LE BEL AND VAN'T HOFF'S HYPOTHESIS
��formula Gabcd corresponds to two non-superposable figures (in space) one of which is the mirror-imago of the other.
In fig. 18, d is supposed above the plane of the paper at the summit of a pyramid. If we wish to trace a path round the base of this pyramid in the direction a — b — c, we must follow the same direction as the hands of a watch for the figure to the left, and the opposite direction for the figure to the right. Each of these figures is the mirror image of the other, and this enantiomorphism is found also in the optical and crystallographic properties of substances of the type Gabcd.
For the sake of simplicity, we have taken the tetra- hedron as regular, but, as the four valencies are saturated by elements or radicals of different weight and volume, it is probable that their centres of gravity are not equidistant from the central carbon atom, and that the molecule should be represented by an irregular pyramid.
The simplification of the figures, however, does not in any way affect our interpretation of the optical activity.
Let us assume (as in fig. 19) the active carbon atom at the centre of a regular tetrahedron with four monovalent elements or radicals at the four corners ; and let us further suppose that the importance (weight and volume) of these elements or radicals decreases in the order 4, 3, 2, 1. We can then construct two non-superposable modifications.
Consider first the plane 1 — 2 — 4 of fig. 19, a.
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