308 , Count de Rournon's Description of 
planes, the apices of which are situated upon those edges of the 
prism which measure 70° 30', making with them an angle of 
125 0 15', and meeting, by their bases, at the top of the crystal, 
in an angle of 109° 30', as in Fig. 51.* 
I also think it right to add, to what the Abb£ Hauy has said 
respecting the colours of this substance, that it is sometimes 
perfectly colourless, sometimes of a yellow colour, and some- 
times of a bluish one. 
We were as completely ignorant of the nature of the stone 
which serves as a matrix to the spinelle, as we were respecting 
that of the matrix of the perfect corundum of Ceylon, when a 
number of specimens were sent from India to Sir John St. 
Aubyn, by Mr. White, amongst which were two pieces of the 
highest value, inasmuch as they served to show' us, for the first 
time, the substance now treated of, inclosed in its matrix. I 
flatter myself a description of these two pieces will be thought 
worthy the attention of the Royal Society, particularly as they 
also contain a species of iron ore hitherto unknown. 
One of these pieces is a calcareous spar, of a granulated 
texture ; the grains are very large, and are intermixed with 
each other, so as to adhere very strongly together, but their 
* The dodecaedron, and the octaedron passing very rapidly to the tetraedron, had 
already been mentioned by Mr. Eslinger, ( Journal de Physique, Vol.-LII. p. 225,) 
as making part of the collection of crystals of this substance in Mr. Werner’s 
possession: the ot’er varieties had not yet been described. According to some of the 
external characters by which Mr. Eslinger describes the spinelle, I am inclined to 
think, that he includes some Ceylanites in that description, and also some oriental 
rubies. Such, for instance, I suspect to be, that which he says has a starry reflection; 
also the hexagonal prism with the alternate angles of the base replaced ; and the cube, 
(without doubt, slightly rhomboidal,) which has a small plane upon two of its solid 
angles diagonally opposite to each other : a form that is very rarely met with, even in 
the oriental ruby. 
