85 



,-itli our ot the i oonlmatr-a\t >, or if it be situated in one of 

 nlinatt --plan. -. etc., or if that face be. parallel to a coordinate- 

 i] to a coordinate-plane, then the symmetrical repetition of 

 it tare will determine a simple form of each crystal-class, which 

 > no longer agree with the most unrestricted, general form of 

 iat class. These new simple forms, on the contrary, will possess less 

 lit ing faces than the most unrestricted one, and therefore will 

 ive a simpler shape and a simpler Millerian symbol. In the next 

 i bit a review is given of the special cases mentioned for every class 

 >i the regular system, and the corresponding Millerian indices for 

 /ery form are there indicated also. 



The constant forms, occurring in all classes of the system, are: 

 cube {100} , and the rhombicdodecahedron {no} , the octahedron 

 eventually being split up into a positive and a negative 

 trahedron. 



Besides the general forms [hkl] , in the fig. 90, the whole seru > 

 of simple forms which occur for determined values of h and k, 

 a iv reproduced here; the crystals in nature are of course generally 

 more or less complicated combinations of several of these simple 

 forms occurring at the same time, and usually with very different 

 relative development of the existent forms. 



