Mindful of Herschel’s observation, Pasteur found that the crystals of certain of the optically active tartrates showed hemihedral faces, whereas those of the corresponding racemates showed no trace of them. On recrystallising the racemates, however, it was noticed that two sets of crystals were formed—enantiomorphic forms—the first set of crystals having hemihedral forms on the right-hand side, and the second set on the left-hand side. The forms, in fact, were so related that one appeared, as if it were the image, as seen in a mirror, of the other. When solutions of these crystals were examined, one set was found to rotate to the right, the other to an equal degree to the left. The dextro-rotatory salt yielded ordinary tartaric acid; the corresponding lævo-rotatory acid was a hitherto unknown modification: the two together, in equal proportions, constituted racemic acid.
In 1863 Wislicenus published a remarkable memoir on the synthesis of lactic acid. The acid in sour milk was discovered by Scheele in 1780. In 1807 Berzelius discovered a similar acid, called _sarcolactic acid_, in muscle juice; this was erroneously pronounced by Liebig to be identical with that of sour milk. Other forms of lactic acid were made known, the structural character of which was not to be explained by current hypotheses. Wislicenus concluded that their differences could be due only to different arrangements of their atoms in space.
In 1874 the conception of atomic grouping received a remarkable development by the publication of two memoirs—one by Van ’t Hoff, and the other by Le Bel—which served to connect molecular structure with optical activity. Confining their attention to carbon compounds, they inferred that all optically active substances contained at least one multivalent atom, united to other atoms or groups, so as to form in space an unsymmetrical arrangement. Van ’t Hoff regarded the carbon atom as occupying the centre of a tetrahedron, to the summits of which its valencies were directed. If different groupings are attached to these summits, the structure is _asymmetrical_, and is optically active. The two forms of lactic acid, for example, may be represented by the following space formulæ: