the Corundum Stone , and its Varieties , &c, 253 
prism,) it often happens that there remains no trace of the 
planes of the primitive rhomboid : the crystal is then a regu- 
lar hexaedral prism. (Fig. 11.) This variety, which is very 
common in the perfect corundum of a red or of a blue colour, 
is also common in the imperfect kind ; it is indeed, in certain 
districts, particularly in the Carnatic, almost the only form that 
is met with. In all these crystals, the prism here spoken of 
differs considerably in its length ; sometimes it is very much 
elongated ; at other times it is very short, as is represented in 
Fig. 12. 
Third Modification. The primitive rhomboid is frequently 
observed to have undergone, in its crystalline lamina?, a de- 
crease at those flat angles which rest upon the common base. 
This decrease occasions, in each of the pyramids of the rhom- 
boid, six new planes equally inclined, which thereby render the 
pyramids enneaedral, (as is seen in Fig. 20,) and which, when 
this modification is perfect, (that is to say, when the planes be- 
longing to it have destroyed every trace of the primitive rhom- 
boid,) change the crystal into a dodecaedron, formed by the 
union, base to base, of two hexaedral pyramids with isosceles 
triangular faces, as in Fig. 13. At present, I shall only take 
notice of the pyramidal form of these crystals, without paying 
any attention to the inclination of the faces of the pyramids ; 
for we shall see, at the end of this modification, that the 
decrease which occasions it is subject to considerable variation, 
changing, at the same time, the inclination the faces of the 
pyramids have to each other. 
It very rarely happens that we find this dodecaedron perfectly 
complete, that is to say, with each of its pyramids terminating in 
a single point, by the exact meeting of all its faces, I know only 
MDCCCII. L 1 
