(/a/) A small portion of this is converted by a reaction which may be variously interpreted as a Cannizzaro transformation or a reductase reaction into glycerol and pyruvic acid. CH{2}:C(OH)·CHO + H{2}O H{2} CH{2}(OH)·CH(OH)·CH{2}(OH) │ glycerol + │ = + CH{2}:C(OH)·CHO O CH{2}:C(OH)·COOH Pyruvic acid (/b/) The pyruvic acid is then decomposed by carboxylase yielding aldehyde and carbon dioxide (equation 2, p. 109). [p114] (/c/) The aldehyde and a molecule of glyoxal then undergo a Cannizzaro reaction and yield alcohol and pyruvic acid, CH{3}·CO·CHO O CH{3}·CO·COOH + │ = + CH{3}·CHO H{2} CH{3}·CH{2}(OH) and the latter then undergoes reaction (/b/). A small amount of glycerol is thus necessarily formed, as is actually found to be the case. The experimental foundation for stages (/a/) and (/c/) will be awaited with great interest, as well as the proof that methylglyoxal is readily fermentable (see p. 104). THE FORMIC ACID THEORY. An interesting interpretation of the phenomena of fermentation was attempted by Schade [1906] based upon the conception that glucose under the influence of catalytic agents readily decomposes into acetaldehyde and formic acid. It was subsequently found that the experimental evidence upon which this conclusion was founded had been wrongly interpreted [Buchner, Meisenheimer, and Schade, 1906; Schade, 1907], but Schade has succeeded in devising an interesting series of reactions by means of which alcohol and carbon dioxide can be obtained from sugar by the successive action of various catalysts. The following are the stages of this series: (1) Glucose, fructose, and mannose are converted by alkalis into lactic acid along with other products. (2) Lactic acid when heated with dilute sulphuric acid yields a mixture of acetaldehyde and formic acid:-- CH{3}·CH(OH)·COOH = CH{3}·CHO + H·COOH. (3) It has long been known that formic acid is catalysed by metallic rhodium at the ordinary temperature into hydrogen and carbon dioxide, and Schade has found that when a mixture of acetaldehyde and formic acid is submitted to the action of rhodium the acetaldehyde is reduced to alcohol at the expense of the hydrogen and the carbon dioxide is evolved:-- CH{3}·CHO + H·COOH = CH{3}·CH{2}(OH) + CO{2}. Schade suggests [1908] that the fermentation of sugar may proceed by a similar series of reactions catalysed by enzymes, the acetaldehyde and formic acid being derived not from the relatively stable lactic acid but more probably from a labile substance capable of undergoing change either into lactic acid or into aldehyde and formic acid.