ISOMETRIC SYSTEM. 



27 



hi figs. 2 to 7. This perfect symmetry is almost never found 

 in actual crystals. 



52. 



53. 



^ H 



^^ 



! H 



H 



...--•"* 



s^ 



A cubic crystal has generally the form of a square prism (tig. 

 51 a stout one, fig. 52 another long and slender), or a rectangu- 

 lar prism (fig. 53). Tn such cases the crystal may still be 

 known to be a cube ; because, if so, the kind of surface and 

 kind of lustre on the six faces will be precisely alike ; and if 

 there is cubic cleavage it will be exactly equal in facility, in 

 three rectangular directions ; or if there is cleavage in four, or 

 six, directions, it will be equal in degree in the four, or the six, 

 directions, and have mutual inclinations corresponding with the 

 angles of the octahedron or dodecahedron; and thus the crys- 

 tal will show that it is isometric in system. 



The same shortening or lengthening of the crystal often dia» 



55. 



56. 



guises greatly the octahedron, dodecahedron, and other forma. 

 This is illustrated in the following figures : Fig. 54 shows the 



