the Corundum Stone , and its Varieties , See. 307 
one in each pyramid ; which last faces have entirely disappeared. 
There are also several crystals in a progressive state, and more 
or less advanced, from the octaedron to this form. (Fig. 4 6.) 
The third form, is a complete dodecaedron, with rhombic 
planes. Fig. 47. It is produced by the enlargement of the planes 
which have replaced the twelve edges of the octaedron ; a mo- 
dification to which the Abb£ Hauy has given the name of 
emarginee. This enlargement is such as to have caused the 
entire disappearance of the eight primitive planes of the octae- 
dron. There are also, in Mr. Greville's collection, crystals 
more or less advanced towards this form, some of which no 
longer show any traces of the planes of the octaedron, except 
by extremely small equilateral triangular planes, as in Fig. 48. 
In these crystals, it is very common to find the decrease of the 
laminae evidently indicated by striae . 
The fourth form, is a rectangular tetraedral prism, terminated 
by two pyramids, also tetraedral, which are situated upon the 
sides of the prism, and have equilateral triangular planes. This 
crystal is produced merely by the edges of the base of the oc- 
taedron being replaced ; which replacement separates the two 
pyramids, by a prism more or less elongated. There are some 
crystals in which this prism is pretty long, as in Fig. 49 ; others 
in which it is, on the contrary, very short, as in Fig. 50. 
Although the Abbe Hauy has described the cuneiform oc- 
taedron, I think it right to add to his description, that, in this 
variety, the separation of the two opposite faces in each of the 
pyramids, becomes sometimes so considerable, that the crystal 
thereby changes its appearance, and acquires that of a rhom- 
boidal tetraedral prism, of 109° 30', and 70° 30'. This prism is 
terminated by two diedral summits, with isosceles triangular 
