Guy, on Crystals of Arsenious Acid. 57 



e. Six sides visible (fig. 22), 



/. The tnacle or twin- crystal of the rhombic 

 dodecahedron (fig. 23). 



ff. The mack or twin-crystal ; another view 



S4. 



(fig- 24). 4^ 



Having now figured some of the leading appearances which 

 the models of the octahedron and rhombic dodecahedron, 

 with their half- crystals, may be made to assume by changes 

 of position, I proceed to give a brief summary of the crystalline 

 forms which I have been able to distinctly recognise in the 

 course of my examinations of the sublimates of arsenious 

 acid. 



1. The crystalline sublimates of arsenious acid consist of 

 regular octahedra, rhombic dodecahedra, cubes, plates, and 

 prisms. 



2. The regular octahedra maybe entire and homogeneous, 

 or they may be variously truncated and notched, mottled and 

 figured ; and they may assume any of the forms depicted in 

 figures 1, 2, 3, 4, and 5. 



3. The entire regular octahedron may also be 

 modified as in the annexed engraving (fig. 25). 



4. The octahedron may present itself as a half- 

 crystal in any of the forms depicted in figures 7 

 to 15, inclusive. 



5. The half-crystals may be combined to form mdcles, or 

 twin-crystals, as in figures 16 and 17. 



6. The entire crystal and the half-ciystal may 



have their edges notched, so as to yield figures ^s 

 resembling the trefoil, or fleur-de-lis, as in the \ y 

 annexed figure (fig. 26). \J 



7. The rhombic dodecahedron may present 



itself entire in any of the forms depicted in figures 18 to 22. 



8. The rhombic dodecahedron may present itself as a half- 

 crystal ; and two half-crystals may be united to produce the 

 mdcles, or twin-crystals depicted in figures 23 and 24. 



9. The cube is a very rare form among the crystals of 

 arsenious acid. 



10. The plates present themselves as hexagons, equilateral 

 triangles, squares, rhombs, and rhomboids ; and they may be 

 of any thickness, from that of thin iridescent films, to the 



