C6H12O6  =2C2H60+2CO2 . These enzymes have an extremely specific

action, producing, for instance, the change in ordinary natural glucose, but not at all in its artificial antipode, and so they are often valuable means of isolating an antipode from the inactive mixtures or racemic compounds; this method has indeed been used for the isolation of the glucose-antipode from the artificial racemic form . The fundamental fact here is due once more to Pasteur, but Emil Fischer added that sugars are acted upon by zymase in an analogous way if their configuration shows a certain amount of identity . For example yeast acts on d-Glucose d-Mannose d-Fructose HCO HCO H2COH HCOH HOCIIH CO HOH HOH HOCH HCOH HCOH HOH HCOH HCOH HCOH H2OH H2COH I-I2COH, and we observe that the three formulae agree indeed in the lower four-carbon chain . This particular behaviour led Fischer to the expression that the enzyme-action on given substances needs a corresponding feature as " lock and key." There are indications that in the synthesis by enzymes, of which examples have been realized in fats, sugars, glucosides and albuminoids, an analogous behaviour prevails . 6 . Mutual Transformation of Antipodes.—Thus far we have supposed the molecule to be stable with atoms in fixed places, as may be the case at absolute zero; in reality, at ordinary temperatures, atoms probably are endowed with movement, and this may be supposed to take place along the fixed places just mentioned as centres, which movement can go so far as to lead to total trans-formation, the one stereo-isomer changing over into the other . These cases may be considered now . As a general rule the liquid, gaseous or dissolved antipode is it itself unstable, tending to be transformed into inactive complexes Temperature may accelerate this, and, as a rule, sufficient heat will. produce the loss of optical activity, half of the original compound: having changed over into its optical antipode . This transformation has been often used for preparing the latter, as was first done by Le Bel with the optically active amyl alcohol, HC(CH3)(C2H6)(CH2OH), rendering it inactive by sufficient heating, and separating from the obtained complex the stereo-isomer . Walden found that in some cases analogous transformations take place at ordinary temperature, as for instance with d-phenylbromacetic acid, which within three years totally lost its considerable rotative power; this transformation has been termed " autoracemization." It explains that till now the most simple compounds with asymmetric carbon have not yet been obtained in antipodes; active CHCIBrF might be obtained by treating chlorobromofiuoracetic acid with potash, but autoracemization, which especially shows itself when halogens are linked to the asymmetric carbon, might, without special precautions, lead to an inactive mixture of antipodes . When two asymmetric carbons are present, four stereo-isomers are possible, which may be represented by: (i) A-I-B, (2) —(A+B), (3) A—B, (4) —(A—B), (I) and (2), as well as (3) and (4), being antipodes .

The stable form will be in this case also the inactive mixture, corresponding in the solid

state either to (I), (2) or (3), (4) . In the last case, suppose the primitive compound is (I), the first step towards stability may be the production of (3), so that practically one stereo-isomer changes over into another Ito of a different type .