377 



VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 



As to the 2d variety of Mr. Werner's ruby, it is equally clear, if in fig. 87, re- 

 ferred to by Rome de l'lsle, represented by our fig. 20, no more of the pyramid was 

 left than the 3 small triangles b, a, c, there would be precisely one of the forms of 

 corundum before described, to which the annexed figure 5 belongs. It may per- 

 haps be objected, that the laminae appear to be parallel to the terminal planes, in 

 the sapphire, and inclined, in the corundum. There are crystals of corundum, 

 in which, very frequently, the laminae appear parallel to the terminal plane ; I was 

 at first, and for some time, deceived by that appearance. In other corundum 

 crystals, the laminae appear to be parallel to the prismatic planes; and, to conclude 

 the instances of analogy, the superposition of rhomboidal laminae is sometimes ob- 

 servable in oriental rubies and sapphires. It was by this appearance, Mr. Greville 

 was led to try the effect of cutting the forementioned stones en cabochon; by which 

 a similar effect of triple reflexion which formed stars of Q rays from a common 

 centre, was produced in the oriental ruby, in the sapphire, and in the corundum. 



Table of the Specific Gravity of the Corundum, Sapphire, Topaz, Ruby, and 



Diamond, on different Authorities. 



Corundum. 



Corundum. 



J 2.768 



Hatchett and 

 Greville . . . 



H.andG 2.785 



Klaproth 3.075 



Klaproth 3.710 



Blumenbach . . 3.808 



Brisson 3.873 



Hatchett & G. 3.876 



Lichtenberg . . 3.908 



*Matrixof corund. Coast. 

 *Lump of corund. Coast. 



*Corone. Bengal. 



Gross 3.935 



Hatchett & G. 3.950 



H. and G. . . . {3.954 



H. and G. . . . 3.959 



H. and G 3.959 



H. and G 3.962 



Klaproth 4.180 



* Crystal of corund. Coast. 

 *Ditto with vitreous cross 



fracture. Coast. 

 *Ruby-coloured. Coast. 

 *Chatoyant. China. 

 *Crystal. China. 



Sapphire. 



Brisson 3.130 Brasilian ; probably a topaz. 



Bergman .... 3.650+ 



Quist 3.800+ 



Bergman 3.940+ 



Klaproth 3.950+ 



Bergman 3.974+ 



Brisson 3-991+ White oriental. 



Brisson 3.994+ 



Blumenbach . . 3-994+ Blue. 



Hatchett and G. 4.000+ * Greyish star-stone. 



Werner 



Blumenbach . . 

 Hatchett and G. 



Brisson 



Blumenbach ... 

 Hatchett and G. 

 Muschenbroeck 

 Blumenbach . . 

 La Metherie . . 

 Quist 



Sapphire. 



4.000+ 



4.010+ 



4.035+ Blue star-stone. 



4.076+ From Puy-en-Velay. 



4.083+ Crimson. 



4.083+ *Pale blue crystal. 



4.090+ 



4.100+ Yellow. 



2.200+ 



4.200+ 



Topaz. 



La Metherie .. 2.690 



Bergman 3.460 



Werner 3.464 



Quist 3.500 



Werner 3.521 



Brisson 3.531 



Brisson 3.536 



Siberia. 



Light blue. Brazil. 



Eibenstocker. 

 Red. Brazil. 

 Dark yellow. Brazil. 



Topaz. 



Werner 3.540 Dark yellow. Brazil. 



Brisson 3.548 Oriental. 



Werner 3.556 Schneckensteiner. 



Brisson 3.564 Schneckensteiner. 



Brisson 4.010+ Oriental 



Bergman 4.560+ 



Ruby. 



Bergman 3.180 



Muschenbroeck 3.180 



VOL. XVIII. 



Quist 



Blumenbach 



3C 



Ruby. 



3.400 Spinel. 

 3.454 Ceylon. 



