28 



CRTSTALLOGEAPHT. 



form of the regular octahedron; 55, an octahedron lengthened 

 horizontally; 56, one shortened parallel to one cf the pairs 

 of faces; 57, one lengthened parallel to another pair, the 

 ultimate result of which obliterates two of the faces, and 

 places an acute solid angle in place of each. The solid is 

 then six-sided, and has rhombic faces whose plane angles are 

 120 s and 60°. The following figures illustrate corresponding 

 ehanges in the dodecahedron (fig. 58). In fig. 59 the dodeca- 



bedion is lengthened vertically, making a square prism with four- 

 sided pyramidal terminations. In 60, it is shortened vertically. 

 In 61 the dodecahedron is lengthened obliquely in the direction 

 of an octahedral axis, and in 62 it is shortened in the same 

 li'ection, making six-sided prisms with trihedral terminations. 



So again in the trapezohedron there are equally deceptive 

 f'rms arising from elongations and shortenings in the same two 

 directions. 



These distortions change the relative sizes of planes, but not 

 the values of angles. In crystals of the several forms repre- 

 sented in figs. 54 to 57, the inclinations are the same as in the 

 regular octahedron. There is the same constancy of angle in 

 Other distorted crystals. 



