COLOUES OF CRYSTALLINE PLATES. 141 



the pyramid "being uppermost), the crystal is right-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 cropping out of the 

 edges of these layers that the undulating appearance peculiar to 

 the fracture of this mineral is owing. The structure itself is dis- 

 played 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 possessing 

 the property. They retain it even when raised to the state of 

 vapour ; and, in general, the rotatory power is independent of the 

 mode of aggregation, provided the molecular constitution is un- 

 changed. Lastly, when two or more liquids possessing this pro- 

 perty are mixed together, the rotation produced 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 rota- 

 tory polarization is inherent in the ultimate particles of bodies, 

 and does not depend on their mutual distance or arrangement.* 

 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 



* Cambridge Trans., vol. i. t &* Trans., vol. ix. 



| M. Biot has recently extended his researches on this subject to a great variety of 

 substances, Annales du Museum d 'Histoire Naturelle, om. ii. In a memoir read to tho 

 French Academy last year he has applied the laws of circular polarization to tho ana- 

 lysis of the process of vegetation in the grasses : and he has shown, in general, the 

 importance of the indications drawn from these phenomena in the researches of organic 

 chemistry. Inttitut, Nos. 1 & 9. 



