the Corundum Stone, and its Varieties, &c. 271 
tore to have the same direction as if the increase of the crystal 
had been produced by a deposition on the faces of the primitive 
rhomboid, it may, I think, be explained by supposing that, in 
this case, the elements of the crystallization ipight already be 
real, though small, secondary crystals, for instance, small hex- 
aedral prisms ; and that the fracture would then be nothing more 
than the result of the sum of all the partial fractures of each of 
them.* 
PHENOMENA WITH RESPECT TO LIGHT. 
The prismatic crystals of corundum, as well as the pyramidal 
ones, when their extremities are terminated by faces which are 
perpendicular to their axes, very frequently have those termi- 
nal faces chatoyant . This property is the natural effect of the 
* I had finished writing this Paper, when Mr. Greville had the curiosity to 
cause one of the hexaedrai prisms of imperfect corundum, from the coast of Malabar, 
the terminal faces of which exhibited the concentric hexagons above spoken of, to 
be cut transversely. This section shewed a very interesting fact, and one that adds 
some probability to what I have said respecting the cause which produces this pheno- 
menon. One of the parts of this crystal (which crystal is sawed into three, and po- 
lished,) exhibits the appearance represented in Fig. 38, A. The whole substance of this 
segment is of a pale purplish-red colour ; but there is, in its centre, a triangular spot, 
similar to that represented in the above figure, which indicates very clearly that the 
section was made below the summit of the primitive rhomboid, and perpendicularly to 
its axis. This spot is also of a purplish-red colour, but much more deep than the rest 
of the crystal, and therefore strikes the eye very forcibly. It is only to be perceived 
upon one of the terminal faces j the other terminal face does not show the smallest 
trace of it. There may, however, be perceived at its centre, a hexagonal plane, nearly 
as large as that represented surrounding the spot in Fig. 38, A ; it is of a different 
colour from the other part of the substance of this segment, being of a dirty gray. 
The spot is also seen, but of a smaller size, upon the terminal face corresponding to 
the segment taken from the top of the preceding ; but there are not any traces of it 
upon the other terminal face. 
N n 2 
