VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 375 



Finally, if, during the formation of the crystal in this modification, it should 

 happen that the laminae deposited on the three planes of the rhomboidal parallelo- 

 piped, on the side where they undergo a greater decrease, do not undergo the de- 

 crease of one row of molecules at the acute angle of the summit, the crystal will 

 be a real hexaedral pyramid, (fig. 17), whose acute angle at the summit, measured 

 on the sides, will be nearly 24°, in one of the varieties; 40° for the most obtuse; 

 and 20° for the most acute variety: the angle of their triangular planes, in the 

 first instance, 13° 4l'; in the 2d, 22° 20'; and 11° 28' in the 3d. 1 have not 

 seen any perfect pyramids ; but in many the hexagonal plane terminating the pyra- 

 mid is so small, that it renders its total suppression probable. This decrease neces- 

 sarily produces a single pyramid, as above-mentioned; yet there are instances of 

 crystals of corundum, belonging to the variety where the terminal planes make, 

 with the planes of the pyramid, a solid angle of about 100°, in which, 2 pyra- 

 mids of the same dimensions, having their summit replaced by a small hexagonal 

 plane, are placed base to base. I have also observed, among the crystals of the 

 obtuse variety above-mentioned, in Mr. Greville's collection, an instance of the 

 decrease taking place by several rows, on 1 three-sided pyramid of the primitive 

 rhomboid, and by single rows on the other. Consequently, the crystal is a short 

 regular hexaedral prism, terminating on one end only by an hexaedral pyramid; 

 the planes of which, as well as of the prism, are alternately broad and narrow, 

 and almost perfect; its apex being replaced by a very small plane. 



I shall conclude, by mentioning a variety of corundum, described by the Abbe 

 Hauy, in the Journ. des Mines, N° 28; in which, the edges of the terminal 

 planes of the hexaedral prism are replaced by planes which form an angle of 

 1 16° 31', with the terminal planes; but, in the numerous collection of Mr. Gre- 

 ville, I have not seen this variety. One crystal had an appearance of such planes; 

 but on examination it was clearly accidental. The authority of Hauy, in crystallo- 

 graphy, is so great, that the existence of such modification ought not to be denied 

 without further examination ; though I cannot in this instance adopt it : he derives 

 this variety, which he calls subpyramidal, from a decrease of 3 rows of molecules 

 at the angles of the base of the 2 pyramids of the primitive rhomboid ; and he 

 seems to attribute the same formation to the pyramidal variety with double pyra- 

 mid, which he supposes may exist. The primitive crystals, and the 1st and 2d 

 modifications of corundum, are from the Peninsula of India. The 3d modifica- 

 tion, or the pyramidal variety, is from China ; nothing approaching this form 

 being among the specimens which Mr. Greville received from the Peninsula 

 of India. 



The preceding observations, and particularly the last-mentioned modification of 

 corundum, compared with the best descriptions of the sapphire, suggest the fur- 

 ther examination of the degree of connection, if not of identity, of these oriental 

 stones. In both, the hexaedral pyramids are usually incomplete in their apex, and 



