MODIFICATIONS OF CRYSTALS 



fez from the inferior in front, two being obtuse and two acute ; 

 consequently, they are independently replaced. Figure 56. 

 shows the replacement of the obtuse basal. So also the front 

 angles differ in the same manner, the upper (left side in fig. 

 57) being independent of the inferior in its modifications. 

 56 57 58 ' 59 



But the four lateral angles are similar (fig. 58.) Two of the 

 lateral edges are obtuse and two acute, as in the right rhom- 

 bic prism, and their secondary planes are therefore unlike 

 (fig. 59.) 



60 In the oblique rhomboidal prism, 



only two diagonally opposite edges 

 or angles are similar, and the modi- 

 fications of one edge are therefore 

 independent of those of all the other 

 edges, except the one diagonally 

 opposite : the same is true of the 

 angles. The difference between this prism and the oblique 

 rhombic will thus be fieen on comparing figures 56 and 60, 

 and also figures 58 and 61 : 



In the rhombohedron, the distinction of vertical and lateral 

 solid angles has already been explained, and also the differ- 

 ence between the terminal and lateral edges. The figures 

 given will show, how these distinctions are carried out in the 

 62 63 64 65 



modifications. In figure 62, the terminal solid angles are 

 replaced, but none of the lateral. In figures 64, 65 and 29, 

 the lateral angles are replaced, but not the terminal. Figure 

 63, has the terminal edges replaced, and figures 68 and 28, 

 the lateral edges. 



Explain the laws with regard to secondary planes from the oblique 

 rhombic prism ; oblique rhomboidal ; the rhombohedron. 



