At first sight this fact appears decisive against the validity of the lactic acid theory, and it is recognised as such by Buchner and Meisenheimer. Wohl has, however, suggested that the non-fermentability of lactic acid by yeast is not really conclusive [1907, 1; see also Franzen and Steppuhn, 1912, 1]. The production of lactic acid from glucose is attended by the evolution of a considerable amount of heat (22 cal.), and it is possible that at the moment of production the molecule of the acid is in a condition of activity corresponding with a much higher temperature than the average temperature of the fermenting liquid. Under these circumstances the molecule would be much more susceptible of chemical change than at a later period when temperature equilibrium had been attained. It has, however, been pointed out by Tafel [1907], that such a decomposition of the lactic acid would occur at the very instant of formation of the molecule, so that no ground remains even on this view for assuming the actual existence of lactic acid as a definite intermediate product. It has also been suggested by Luther [1907] that an unknown isomeride of lactic acid is formed as an intermediate product and fermented, and that traces of lactic [p104] acid are formed by a secondary reaction from this, but no satisfactory evidence for this view is forthcoming. There still remains a doubt as to whether the living yeast-cell is permeable to lactic acid, a fact which would of course afford a very simple explanation of the non-fermentability of the acid. Apart from this, however, it is difficult, in face of the evidence just quoted, to believe that lactic acid is in reality an intermediate product in alcoholic fermentation. METHYLGLYOXAL, DIHYDROXYACETONE AND GLYCERALDEHYDE. As regards the fermentability by yeast of compounds containing three carbon atoms, which may possibly appear as intermediate products in the transformation of sugar into carbon dioxide and alcohol, many experiments have been carried out, with somewhat uncertain results. Care has to be taken that the substance to be tested is not added in such quantity as to inhibit the fermenting power of the yeast or yeast-juice, and further that the conditions are such that the substance in question, often of a very unstable nature, is not converted by some chemical change into a different fermentable compound. It is also possible that the substance to be tested may accelerate the rate of autofermentation in a similar manner to arsenates (pp. 80, 126) and many other substances. These are all points which have not up to the present received sufficient attention. In the case of living yeast the further question arises of the permeability of the cell.