282 



SALTS. 



Define the 705. A third system is characterized by 



fourth^ inequality in three principal dimensions. 

 systems. The axis or lines connecting the solid an- 



gles in the octahedron, and joining the faces in the 

 prism, are all unequal. As each axes may be indefi- 

 nitely varied in this system, there is room wthin its 

 limits for still greater variety than before. The fourth 

 system differs from the third in an oblique position of 

 some one of the unequal axes. The student will 

 readily imagine certain oblique forms which it in- 

 cludes. The fifth system is characterized by an ob- 

 lique position of three unequal axes.* 



706. The regular system, which is pro- 



What are the f 



characteristics perly the first, has all its axes equal, and all 



/yst k emT Ular its an g les ri g ht angles.f The figures 

 which precede this paragraph, represent 

 some of its simpler forms. Those which follow, are 

 among its most interesting combinations. In the 

 last, the student will be able to select three distinct 

 kinds of surfaces. One of these sets, if enlarged to 

 the exclusion of the others, would produce a cube, 



* The variations of length and inclination of axis which correspond 

 to the different systems, may be beautifully illustrated to the eye by a 

 wooden frame work movable at the centre with threads connecting 

 the arms. 



f The first and sixth systems are made to change places in the above 

 ai'rangement, for the convenience of illustration, from the quartz crystal. 



