78 FERMENTS AND MICRO-PARASITES. 



the form of ashes of yeast) and sugar form the only 

 necessary ingredients for a cultivating fluid for yeast, 

 and in solutions of this simple composition fermentation 

 goes on well. These experiments were completely con- 

 firmed by Cohn, Duclaux, &c., and they render it quite 

 impossible to ascribe to the albuminous materials in the 

 fermenting fluids such an important role in the fermen- 

 tative process as Liebig, for example, has done. (See 

 below.) 



The following observations on its relation to oxygen 

 were also very important for the knowledge of the tissue 

 change of the yeast. Pasteur found that the fermentative 

 organisms during their growth took up a large quantity 

 of oxygen and gave off carbonic acid ; this fact was con- 

 firmed by Schiitzenberger, who further ascertained that 

 the more oxygen was used the more active was the 

 vegetation of the yeast cells. Several other investiga- 

 tions led to similar results (Traube, Brefeld), and 

 thereby the biological behaviour of the fermentative 

 organisms seemed to have been made clearer and the 

 direct dependence of fermentation on the nutritive 

 processes of yeast rendered more certain. 



But further investigations by Pasteur led to very 

 different results. He found that if the access of air 

 were hindered alcohol was formed in large amount, 

 while if oxygen was admitted but little sugar was broken 

 up. Pasteur made the same observation in the case of 

 other fermentative processes, for example in butyric 

 fermentation and in putrefaction ; active fermentation 

 only occurred where the amount of oxygen was deficient ; 

 the admission of oxygen seemed in fact to be detri- 

 mental to the fermentative processes, although growth 

 and multiplication of the yeast cells could take place. 

 Certain exciting agents of fermentation and putrefaction 

 did not however seem to be able to exist without free 

 oxygen ; Pasteur distinguished these organisms as 

 aerobes from the anaerobes which are killed by free 

 oxygen, or at any rate are active only in the absence of 

 oxygen. 



Deficiency of oxygen seemed to Pustear to be the 



