Mr. GreviEle on the 
442 
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 Abb£ Hauy, in the Journal de Physique , August, 1 793, 
names this variety. Orientate Enneagone, which is represented 
in the annexed Plate, (fig. 18.) and says, that the small triangu- 
lar planes make, with the terminal planes, an angle of 122 0 18': 
and, in the description of the same triangular planes in the 
Corundum, (fig. 16.) it appears, that these planes are the re- 
mains of the planes of the primitive rhomboid, and form, with 
the terminal planes, an angle of i22°34'. 
Perhaps the rhomboidal crystal, which Rome' de l'Isle had 
given as one of the forms of the sapphire, should be restored 
to it. He had examined it at M. Jacquemin's, jeweller to the 
crown, ( Cristallographie , 1. edit. p. 221.) and he suppressed it 
in his second edition, but often expressed to me his regret in 
having made the alteration, I have before me a letter from 
that celebrated naturalist, dated September, 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, two 
called by him rubies ; one a rhomboid, of which the angles of 
the summit are substituted by planes, (fig. 19.) the othef is 
precisely the same as fig. 3, 4, and 5, of the annexed Plate. 
The following is a translation of Rome' de 1/ Isle's words : 
“ The first of these rubies has exactly the same form as I have 
“ represented in Plate IV. fig. 60. of my Cristallographie , viz. 
* A letter to the same effect was written to M. la Methbrie, and published in 
the Journal de Physique, May, 1787. 
