406 FOURTH UEPORT — 1834. 



same direction ; and it is found that when that direction is to 

 the right, (the apex of the pyramid being uppermost,) the cry- 

 stal is rif/ht- handed; and that on the contrary it is left-handed, 

 when the planes lean in the opposite way *. Sir David 

 Brewster has shown that the amethyst, or violet quartz, is 

 actually composed of alternate layers of right-handed and left- 

 handed quartz. It is to the crojjping out of the edges of these 

 layers, that the undulating appearance peculiar to the fracture 

 of this mineral is owing. The structure itself is displayed in 

 the most beautiful manner in polarized light f. 



Some liquids, and even gases, have been found by MM. Biot 

 and Seebeck to possess the same property as quartz, though in 

 a much feebler degree ; and to impress a rotation on the plane 

 of polarization of the intromitted ray, which is proportional to 

 the thickness of the substance traversed. These liquids do not 

 lose their rotatory power by dilution with other liquids not pos- 

 sessing the property. They retain it even when raised to the 

 state of vapour ; and, in general, the rotatory power is inde- 

 pendent of the mode of aggregation, provided the molecular 

 constitution is unchanged. Lastly, when two or more liquids 

 possessing this property are mixed together, the rotation pro- 

 duced by the mixture is the sum of the rotations produced by 

 the ingredients, in thicknesses proportional to the volumes in 

 which they are combined. From these and other facts M. Biot 

 concludes that the property of rotatory polarization is inherent 

 in the ultimate particles of bodies, and does not depend on their 

 mutual distance or arrangement X- On the other hand, quartz 

 is found to lose the property when deprived of its crystalline 

 structure. Thus Sir John Herschel observed that quartz held 

 in solution by potash did not possess the property ; and the 

 same thing has been remarked by Sir David Brewster with 

 respect to fused quartz. 



The phenomena of rotatory polarization in rock crystal, M. 

 Biot ascribed to a continued rotation of the molecules of light 

 round their centres of gravity, produced by the operation of 

 some unknown forces. Fresnel has proved that they arise from 

 the interference of tivo circularly-polarized pencils whicli are 

 propagated along the axis with unequal velocities, otie revolving 



* Cambridge Trans., vol. i. t J^din. Trans., vol. ix. 



X M. Biot has recently extended his researches on this subject to a great 

 variety of substances, Annates du Museum d'Histoire NutureUe, toni. ii. In a 

 memoir read to the French Academy last year he has applied the laws of cir- 

 cular polarization to the analysis of the process of vegetation in the grasses ; and 

 he has shown, in general, tiie importance of the indications drawn from these 

 phenomena in the researches of organic chemistry. InstUut, Nos. 1 & 9. 



