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: [Illustration] It will be seen from an inspection of the figures that the one is the image-form of the other, and, no matter how they are turned, they are not superposable; they are right- and left-handed, or, as it is termed, enantiomorphs. There is no fundamental distinction between the hypothesis of Van ’t Hoff and Le Bel as to the effect of asymmetry on optical behaviour. Le Bel regards the effect of asymmetry simply as a necessary consequence of the presence of four dissimilar groupings, and as independent of valency and the geometrical form of the molecule. [Illustration: JACOBUS HENRICUS VAN ’T HOFF.] It was surmised by Pasteur that every liquid or solid in solution showing optical activity, if crystallisable, would be found to manifest hemihedral faces; but this has not been generally established. Further, it does not always happen that an optically active substance in solution is so when solid. Lastly, optical activity may be latent even in asymmetric carbon compounds if dextro- or lævo-modifications are present in equal proportions, as in racemic acid. Such compounds are, in fact, termed “racemic,” or _racemoids_; and they may be separated occasionally by crystallisation, as in Pasteur’s method with the tartrates; or as shown by him by the action of the racemoid upon another optically active substance; or, lastly, by taking advantage of the specific action (specific assimilation) of organisms—Pasteur’s so-called biochemical method. It is a physiological fact of great interest that the behaviour of enantiomorphs towards the animal organism is frequently markedly different. Lævo-tartaric acid administered to guinea-pigs is found to be twice as poisonous as the dextro-acid; dextro-asparagine possesses a sweet taste, but lævo-asparagine is tasteless; lævo-nicotine is more poisonous than the dextro-alkaloid.