26 



STRUCTURE OF MINERALS. 



lateral : that is, the width and breadth are equal, but the 

 height is varying. All the axes are at right angles with one 

 another. Fig. 4 is a square prism higher than its breadth, 

 and fig. 5 is one shorter than its breadth. 



The right square prism and square octahedron may be of 

 any height, either greater or less than the breadth ; but the 

 dimensions are fundamentally constant for the same mineral 

 species. The square prism has its base a square. The 

 square octahedron has its base (bb) a square, and its 8 faces 

 equal isosceles triangles. The lateral edges of the prism 

 differ in length from the basal ; and the terminal or pyra- 

 midal edges of the octahedron differ in length from the basal. 



III. The third system includes the rectangular prism 

 (fig. 7,) the rhombic prism (fig. 8,) and the rhombic octahe- 

 7 8 9 



dron (fig. 9.) They are similar in having the three dimen- 

 sions, or the three axes, unequal ; and the axes at right an- 

 gles with one another. 



The rectangular prism has a rectangular base, and the 

 axes connect the centers of opposite faces. The rhombic 

 prism and rhombic octahedron have each a rhombic base, 

 the angle of which differs for different species. The lateral 

 axes of the prism connect the centers of opposite edges, 

 and in the octahedron they connect the apices of opposite 

 angles. 



IV. The fourth system includes the right rhomboidal prism 

 10 11 12 13 



(figs. 10, 11,) and the oblique rhombic prism (figs. 12, 13.) 

 The lateral axes are unequal, and at right angles as in the 



What forms are included in the third system and how are they rela- 

 ted? Describe the forms. What forms does the fourth system include 

 and how are they related ? 



