84 OUTLINES OF PHYSICAL CHEMISTRY
It is, therefore, the same as the rotation for unit length multiplied by the cube root of the molecular volume. The molecular rotation evidently reduces the rotation to that of columns of liquid proportional to the volumes of the different molecules ; but, since the rotation is dependent on the thickness of substance through which the light passes, the rotation ought to be reduced to lengths corre- sponding to the diameters of the molecules, which is obviously accomplished by using the cube root of the molecular volume as in the formula for molecular deviation.]
Cause op Rotation. — Beusch has shown (1869) that it is possible to artificially construct a system which possesses optical activity, by superposing thin sheets of biaxial mica in such a way that the optical axes of the different sheets are arranged spirally. A similar spiral arrange- ment of the crystalline particles may be supposed to exist in quartz and in other optically active crystals. But it would be difficult to make the same supposition for liquids or solutions. In them the spiral structure must be the property of each molecule itself. This view is confirmed by an experiment of Biot, who showed that the vapour of an optically active liquid is itself optically active.
��Le Bel and Van't Hoff's Hypothesis
Stereo-Chemistry of Carbon
An asymmetric carbon atom is one which is united by its four valencies to four different elements or radicals. The chemical examination of optically active organic substances has shown that every one of them contains at least one asymmetric carbon atom. We may therefore find the cause of optical activity in the properties of the asymmetric carbon atom.
If we admit that the four valencies of a carbon atom are directed from the centre towards the four corners of a tetrahedron, it is easy to perceive that a substance of the
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