376 PHILOSOPHICAL TRANSACTIONS. (*ANNO 1798. 



they vary in acuteness. I have stated the degree in which the solid angles of the 

 pyramid, taken as complete, vary in corundum, to be from 20° to 40°. 



Rome de l'lsle states, that the sapphire varies from 20° to 30°. The Abbe 

 Hauy (Journ. de Phys. Aug. 1793), mentions 2 varieties of the sapphire, one 

 measuring at the solid angle of the pyramid 40° 6', the other 57° 24'. I never 

 saw a sapphire with so obtuse an angle as the last ; but many, whose angle at the 

 top, if the pyramid had been complete, would have been the same as that of the 

 corundum. Besides the analogy between the crystals of corundum, and the sap- 

 phire, by the union of 2 hexaedral pyramids at their base, it also exists by the 

 measure of their angles ; and both substances are subject to the same irregularity, 

 sometimes appearing as a single hexaedral pyramid, and sometimes as an hexaedral 

 prism ; moreover, the sapphire sometimes has on its solid angles, alternately, the 

 same triangular planes, (fig. 5), and also the prominent triangles on the planes of 

 the extremities, (fig. 10), which often appear in the crystals of corundum. The 

 Abbe Hauy, in the Journ. de Phys., Aug., 17Q3, names this variety, Orientale 

 Enneagone, which is represented fig. 18, and says, that the small triangular planes 

 make, with the terminal planes, an angle of 122° 18'; and, in the description of 

 the same triangular planes in the corundum, fig. 1 6, it appears, that these planes 

 are the remains of the planes of the primitive rhomboid, and form, with the ter- 

 minal planes, an angle of 122° 34'. 



Perhaps the rhomboidal crystal, which Rome" de l'lsle had given as 1 of the forms 

 of the sapphire, should be restored to it. He had examined it at M. Jacquemin's, 

 jeweller to the crown, (Cristallogr. 1. edit. p. 221,) and he suppressed it in his 2d 

 edition, but often expressed to me his regret in having made the alteration. I have 

 before me a letter from that celebrated naturalist, dated Sept. 1784, in which he 

 inclosed, for my opinion, a copy of a letter he had received from Mr. Werner, with 

 models of some crystals ; among them, 2 called by him rubies ; one a rhomboid, 

 of which the angles of the summit are substituted by planes, (fig. 19), the other is 

 precisely the same as fig. 3, 4, and 5, of the annexed plate 6. 



The following is a translation of Rome de l'lsle's words : " The first of these 

 rubies has exactly the same form as I have represented in plate 4, fig. 6*0, of my 

 Cristallographie, viz. a rhomboidal parallelopiped, truncated at each of its obtuse 

 angles, by an equilateral triangular plane. You will have a correct idea of the 

 other crystal, if you suppose the crystal represented in pi. 4, fig. 87, truncated at 

 each of the summits of its pyramids, by an equilateral triangular plane, as in the 

 preceding modification, but deeper, and in so great a degree, that the 3 rhombic 

 planes of each pyramid disappear, with the exception of 3 isosceles triangles ; this 

 modification differs from the first, only by the hexaedral prism, and the deeper 

 truncature at the summits of the pyramids." It is therefore clear, that if the pri- 

 mitive rhomboid of corundum decreased only at the superior angles of its laminae, 

 it would exhibit exactly the first of these varieties of Mr. Werner's ruby, as in the 

 annexed fig. 19. 



