the Corundum Stone , mid its Varieties , See. 315 
one more acute* the other more obtuse, than the primitive 
rhomboid. 
The first of these pyramids, is the natural produce of the in- 
crease of the planes which have replaced the acute angles of the 
rhombic planes of the primitive crystal : these planes are repre- 
sented at the letter 0, in Figs. 114, 115, 11 6, and 121, Plate LIL 
of the Mineralogy lately published by the Abbe Hauy. This 
learned mineralogist has indeed represented a considerable in- 
crease, but not a complete one, of the above-mentioned planes, 
in Fig. 121, which he says was communicated to him by Mr. 
La Metherie. From the appearance of this form, I think it 
probably belongs to the tourmalins of Regensberg, in the Upper 
Palatinate ; for many crystals of tourmalin from that place ex- 
hibit, at one of their extremities, the pyramid represented at Fig. 
121 of the work just mentioned, and the pyramid I have here 
described, at the other. This triedral pyramid measures 107°, at 
the solid angle of its summit. 
The second of the pyramids, is produced by the increase 
of the planes which have replaced the edges of the pyramids 
of the primitive rhomboid : these planes are represented by the 
Abb£ Hauy at letter n, in Figs. 118, up, and 120, also of 
Plate LI I. The triedral pyramid which these planes produce, 
after having caused every trace of the planes of the primitive 
rhomboid entirely to disappear, has, very nearly, 159 0 for 
the measure of the solid angle of its summit. I have seen 
this variety among the tourmalins from the Ural mountains, in 
which, very often, the solid angle of their summit is replaced by 
a plane, of greater or less extent, which is perpendicular to 
their axis. 
Among the various colours exhibited by the tourmalins which 
