Guy, on Crystals of Arsenious Acid. 
57 
e. Six sides visible (fig. 32). 
/. The made or twin- crystal of the rhombic r-- 
dodecahedron (fig. 23). 
ff. The made or twin-crystal ; another view 
(fig. 24). 
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 may be 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) . A -Vk 
4. The octahedron may present itself as a half- \ r 
crystal in any of the forms depicted in figures 7 
to 15, inclusive. 
5. The half-crystals may be combined to form mddes, or 
twin- crystals, as in figures 16 and 17. 
6. The entire crystal and the half-crystal may 
have their edges notched, so as to yield figures 
resembling the trefoil, or fleur-de-lis, as in the 
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 
mddes, 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 
