considerable size, almost covered with well defined crystals represented by figs. 21. and 25.
Neither the planes of this, nor of any other modification have I believe been found in simple combination with those of the primitive crystal. They are thus given preceding the series of each modification, in the hope of thereby rendering each the more intelligible; to this I have generally added a figure representing their combination with the planes of the secondary octohedrons, being those of the second modification, because the planes of that modification form the pyramid most commonly found on the crystals of this substance.
Each of the four solid angles formed by the meeting of the two pyramids of the primitive form base to base, is by this modification replaced by two triangular planes; each plane being placed on an edge of the pyramid, but inclining on the axis passing through the solid angles, fig, 22. Pl. 16.
This modification is represented in an advanced state, by the dotted lines of fig. 23. shewing by the lines within it, that when complete, it produces a secondary pyramid considerably more acute than that of the primitive form. The secondary pyramid produced by this modification is that commonly observed on the crystals of this substance, and by fig. 24. is represented within the dotted lines of the primitive form.
Fig. 25. shews the first and second modifications in combination with the planes of the primitive crystal. The lines on the faces of this figure denote the direction in which the striæ are sometimes to be observed on the crystals.