10 On the cryJlalUne Fortns of Corundum. 



The following is a tranflation of Rome de L'Ifle's words : " The firft oF thefe rubies has - 

 *< cxadtly the fame form as I have, reprefented in plate IV. fig. 60, of my Criftallographie, 

 *' viz. a rhomboidal parallelepiped, truncated at each of its obtufe angles, by an equilateral 

 *' triangular plane." 



" You will have acorreft idea of the other cryftal, if you fuppofe the cryftal, reprefented in 

 **iPlate IV. fig. 87, truncated at each of the fummits of its pyramids, by an equilateral triangu- 

 ".lar Plane, as in the preceding modification, but deeper, and in fo great a degree, that the 

 « three rhombic planes of each pyramid difappear with the exception of three ifofceles trian- 

 " gles : this modification differs from the firft only by the hexaedral prifm, and the deeper 

 *' truncature at the fummits of the pyramids." 



It is therefore clear, that if the primitive rhomboid of corundum decreafed only at the fupc- 

 rior angles of its laminae, it would exhibit exaftly the firft of thefe varieties of Mr. Werner's 

 ruby, as in the annexed fig. 19. 



As to the fecond variety of Mr. Werner's ruby, it is equally clear, if in fig. 87, referred 

 to-by Rome de L'lfle (reprefented by the annexed fig. 20.), no more of the pyramid was left 

 than the three fmall triangles b, a, c, there would be precifely one of the forms of corundum 

 before defcribed, to which the annexed figure 5 belongs. 



It may, perhaps, be objedted, that the laminae appear to be parallel to the terminal planes, 

 in the fapphire, and inclined in the corundum. There are cryftals of corundum, in which, very 

 frequently, the laminas appear parallel to the terminal plane ; I was at firft, and for fome time, de- 

 ceived by that appearance. In other corundum cryftals, the laminae appear to be parallel to the 

 prifmatic planes ; and to conclude the inftances of analogy, the fuperpofition of rhomboidal 

 lamina; is fometimes obfervable in oriental rubies and fapphires. It was by this appearance, 

 Mr. Greville was led to try the effect of cutting the forementioned ftones en cabochon ; 

 whereby a fimilar effe<5t of triple reflection, which formed ftars of fix rays from a common 

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



It is to be lamented that Mr. Werner did not fend, with his models, the fpecific gravity of 

 each o^the rubies, and the meafures of their angles : we fhould then have had data to decide 

 whether the rubies, fent by Mr. Werner, were, as I fuppofe them to be, oriental rubies, or 

 fapphires ; and, with equal certainty, whether the parallelepiped rhomboid correfponds precifely 

 with that of the corundum : by this the perfecS identity or analogy, between the corundum and 

 the fapphire, would have been no longer doubtful. 



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