56 



Guy, on Crystals of Arsenious Acid. 



The first section supplies the following forms : 



a. The equilateral triangle (fig. 10). 



b. The equilateral triangle resting on 

 half the adjoining triangle as a 

 base (fig. 11). 



This is a very common aspect of the 

 half-crystal. 



c. The hexagon, (fig. 12.) 



d. The hexagon with the three small 

 triangular facettes in shadow (fig. 

 13). 



This also is a very common aspect of the 

 half- crystal. 



e. The half- triangle (fig. 14.) 

 /. The edge of the half- crystal tilted 



forwardjSO as to give the appearance 

 of a triangular prism (fig. 15). 

 -A xifA This again is a very common aspect of 



H ^i/jH ^^3 half-crystal. 



P^ ^|m • g rjAj^g rndcle or twin -crystal, formed by 



the partial rotation of two half- 

 crystals on each other (fig. 16). 

 h. The same, with the triangular face of 

 one half- crystal visible (fig. 17). 

 The second and third sections are of rare occurrence, and 

 do not assume appearances requiring more minute description. 



3. The rhombic dodecahedron. 



a. Three sides visible, so as to resemble the 

 perspective of a cube (fig. 18). 



b. Four sides ^isible, and a solid angle pro- 

 jected forward, (fig. 19) . 



27 



« 



c. Five sides visible (fig. 20). 



d. Five sides visible ; another aspect of the 

 crj^stal (fig. 21). 



