Corundum Stone from Asia. 435 
form equilateral triangles with one another, as may be seen in 
%. 10. 
The cause of these small equilateral triangles, which some- 
times project a little over the planes on the ends of the prism, 
must now be obvious. If, during the superposition of the crys- 
talline laminae on all the planes of the rhomboidal parallele- 
piped, it has happened, from any cause whatever, that the la- 
minae deposited on the three faces of the same summit, have 
not fallen exactly on those which preceded them, or that they 
have experienced some deviation, or have not had the same 
decrease as all the others, at the angle of 86°, these triangles 
must necessarily occur ; in the same manner it must be ob- 
vious, why these small equilateral triangular projections are 
frequently placed on one of the sides of the crystal. 
The primitive form of the Corundum crystal is therefore a 
rhomboidal parallelopiped, whose solid angle at the summit is 
84° 31', and that formed by the re-union of the bases is 
95 ° 29'. 
The crystalline laminae are rhombs of 86° and 94 0 : these, in 
my opinion, are double crystalline molecules ; the single mole- 
cules I apprehend to be isosceles triangles, of 86° at the angle 
of the summit, and of 47° at those of the base * 
* I am at present preparing a work, in which I shall, if circumstances permit me 
to finish it, give the result of my observations, and my own opinion on this interesting 
part of mineralogy. I shall only observe here, that although double molecules, square 
and rhomboidal, are frequently formed in the process of crystallization, yet the real 
form of the crystalline molecules seems to be triangular. By observing the pro- 
gress of the rhomboidal parallelopiped, in its passage to the form of an hexaedral 
prism, (fig. 4. and 5.) and by considering the prism terminated, it seems evident, that 
the last lamina which had been deposited, after the progressive decrease in the rows 
of crystalline molecules to one single molecule, must necessarily have been triangular, 
3 K 2 
