CRYSTAL SYSTEMS. 429 



thus obtain figs. 443 and 444. These are known as tetrahedron. The 

 pentagonal dodecahedron is another secondary form (fig. 445). 



Fig. 437. Octohedron Fig. 439. Cube with 



angles removeu. angles removed. 



Fig. 438. The cube. 



The cube, or hexahedron, the octohedron, and the rhombohedron are all 

 simple forms, being each bounded by equal and similar faces, or surfaces. We 

 can thus understand how certain primary or original natural forms of crystals 

 can be changed in appearance by connection. Of the various substances 

 crystallizing in this system we find salt, iron pyrites, gold, silver, copper, 

 and platinum, and the sulphide of lead called galena, in the cube or hexa- 





hedron form. The diamond and fluor-spar, alum, etc., appear in the first 

 form (i), fig. 436 (octohedron). The cube, we see, has six equal faces, eight 

 equal angles, and twelve equal edges* Galena, as will be observed from the 

 illustration herewith, shows this peculiarity in a very marked manner (fig. 446). 



,43 and 444.- Secondary forms of first system. ^ 445 _p entagonal do . 



decahedron. 



The .ttYW/^crystalline form is the HEXAGONAL, and in this system three 

 of the four axes are equal and in the same plane, inclined at an angle of 

 60, with a principal axis at right angles to the others. In crystals of this 

 system are found quartz and calc-spar. 



The third system is termed the QUADRATIC or the diametric. This form 

 has three axes, all at right angles, two beiiig equal and the other longer 



