The primitive Crystal and its varieties.
Fig. 1 is the primitive crystal; the angle formed by the meeting of the two faces PP has been ascertained by Haüy to be 109d. 28′, 16″. Fig. 2 is the result of a [1] decrease on one face of the upper, and on the opposed face of the lower pyramid. This crystal has aptly been called the segment of the octahedron, as the explanatory fig. 3, will evince. Fig. 4 is the primitive crystal elongated; this elongation arises, as is obvious, from a regular increase of the crystalline laminæ on one face of the upper pyramid, and on that face of the lower to which it is united at their common base. Fig. 5, is the same occurring in capillary crystals. Fig. 6 differs from fig. 4 in the upper pyramid only; on the already increased face of which, a still further increase of laminæ, has taken place, as well as a similar deposition on the opposite face of the same pyramid. Fig. 7 is produced by an increase on one face of the upper, and on one face of the lower pyramid of fig. 4, as will be obvious on consulting fig. 8. Fig. 9 is the consequence of an increase on two opposed faces of the upper, and on the other two opposed faces of the lower pyramid of the primitive crystal, as will be seen by fig. 10. Fig. 11 is the result
- ↑ In the present imperfect state of mineralogical language, it is difficult on every occasion to find terms by which even facts can be accurately defined. For instance, in that modification of the octahedron, where the solid angle is wanting, and which though differing inform from the primitive Crystal, can scarcely be called an imperfection in crystallization, there is no term in use justly descriptive of the fact. The terms truncation and decrease are obviously inaccurate. I have used the latter as perhaps the least exceptionable. It is with deference to the opinions of the Count de Bournon, I venture to observe that the term retrogradation lately adopted by him seems equally objectionable with either of the preceding.
