The cube, fig. 15, or solid, composed of six quadrilateral planes united at right angles. 4th. The rhomb, fig. 16, or solid, of six quadrilateral planes united at ob lique angles.' 5th. The prism, or solid, of two terminal planes, parallel, equal, and similar, connected by quadrangular lateral planes, having one direction ; the number of lateral planes may of course be various ; the usual form observed in crystals are, the four-sided rectangular prism., fig. 17 ; and the six-sided equian gular prism, fig. 19. 6th. The pyramid, or solid, the base of which is a plane of an indeterminate number of sides, and the sides triangles, the vertices of which meet in one point, forming the summit : the more common varieties of this figure, as forms of crystals, are the three-sided pyramid, or tetraedron, fig. 20, and the four-sided pyramid, fig. 21. 7th. The ta ble, which, strictly speaking, is nothing but a very compressed prism ; it is de fined as composed of two parallel lateral planes and of an intermediate number of terminal planes, connected with the late ral planes and with each other, and small, compared with the lateral ones ; the prin cipal varieties are, the oblique-angular, or rhomboidal four-sided table, fig. 23, the rectangular four-sided table, fig. 24, and the six-sided table, fig. 24. Lastly, The lens, fig. 25, a solid, consisting only of two planes which are curved, of which there are two varieties, one composed of two convex planes, and another compos ed of a convex and a concave plane. These simple figures are modified by combination, by truncation, by bevel ment, and by acumination.
The modifications by combination are confined to the pyramids, and these are frequent, two pyramids being joined by the base ; the lateral planes of the one being set either directly on the lateral planes of the other, as in the double four sided pyramid, or octaedron, fig. 26, or obliquely, as in the double four-sided py ramid, fig. 27. Fig. 28, is the double six sided pyramid.
A crystal is said to be truncated, when any or all of its solid angles or edges ap pear cut off,so that where there would have been an edge or angle we have a plane, as has already been represented in fig.
2 and 3. These two figures represent forms arising from the truncation of the cube : fig. 29, shews the cube with the angles and edges truncated : fig. 30, the six-sided prism, with truncated termi nal edges : fig:ftl, the same prism, with both the lateral and terminal edges trun cated.
A crystal is said to be bevelled, when its edges, angles, or terminal planes, are so altered, that instead of an angle edge or terminal plane, there appear two smaller converging planes, which terminate in an edge : fig. '32, shews the cube with bevelled edges : fig. 33, the three-sided pyramid with bevelled edges : fig. 34, the oblique four-sided prism, bevelled on both extremities.
Lastly, the forms of crystals are altered by acumination. This is that kind of al teration, in which, in place of the angles or terminal planes of a crystal, there are three or more planes converging, and forming a point or edge : fig. 35, shews the cube, with angles acuminated by three planes set bn the lateral planes ; fig. 36, the rectangular four-sided prism, acu minated by four planes set on the lateral planes : fig. 37, the six-sided prism, acu minated by six planes set on the lateral planes. This kind of modification is often described as consisting of the primary form, with pyramidal terminations.
The forms of crystals from the preced ing modifications are frequently still more altered, and rendered complicated, by being super-added and combined ; and by the extent of the modifications, one form frequently passes into another. The figures of crystals are likewise rendered complicated by aggregation, two or more crystals of the same substance being more or less closely united.
For the more minute details of this sub ject, particularly as relatnig to mineralogy, reference may be had to Weaver's trans lation of the " External Characters of Minerals," by Werner ; or the treatise on the same subject by Professor Jameson. See CRYSTALLIZATION.