ALCOHOLIC FERMENTS. 133 



method as an advance, in that he wrote, " HANSEN was the first 

 to perceive that beer-yeast should be pure, and not only as 

 regards microbes and disease-ferments in the narrower sense, 

 but that it should also be free from the cells of wild yeasts." 



As, however, PASTEUR'S work always retains its technical 

 importance, on account of the force with which the influence 

 of bacteria in the fermentation industries is asserted, so it also 

 possesses great theoretical interest, especially from the new 

 theory of fermentation enunciated therein, which at the time 

 necessarily attracted much attention. 



Contrary to BREFELD, who asserted that yeast could not 

 multiply without free oxygen, and TRAUBE (1858), who indeed 

 granted that yeast was able to develop without free oxygen, 

 but maintained that it then required for its cell-formation 

 soluble albuminoids in the liquid, and that the activity of the 

 yeast-cell as an alcoholic ferment depends on chemical com- 

 bination, an enzyme being contained in the plasma, PASTEUR 

 stated that the organisms of fermentation constitute a group of 

 living beings, whose function as ferments is " a necessary con- 

 sequence of life without air, of life without free oxygen"; and, 

 further, that such a fermentation can take place in a pure 

 sugar solution. He maintains that the reason why BREFELD 

 could not get yeast to develop in a moist chamber in an atmos- 

 phere of carbon dioxide was because he was working with old 

 yeast-cells, whilst it is only possible for yeast to multiply in 

 the absence of free oxygen when the cells are very young. 

 The minute quantity of free oxygen which is present in the 

 liquid to which the yeast is added " rejuvenates the cells and 

 makes it possible for them to resume the power to bud, to 

 preserve life, and to carry on their multiplication without 

 access of air." 



Hence PASTEUR makes a distinction between two classes of 

 organisms : aerobic, those which cannot live without the 

 presence of free air; and anaerobic, those which can exist in 

 the absence of air. According to his view, these latter 

 constitute " ferments in the true sense of the word." 



It would be incorrect to assume that the presence of alcohol 

 and carbon dioxide among the products of the fermentation 

 unconditionally presupposes the influence of " organisms of 



