OF AETS AND SCIENCES. 65 



The rhombic angle of witherite (native baric carbonate) is 118° 30', 

 and the all but universal hexagonal macling of crystals of this species is 

 a vrell-known fact.* The rhombic angle of aragonite (the corresponding 

 form of calcic carbonate) is 116° 10', and the much greater divergence of 

 this angle from 120° determines, as is also known, a style of macling 

 which is usually quite different from that of witherite. In the isomeric 

 calcite, however, we have the type of all hexagonal forms. Hitherto 

 the crystalline forms of calcite and aragonite have been regarded as 

 being as widely separated as possible, and a comparison of these two 

 well-known mineral species has furnished one of the most striking 

 illustrations of demorphism. But may not, after all, the comparatively 

 small physical differences between these two minerals correspond to a 

 crystallographic difference no greater, fundamentally, than the difference 

 between the rhomb of 116° 10' and the rhomb of 120° ? 



The macles of chrysoberyl and witherite are illustration of a general 

 truth, fully recognized in mineralogy, that all rhombic crystals, whose 

 angles appi'oach 120°, tend to form hexagonal macles. The optical 

 phenomena described in this paper certainly suggest the theory that 

 a perfect hexagonal form and structure may be the result of a more 

 fundamental and molecular macling, which results when the angle is 

 exactly 120°. 



Observations of Senarmont. — The only previous observations which 

 we have been able to find bearing on the subject of this paper are 

 those of the late eminent mineralogist, H. de Senarmont, of Paris. In 

 a well-known paper (Ann. Cliim. et Phys. 3? xxxiii. 391), Senarmont 

 showed that salts which were both geometrically and chemically iso- 

 morphous might have very different optical I'elations ; for example, 

 that, while the biaxial crystals of two such salts might have the same 

 bisectrix, the plane of the optical axes in the crystals of one might be 

 perpendicular to the corresponding plane in those of the other. He 

 further proved, by crystallizing together two salts so related, that in 

 the crystals of the isomorphous mixtures thus obtained the ojjtical 

 angle varied with the varying proportions of the constituents, between 

 the extreme conditions in the crystals of either salt ; and by trial he 

 succeeded in forming from two biaxial salts crystals which, in mono- 

 chromatic light at least, ajjpeared uniaxial. In a later paper (Ann. 

 Chim. et Phys. 3f xxxiv. 171), Senarmont applied the principles, 

 which he had thus experimentally verified, to explain the variation of 

 the optical angle of the micas. In this paper he seeks to prove, first, 



* See figures, Dana's System of Min., p. 697. 

 VOL. I. 9 



