262 Count de Bournon’s Description of 
be 6o° 4,6'; the obtuse ones 11 9® 14' ; and the solid angle of the 
summit 165°. The crystal I have just described, would then be 
nothing more than the prismatic modification, combined with 
that which occasions this rhomboid ; at both extremities of which, 
one of the faces of each of the obtuse triedral pyramids, be- 
longing to the new rhomboid, would have acquired (in a con- 
trary direction with respect to its extremities) such an increase 
as would cause the other faces to disappear. These two faces, 
having now become the terminal ones of the hexaedral prism, 
would in fact make, with those edges of the prism on which 
they would incline, angles of ioo° and 8o°. This very obtuse 
rhomboid would be the result of a decrease analogous to 
the two preceding ones, but still more rapid. Many pyra- 
midal crystals of this kind of corundum, present such inclined 
terminal faces ; but with a difference, in the measure of their 
angles, conformable to the inclination of the edges of the 
pyramids. 
Seventh Modification. The primitive rhomboid of this sub- 
stance also undergoes sometimes, though very rarely, a decrease 
at those acute angles which rest upon the base ; and this de- 
crease is such, that it replaces each of the solid angles of this 
same base, by a plane which is parallel to the axis of the rhom- 
boid. If this modification were complete, it would give rise to 
a regular hexaedral prism, which would differ from the prism of 
the second modification, in having its sides corresponding with 
the solid angles of the base of the rhomboid ; whereas the sides 
of the other correspond with the edges of the said base. I know 
this modification only by a single crystal, which is in the col- 
lection of Mr. Greville; in it is combined the modification 
here spoken of with the three first. This crystal, which is 
