Fig. 49
5. Triclinic system (Fig. 50). Three axes, all inclined to each other.
Fig. 50
6. Hexagonal system (Fig. 51). Three equal axes in the same plane intersecting at angles of 60°, and a fourth at right angles to all of these.
Fig. 51
Every crystal can be imagined to have its faces and angles arranged in a definite way around one of these systems of axes. A cube, for instance, is referred to Plan 1, an axis ending in the center of each face; while in a regular octohedron an axis ends in each solid angle. These forms are shown in Fig. 46. It will be seen that both of these figures belong to the same system, though they are very different in appearance. In the same way, many geometric forms may be derived from each of the systems, and the light lines about the axes in the drawings show two of the simplest forms of each of the systems.
In general a given substance always crystallizes in the same system, and two corresponding faces of each crystal of it always make the same angle with each other. A few substances, of which sulphur is an example, crystallize in two different systems, and the crystals differ in such physical properties as melting point and density. Such substances are said to be dimorphous.