FUNDAMENTAL FORMS OF CRYSTALS. 



2fi 



I. The first system includes the cube (fig. 1 or la, the lat- 

 ter in outline ;) regular octahedron (fig. 2 ;) and the rhombic 

 1 la 2 



^^1~7P~ 



^ 



\ i 





_y 



^J i 







^^ 



dodecahedron (fig. 3 or 3a.) They are symmetrical solids 

 .hroughout, in all positions, being alike in having the height, 

 breadth and thickness equal ; their three axes, represented by 

 the dotted lines in the figures, are at right angles with one 

 another and equal. In the cube, the axes connect the cen- 

 ters of opposite faces ; in the octahedron and dodecahedron, 

 they connect the apices of solid angles. This is more fully 

 explained on a following page. 



The cube has its faces equal squares, and its angles all 

 right angles. 



The octahedron has its 8 faces equal equilateral triangles : 

 its edges are equal ; its plane angles are 60° ; its interfacial 

 angles (angles between adjacent faces) 109° 28'. 



The dodecahedron has its 12 faces equal rhombs ; the 

 edges are equal ; the plane angles of the faces are 109° 28' 

 and 70° 32' ; its interfacial angles are 120°. 



II. The second system includes the right square prism 

 4 5 6 





i p 



^1 





mL 



M 





1 



"X 



(figs. 4 and 5,) and square octahedron (fig. 0.) They have 

 wo equal lateral axes, and a vertical axis unequal to the 



What forms does the first system include 1 How are these forms 

 related 1 Describe the forms. What forms does the second system 

 »clude, and how are they related ? Describe the. forms. 



3 



