Corundum Stone from Asia. 431 
piped, (fig. 2.) that nature has formed the regular hexaedral 
prism (fig. 1.) which this substance presents. 
For, if we conceive, that in any period whatever of the in- 
crease of the rhomboidal paralielopiped, a series of laminae or 
crystalline plates has been deposited on all the sides of the pa- 
ralielopiped ; and that these laminae have all undergone a pro- 
gressive decrease of one row of crystalline molecules, at the acute 
angle which tends to form the summit, and also along the sides 
of the opposite acute angle, (fig. 3. and 4. ) there will necessarily 
result from the continuation of this superposition, to a certain 
period, an hexaedral prism, terminated by two triedral pyra- 
mids, placed in a contrary direction ; and their planes or faces, 
which form a solid angle of 147 0 2 6', with the sides of the 
prism, will be either pentagonal, (fig. 3.) or triangular. (Fig. 4.) 
They will also have, in place of a summit, an equilateral trian- 
gular plane, sometimes greater and sometimes smaller. 
If the superposition continues, the equilateral triangular 
plane on the summit will become nonagonal, and there will 
remain no other traces of the primitive planes of the rhom- 
boidal paralielopiped, than small isosceles triangular planes : 
(fig. 5.) if the superposition still continues, until the last 
crystalline lamina is reduced to a single molecule or point, 
no appearance of the rhomboidal paralielopiped will then re- 
main ; and the crystal resulting from this operation of nature 
will be a regular hexaedral prism. (Fig. 1.) 
In the same manner, viz. by a decrease on the lower edges 
of the laminae, the primitive rhomboidal paralielopiped of cal- 
careous spar passes to a regular hexaedral prism of that sub- 
stance; though more frequently it does so by a decrease on the 
lower angles of the laminae. 
