FIFTH MODIFICATION.
This interesting modification is found in combination with each of the preceding, but is rarely seen displayed by itself on the primitive crystal as in fig. 74. It arises from a decrease of the crystalline laminæ on that part of each face forming the solid angle, by which each is replaced by an obtuse quadrilateral pyramid; the faces of which incline on the axis that passes through the angles. The angle formed by the meeting of P and 5 is about 160 d. and of 1 on 5 about 144 d.[1]
Fig. 78 seems to be the result of a partial after-deposition on the faces of the primitive crystal, by which each face of it (such parts of of them excepted as contribute to form the solid angle,) is brought in form to resemble fig. 77, except only that in this the solid angles of the primitive crystal remain.
It may perhaps be imagined that some of the latter figures in the series of this modification, exhibiting its combination with some of the preceding modifications might have been omitted; but as the term variety is used to signify combination of two or more modifications, as well as those differences in crystals which arise from the various proportions of the faces to each other, by which variations in the form of the face are produced; and as each of these differs in one or more of these respects, and actually exists, it seemed proper not to omit them.
- ↑ There are other modifications, and their combinations delineated in the series of the crystallization of this substance, the angles of which it would have been desirable to here given; in most, if not all of which, it is, from their exceedingly small size, to be regretted that it would have been difficult, if not impossible, even for the most skilful and practised hand, to have subjected them to the goniometer.
