28 STRUCTURE OF MINERALS. 



The lateral edges in figure 17a, are distinguished fkip 

 the terminal by being made heavier. Figure 19 repie- 

 sents a*vertical view of fig. 16 ; 

 the three edges meeting at center 

 are the terminal edges of one ex- 

 tremity : the exterior six are the 

 lateral edges ; and the six lateral 

 angles are seen at their intersec- 

 tions. In fig. 19a, the same is 

 seen in outline, and the dotted lines represent the three late 

 ral or transverse axes, connecting the centers of opposite 

 lateral edges. The lateral and terminal edges differ in one 

 set being acute and the other obtuse ; in the obtuse rhombo- 

 hedron (fig. 16) the terminals are obtuse, and in the acute 

 rhombohedron (fig. 17) they are acute. 



Several of the primary forms are easily cut from wood oi 

 chalk. Cut out a square stick, and then saw off a piece 

 from one end as long as the breadth of the stick : this is the 

 cube. Saw off other pieces longer or shorter than this, anc 

 they are different right square prisms. Shave off a piece 

 of more or less thickness from one side of the square stick, 

 and it then becomes a rectangular stick. From it, pieces 

 may be sawn off, of different lengths, and they will be right 

 rectangular prisms. Next cut a stick of a rhombic shape, 

 (a section having the shape in figure C, page 26,) from it 

 right rhombic prisms may be cut, of any length. Shave off 

 more or less from one side of the rhombic stick, and it is 

 changed to a rhomboidal form, (section as in fig. D, page 26,) 

 and rhomboidal prisms may be sawn from it of any length. 

 Take a rhombic stick again ; and instead of sawing it off 

 straight across, as before, saw off the end obliquely from one 

 side -edge to the opposite ; the base thus formed is oblique 

 to the sides : then saw the stick again in parallel oblique di- 

 rections, (accurately parallel,) and an oblique rhombic prism 

 will be obtained. If the oblique direction is such that the 

 basal plane equals the lateral, the solid is a rhombohedron. 

 Proceeding in the same way with a rhomboidal stick, oblique 

 rhomboidal prisms may be made. The student is advised to 

 make these solids, either from wood, raw potatoes, or chalk,* 

 in order to become familiar with them. 



What is said of the lateral edges and angles of the rhombohedron ? 



* Models made of chalk become quite hard if washed over with a 

 Birong solution of gum Arabic, or varnish. 



