428 



SCIENTIFIC RECREATIONS. 



,at right angles, is represented by fig. 436. This crystal is limited by eight 

 equilateral triangles. It has twelve edges and six angles. If we describe a 

 line from any one angle to an opposite one, that line is called an axis, and 

 in the case before us there are three such axes, which intersect each other at 

 right angles.* Such crystals are regular octohedra. There are irregular 

 forms also, whose axes do not come at right angles, or they may be of 

 unequal length. The substances which we find crystallized in this form or 



Fig. 435. Stone quarry. 



system are the diamond, nearly all metals, chloride of sodium (salt), fluor- 

 spar, alum, etc. 



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 experi- 

 ment 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. 



We will exhibit the gradations. Suppose we cut 

 fig- 437 J 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 



* A crystal should be held so that one of its axes is vertical to the spectator. This axis 

 *' termed the principal axis, and when there is inequality the longest axis is the principal. 



Fig. 436. Regular octahe- 

 dron first system. 



