base to base, by which each is replaced by a plane, perpendicular to the axis passing through those edges, fig. 34. Pl. 17.
The planes of the primitive crystal are rarely found in combination with those of this modification, except when they seem only to be the result of a decrease on the edges of the secondary pyramid as in fig. 39. Fig. 35. shews the combination of the planes of this modification with those of the secondary pyramid, which is thus given, because, as the secondary pyramid is that commonly observed on the crystals of this substance, it seems to facilitate the tracing of the various combinations in the succeeding figures.
Fifth Modification.
The fifth modification arises from a decrease on each of the solid angles caused by the meeting of the two pyramids base to base, by which each is replaced by two triangular planes placed on the edges formed by the meeting of the two pyramids, but inclining on the axis passing through the lateral solid angles, fig. 45. Pl. 17.
The planes of the primitive crystal are also shewn by the dotted lines of fig. 46. together with the planes of this modification in a more advanced state. The latter are also exhibited in combination with those of the secondary pyramid, by the lines within that figure.
It will be noticed how nearly the crystals given by fig. 55. approach the cube, and that of fig. 47. the secondary octahedron. The crystal represented by the latter figure does not exceed in site the head of a small pin, but all its planes are remarkably brilliant and well defined.
