Corundum Stone from Asia. 439 
than one row, on the planes of its other pyramid. This general 
observation, on the manner in which this primitive crystal of Co- 
rundum passes to the different varieties just mentioned, is the 
only one I have established with any great degree of certainty 
at present. Specimens with perfect crystals, whose angles may 
be measured with accuracy, will probably arrive from India, 
and give further demonstration, as to these and other varieties 
of modifications of Corundum. We may conceive, that if, in 
this modification, the crystallization had ceased before the en- 
tire formation of the crystal, there would have remained small 
isosceles triangular planes, on three of the alternate solid angles, 
formed by the junction of the planes on the ends, with the 
edges of the truncated pyramid. These isosceles triangular 
planes resemble those we have seen in the first modification; 
(fig- 4- an( f 5-) an( f form, in the same manner, solid angles of 
122° 34', with the planes on the ends of the prism. (Fig. 16.) 
Finally, if, during the formation of the crystal, in this modi- 
fication, it should happen that the laminae deposited on the 
three planes of the rhomboidal parallelepiped, on the side 
where they undergo a greater decrease, do not undergo the 
decrease of one row of molecules at the acute angle of the sum- 
mit, the crystal will be a real hexaedral pyramid, (fig. 17.) 
whose acute angle at the summit, measured on the sides, will 
be nearly 24 0 , in one of the varieties; 40° for the most obtuse; 
and 20 0 for the most acute variety : the angle of their triangu- 
lar planes, in the first instance, 13 0 41 7 ; in the second, 22 0 20'; 
and 1 1° 28' in the third. I have not seen any perfect pyramids; 
but, in many, the hexagonal plane terminating the pyramid is 
so small, that it renders its total suppression probable. 
