433 
Corundum Stone from Asia. 
6,45 : 5 ; and, consequently, this prism would have been longer 
than that of the rhomboidal parallelopiped which served as its 
nucleus. 
On the other hand, if the increase of the rhomboidal paral- 
lelopiped had taken place by a superposition of crystalline 
laminae, decreasing at the acute angle of the summit, and, 
some time after, decreasing also along the sides of the acute 
angle of the base, (fig. 7.) the regular hexaedral prism resulting 
from this process would have been shorter, in proportion to the 
duration of the mode of decrease in the crystalline laminae 
which were first deposited. There are some of the hexaedral 
prisms, in Corundum crystals, which are so short, that they 
appear no more than segments. Calcareous spar offers the 
same phenomenon ; as do likewise all the substances in which 
the hexaedral prism has any analogy of formation with that 
which we have here described. 
It happens frequently, when the superposition of the crystal- 
line laminae does not go on equally on all the faces of the 
rhomboidal parallelopiped, that one or two only of the solid 
angles of the hexaedral prism, taken alternately, still shew, by 
small isosceles triangular planes, some remains of the faces of 
the parallelopiped, while the others do not shew any at all. 
Mr. Grevillr, in his collection of this substance, has a 
crystal of Corundum, upon one side of which, only two of the 
planes of the rhomb have experienced an equal and perfect 
superposition, while there has been but a very small number 
of crystalline laminae deposited on the third plane. Conse- 
quently, this crystal presents a regular hexaedral prism, one of 
whose solid angles is so much truncated, that the half of the 
plane of the end of the hexaedral prism disappears; (fig. 8,) 
MDCCXCVIII. 3 K 
