The discovery of optical rotation in vapours was made in 1818 by Biot, who detected the existence of optical rotatory power and of a normal type of rotatory dispersion in a 30-metre column of turpentine-vapour prior to the conflagration which destroyed his apparatus. A quantitative study of the specific rotatory powers of liquids and vapours was made in 1864 by Gernez, who compared the rotations produced by 4-metres of the vapours of the essential oils of orange, Seville orange, and turpentine with those produced in the liquids. These rotations were substantially equal in the case of turpentine, where the specific rotatory power of the liquid was almost independent of temperature, although a small progressive decrease was observed both on heating and on vaporisation. In the other two cases, however, a rapid decrease of specific rotatory power with rise of temperature in the liquid was followed by a slighly more rapid decrease on passing from liquid to vapour, so that the specific rotatory powers in the vapour were definitely smaller than in the liquid state. During the half century which has elapsed since Gernez worked on the subject, no further measurements appear to have been made of the rotatory powers of vapours. It is obvious, however, that the difficulty of predicting optical rotations would be substantially reduced if the disturbing influence of contiguous molecules could be eliminated by observing the rotations in a dilute vapour instead of in solution, and that progress in the theoretical study of this problem may therefore depend fundamentally on experimental work of this kind. The primary object of the experiments now described was therefore to develop a method for measuring the optical rotatory powers of vapours, and to apply it to the typical cases of camphor and of camphorquinone. The principal interest of the work, however, depends on the fact that observations of rotatory dispersion were made in the region of absorption, covered by the well-known ketonic and quinonoid bands, as well as in the range of wavelengths within which these compounds are completely transparent. The combination of these two lines of investigation is indeed a perfectly logical procedure, since the extreme dilution, which is often required in order to penetrate the region of absorption, is provided very readily by a vapour, the concentration of which is already limited by its vapour-pressure, and can be reduced to an indefinite extent by further reductions of pressure.
Optical rotatory power. I—A theoretical calculation for a molecule containing only isotropic refractive centres
