YEAST. 581 



and that yeast does not excite fermentation unless it contains living 

 Torulce, stands fast. Moreover, of late years, the essential participa- 

 tion of living organisms in fermentation other than the alcoholic, has 

 been clearly made out by Pasteur and other chemists. 



However, it may be asked, Is there any necessary opposition be- 

 tween the so-called " vital " and the strictly physico-chemical views of 

 fermentation ? It is quite possible that the living Torula may excite 

 fermentation in sugar, because it constantly produces, as an essential 

 part of its vital manifestations, some substance which acts upon the 

 sugar, just as the synaptase acts upon the amygdaline. Or it may be 

 that, without the formation of any such special substance, the physical 

 condition of the living tissue of the yeast-plant is sufficient to effect 

 that small disturbance of the equilibrium of the particles of the sugar 

 which Lavoisier thought sufficient to effect its decomposition. 



Platinum in a very fine state of division known as platinum black, 

 or noir de platine has the very singular property of causing alcohol 

 to change into acetic acid with great rapidity. The vinegar-plant, 

 which is closely allied to the yeast-plant, has a similar effect upon 

 dilute alcohol, causing it to absorb the oxygen of the air, and become 

 converted into vinegar ; and Liebig's eminent opponent, Pasteur, who 

 has done so much for the theory and the practice of vinegar-making, 

 himself suggests that, in this case 



The cause of the physical phenomenon which accompanies the plant's life 

 is to be attributed to a peculiar physical state, analogous to that of platinum 

 black. It must, however, be observed that this physical state of the plant is 

 closely connected with the plant's life. 1 



Now, if the vinegar-plant gives rise to the oxidation of alcohol, on 

 account of its merely physical constitution, it is, at any rate, possible 

 that the physical constitution of the yeast-plant may exert a decom- 

 posing influence on sugar. 



But, without presuming to discuss a question which leads us into 

 the very arcana of chemistry, the present state of speculation upon the 

 modus operandi of the yeast-plant in producing fermentation is repre- 

 sented, on the one hand, by the Stahlian doctrine, supported by Liebig, 

 according to which the atoms of the sugar are shaken into new combi- 

 nations, either directly, by the Torulce, or indirectly, by some substance 

 formed by them ; and, on the other hand, by the Thenardian doctrine, 

 supported by Pasteur, according to which the yeast-plant assimilates 

 part of the sugar, and, in so doing, disturbs the rest, and determines 

 its resolution into the products of fermentation. Perhaps the two 

 views are not so much opposed as they seem at first sight to be. 



But the interest which attaches to the influence of the yeast-plants 

 upon the medium in which they live and grow does not arise solely 

 from its bearing upon the theory of fermentation. So long ago as 



1 " Etudes sur les Mycoderraes," Comptes Rendus, liv., 1862. 



