absence of oxygen 

 reaches its maximum. 



It is thus evident that the processes which go on in fermen- 

 tation are similar in kind to those chemical metamorphoses 

 that are constantly being brought about in the animal and 

 vegetable organisms ; the only difference being, first, in the 

 nature of the material that is broken up ; second, in the 

 nature of the cells that bring about the metamorphoses ; and 

 third, in the exact nature of the ultimate products of the 

 various processes ; the difference in all cases being not so 

 much in kind as in degree. As to the exact nature of the 

 process, there is much difference of opinion, some authorities 

 holding that it is necessary for the whole of the sugar which 

 is altered by the living cell of beer-yeast to penetrate or 

 pass, by a process of endosmosis, through the membranous 

 envelope of the cell and become an integral part of the 

 cell protoplasm, and that, unless this takes place, the 

 resolution of sugar into alcohol, glycerine, carbonic acid 

 gas, succinic acid, &c., cannot take place. It is necessary, 

 in fact, that the whole of the sugar should be, as it 

 were, digested by the yeast-cells, and combined in great 

 part into protoplasm before it can be converted into the 

 various substances above mentioned, which are, on this 

 assumption, merely the excretory products of the vegetable 

 cells feeding on a definite kind of nutrient material. We 

 have seen, however, that the cells of yeast secrete a definite 



