and that yeast does not excite fermentation unless it contains living Torulae, stands fast. Moreover, of late years, the essential participation 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 between 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 decomposing 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 represented, on the one hand, by the Stahlian doctrine, supported by Liebig, according to which the atoms of the sugar are shaken into new combinations, either directly, by the Torulae, or indirectly, by some substance formed by them; and, on the other hand, by the Stahlian 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
- ↑ "Études sur les Mycodermes," Comptes Rendus, liv., 1862.
