A Treatise on the External, Chemical, and Physical Characters of Minerals (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1816 edition. Excerpt: ...parallel to the edges of the square touch? by one of their faces, and the ranges themselves are simply placed contiguous to each other. The molecules pa rallel to the diagonals touch only by an angle, and the ranges are indented into each other. When secondary crystals are formed by this last kind of decrement, the new faces are not merely channelled, as happens in the case case of decrements on the edges; they are' all bristlell 'with points, which being exceedingly minute, and all in the same plane, escape the eye, so that the faces appear Smooth. Having thus explained the meaning of the terms, let us illustrate this kind of decrefiient by an example; and we cannot get a better than the formation of a regular octahedron from a cubic nucleus. This is the conse'quence of the superposition of plates upon each face of the cube with decrements of a single range of molecules on the angles. Let AEOI, Fig. 44. Pl. V. A, be one ofthe faces of the cubic nucleus subdivided into 81 little squares, which are the bases of so many molecules, of which the face is conceived to be composed. Fig. 4-4. B, represents the first plate of superposition, which ought to be placed above AEOI, Fig. 44-. A, in such a' manner, that the point c corresponds with the point e; the point a with the point a; the point 0' with the point 0; and the point i with the point 2." It is obvious, from this manner of placing it, that the squares E e, A a, I 1', O 0, Fig. 44-A, remain uncover'ed; which is the initial effect of the decrement on the angles. We see likewise, that the edges QV, PN, LC, FG, Fig. 44. B, exceed by a range of molecules the edges EA, -EO, -OI, IA, Fig. 44. A. This is necessary to prevent ...