A short time after Cagniard de la Tour discovered the yeast plant, Liebig, struck with the similarity between this and other such processes and the fermentation of sugar, put forward the hypothesis that yeast contains a substance which acts upon sugar, as synaptase acts upon amygdalin.  And as the synaptase is certainly neither organized nor alive, but a mere chemical substance, Liebig treated Cagniard de la Tour’s discovery with no small contempt, and, from that time to the present, has steadily repudiated the notion that the decomposition of the sugar is, in any sense, the result of the vital activity of the Torula.  But, though the notion that the Torula is a creature which eats sugar and excretes carbonic acid and alcohol, which is not unjustly ridiculed in the most surprising paper that ever made its appearance in a grave scientific journal,[4] may be untenable, the fact that the Toruloe are alive, and that yeast does not excite fermentation unless it contains living Toruloe, 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.

[Footnote 4:  “Das entraethselte Geheimniss der geistigen Gaehrung (Vorlaenfige briefliche Mittheilung)” is the title of an anonymous contribution to Woehler and Liebig’s Annalen der Pharmacie for 1839, in which a somewhat Rabelaisian imaginary description of the organisation of the “yeast animals” and of the manner in which their functions are performed, is given with a circumstantiality worthy of the author of Gulliver’s Travels.  As a specimen of the writer’s humour, his account of what happens when fermentation comes to an end may suffice.  “Sobald naemlich die Thiere keinen Zucker mehr vorfinden, so fressen sie sich gegenseitig selbst auf, was durch eine eigene Manipulation geschieht; alles wird verdant bis auf die Eier, welche unveraendert durch den Darmkanal hineingehen; man hat zuletzt wieder gaehrungsfaehige Hefe, naemlich den Saamen der Thiere, der uebrig bleibt.”] 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 amygdalin.  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—­