Count de Bournon’s Description of 
new rhomboid, which is much more obtuse than the former one. 
(big. 28.) The rhombic planes have 117 0 , for the measure of 
their obtuse angles; and 63°, for the measure of their acute 
ones. The solid angle of the summit of the pyramid is very 
nearly 150° 30'; consequently, the angle formed by the meeting 
of the bases is about 2 g° 30-'.* 
There are, in Mr. Greville's collection, two oriental rubies 
which exhibit this rhomboid completely formed ; its planes 
are deeply striated, in the direction of the decrease; a circum- 
stance which is very common in all planes that are the result 
of a rapid decrease, or in which the edges of the laminae last 
deposited, deviate considerably from the edges of those which 
were already formed. 
There are also, in the same collection, two perfect hexaedral 
prisms of corundum from the Carnatic, in which this modi- 
fication shows itself by small isosceles triangular planes, si- 
tuated upon three of the alternate solid angles of each extre- 
mity. (Fig. 23.) These planes may easily be distinguished 
* After having, in this substance, met with a secondary rhomboid that exactly agrees 
with one of those belonging to calcareous spar, (although the planes which pro* 
duce it are differently situated upon the primitive crystal,) it appeared to me very- 
extraordinary to meet with a second, which had exactly the same proportions as 
another of the obtuse rhomboids of the abovementioned substance. In fact, there 
exists in calcareous spar, a rhomboid much more obtuse than that which Rome" 
de Lisle named lenticular, (called equiaxe by the Abbe Hauy,) of which the 
measures are exactly the same as those which have just been assigned to the rhomboid 
of corundum ; but there is the following difference between them, viz. in calcareous 
spar, this rhomboid is the result of a decrease along the edges of the pyramid 
belonging to the primitive rhomboid; whereas, in corundum, it is the result of a 
decrease at the angles which contribute to the formation of the solid angle of the 
summit. This modification of calcareous spar has not yet been described ; but, indeed, 
the same thing may be said of many other modifications of that substance. 
