In plates of Rock-Crystal cut perpendicular to the axis of the prism, an unusual kind of polarisation had been observed in the colours seen along the axis. The phenomena were subsequently analysed by M. Biot, who remarked, that in some specimens of quartz, the succession of tints appeared by turning the doubly refracting prism from right to left, while in other specimens the same succession was developed by turning the prism from left to right; and he concluded from his experiments, that the quartz impressed upon the particles of light a rotatory motion, and that this property belonged to the ultimate particles of silex, and was independent of their mode of aggregation.The same species of colours was afterwards observed, about the same time, by MM. Biot and Seebeck, in transmitting polarised light through considerable thicknesses of some essential oils, and solutions of sugar and camphor; and this new fact seemed to confirm the supposition that the colours were not dependent upon crystallisation.