PRIMARY FORMS. 



Fig. 23. 



Its plane angles =60. The inclination of the faces united by 

 its edges, as P on P', or P", Fig. 23. = 109 28' 16". The incli- 

 nation of the faces united by their angles, 

 as P on B = 70 31' 43". 



All its solid angles being similar, the regu- 

 lar Octahedron has a similar axis in three di- 

 rections. 



The regular Octahedron is one of the most 

 abundant forms among crystals. It is com- 

 mon in Magnetic Iron Ore, Spinel and Red 

 Oxide of Copper. 



<. 36. THE RHOMBIC DODECAHEDRON. 



The rhombic Dodecahedron is contained under twelve 

 equal rhombic faces. 



This solid has two kinds of solid angles, six Fig. 24. 



acute, and eight obtuse ; and two varieties of 

 plane angles. The six acute solid angles, and 

 which consist each of four acute plane angle?, 

 are opposite, two and two ; and the eight ob- 

 tuse solid angles, consisting each of three ob- 

 tuse angles, are also opposite, two and two. 



The mutual inclination of those faces uni- 

 ted by their edges, as P on P" Fig. 24. = 

 120. The inclination of those faces united by their acute angles, 

 as P on P' = 90. Its plane angles = 109 28' 16" and 70 31' 43". 



It has two dissimilar sets of axes passing through its centre ; one 

 set, as ab y passes through the pairs of opposite, acute solid angles; 

 the other, as cd, passes the obtuse solid angles. The former are 

 three, and the latter four, in number. When either of the axes 

 passing through the acute solid angles, is in a vertical situation, the 

 solid is in position. 



The rhombic Dodecahedron is of frequent occurrence among 

 crystal?, of which Garnet and Blende are examples. 



this denomination, the modifications they undergo furnish reasons for 

 adopting a particular base, and a particular axis in preference to the 

 others, except in the case of the regular Octahedron. The base being 

 determined upon, when we speak of the axis, we of course refer to the 

 one which is vertical ; otherwise, we mean any one of them... 



