208 LECTURE XI. 



others take place independently of it. A familiar example 

 of the former kind of the oxidative decompositions, as we 

 may term them is afforded by the formation of acetic acid 

 from ethyl-alcohol under the influence of the Fungus known 

 as Mycoderma Aceti, a plant which is allied to Yeast. The 

 process may be roughly represented by the equation 

 C 2 H G O + O 2 =C 2 H 4 O 2 + H 2 O. 



Another similar Fungus, the Mycoderma Vim, induces 

 more complete oxidation and decomposition of alcohol, carbon 

 dioxide and water being the products of its action. It is 

 probable that processes of this kind go on to a greater or 

 less extent in all living cells; 



Of the fermentative processes which can go on in the 

 absence of oxygen the most familiar is the decomposition 

 of sugar into alcohol and carbon dioxide which is effected 

 by the Yeast, and which is known as the alcoholic fermen- 

 tation. It may be roughly represented by the equation 



C 6 H 12 6 =2C 2 H 6 0+2CO* 



It is seen from the equation that the process does not 

 involve the presence of oxygen, and, as a matter of fact, 

 Pasteur, Hoppe-Seyler, and Brefeld have shewn that when 

 the supply of oxygen is abundant the alcoholic fermentation 

 is reduced to a minimum. 



In a series of experiments instituted with the view of determining this 

 point Pasteur found that whereas, in the absence of oxygen, the propor- 

 tion between the weight of Yeast formed in the fermenting liquid to the 

 weight of sugar decomposed was as I : 176, in the presence of oxygen the 

 proportion in an especially successful experiment was as I : 4. P'rom 

 this he concludes that in the presence of oxygen the fermentative action 

 of Yeast is diminished. 



Although we usually associate the alcoholic fermentation 

 with the Yeast-plant, yet we must not conclude that it is peculiar 

 to it. Pasteur and Brefeld have observed that it is excited, 

 in the absence of oxygen, by various other Fungi (species of 

 Mucor, Penicillium, Mycoderma) growing in saccharine solu- 

 tions ; whereas, when free oxygen is present, no alcoholic 

 fermentation occurs. 



