CRYS′TAL

A solid body, having a regular geometrical form. The plane surfaces by which a crystal is bounded are termed faces; these intersect in straight lines or edges; and these again meet in points, and form angles. The axis of a crystal is an imaginary line pa ing through its centre, and terminating either in the middle of two faces or of two edges, or in two angles; and axes terminating in similar parts of a crystal are named similar axes. When the axes of a crystal are properly chosen, and placed in a right position, the various faces are observed to group themselves in a regular and beautiful manner around these axes, and to be all so related to them as to compose a connected series, produced according to definite laws. The multitudinous forms of crystals have been systems, distinguishable from one another by the relative positions and lengths of the three axes about which the planes or faces are arranged; while the different figures of any particular system are distinguishable by the arrangement of the planes in respect to the axes. Thus, the cube or hexahedron, the rhombic dodecahedron, and the octahedron all belong to the regular system, which is characterised by 3 equal axes cutting one another at right angles. But in the cube each plane cuts 1 axis, and is parallel to 2 axes; in the dodecahedron each plane cuts 2 axes, and is parallel to a third; while in the octahedron each plane cuts the 3 axes. The names and definitions of the six crystalline systems are given below:— | 1. Regular system. | The 3 axes equal and rectangular. | | 2. Square prismatic s. 2 equal axes. | The 3 axes unequal, and rectangular. | | 3. Right prismatic s. All unequal. | | 4. Rhombohedral s. | The 3 axes equal, but not rectangular. | | 5. Oblique prismatic s. 1 axis rectangular to 2. | The 3 axes not equal, and not rectangular. | | 6. Doubly o.p.s. None rectangular. |