Fig. 435.—Stone quarry.
When we say in this form we do not mean that all the minerals are shaped like the illustration (fig. 436). We shall at once see that the system admits of other shapes. For instance, a regular crystal may have been cut or rubbed (and the experiment can be made with a raw turnip). Suppose we cut off the angles in fig. 436; we then shall have a totally different appearance, and yet the crystal is the same, and by cutting that down we can obtain a cube (fig. 437). Take off its angles again we obtain a regular octohedron once more, as shown in the diagram opposite.
Fig. 436.—Regular octahedron—first system.
We will exhibit the gradations. Suppose we cut fig. 437; we will obtain (fig. 438) the cube. The next is merely the cube with angles and edges cut off; and if we proceed regularly we shall arrive at fig. 442, the rhombic dodecahedron, or twelve-sided figure, whose equal planes are rhombs.
We can, by taking away alternate angles or edges situated opposite, arrive at other secondary crystals. From the original octohedron we can thus obtain figs. 443 and 444. These are known as tetrahedron. The pentagonal dodecahedron is another secondary form (fig. 445).
Fig. 437.—Octohedron angles removed.
Fig. 438.—The cube.