43^ Mr. Greville on the 
ever seen of this substance. The surface of the faces of the 
prism, although rough, is infinitely less so than that of the 
others, and much more brilliant. The planes on the ends have 
the usual polish of crystals ; its colour is a pale red, and its 
transparency may be compared to that of wax. 
This substance presents a third modification, in which the 
hexaedral prism diminishes in diameter, as is apparent by com- 
paring the diameters of its two ends ; in some, it appears like a 
regular hexaedral pyramid truncated. (Fig. 15.) The crystals 
of this modification are usually irregular, and seldom admit of a 
certain measure of their angles; but, among the numerous 
specimens in Mr, Greville’s collection, I have been able to 
ascertain, in the greater part, that the hexagonal plane at the 
top forms angles of about 120°, with the planes of the pyramid; 
and the hexagonal plane at the base forms angles of about 78°, 
with the planes of the pyramid. In other instances, the form of 
the pyramid varies greatly ; in some, the angle at the upper 
plane was 110°, and the angle at the base about 70°; in others, 
the angle at the upper plane was about ioo°, and the one at 
the lower plane about 8o°. 
In these three varieties, the crystalline laminae can be sepa- 
rated, as in the hexagonal prism, at the three solid alternate 
angles of each end, but in a contrary direction to each other. 
The planes which appear when the laminae are detached re- 
gularly, form solid angles of 22 0 34', with the planes of the ex- 
tremity : this arrangement is analogous to that of the hexae- 
dral prism. The difference of form arises from the crystalline 
laminae deposited on the planes of the primitive rhomboid, 
decreasing by more than one row of molecules, on the planes 
of one of the triedral pyramids of the rhomboid, and by less 
