58 G-UY, on Cryslala of Arsenious Acid. 



third or the half of the diameters of the faces of the plates. 



They may also greatly exceed in size the largest crystals of 



2 7 the groups in which they 



O* I 1 y\ A ry are found. The principal 

 / \ \^ I J // forms are shown in the 



annexed figure (fig. 27). 



11. Sometimes compound plates of large size 

 and symmetrical form are found united at 

 angles corresponding with those of the faces 

 of the octahedron, as in fig. 28. At other 

 times they are grouped with great irregularity. 

 In other instances plates, such as the equi- 

 lateral triangle, are found built up by a hexa- 

 gonal plate symmetrically joined to three equi- 

 lateral triangles, as in fig. 29. 



12. The prisms are either four-sided prisms of 

 small size, or they are large four-sided rectangular 

 prisms terminated by four-sided pyramids 

 (fig. 30). 



13. Sometimes the prisms are shorter, and 

 present the form depicted in the subjoined figure 

 (fig. 31). 



To this detailed description it is only necessary to add that 

 there is great variety to be found in groups of crystals of 

 arsenious acid produced at the same time and in the same way. 

 In some groups the crystals are perfect, free from spot or 

 blemish, transparent, and brilliant ; in others, notched or 

 truncated, mottled and figured, and translucent; in some 

 the regular octahedron is the prevailing form, other forms be- 

 ing exceptional ; in others, plates predominte, and are nearly as 

 numerous as the crystals themselves ; occasionally equilateral 

 triangular plates occupy the whole field, to the exclusion of 

 almost all other forms. The lithographic plate (PL VI) ap- 

 pended to the paper, and showing the sublimates as they appear 

 by the monocular and binocular microscope, by transmitted 

 and reflected light, will give some idea of the variety of forms 

 which the crystals assume. 



The best mode of obtaining the crystals of arsenious acid 

 may be described in a few words. The apparatus required 

 consists of a spirit-lamp with small flame, specimen tubes of 

 small diameter and not exceeding an inch in length, and 

 slides or discs of crown glass. A few grains of arsenious acid 

 are placed in a clean and dry specimen tube, and this in a con- 

 venient holder, consisting of a slip of copper or brass punched or 

 drilled to receive it. The tube is to be held over the flame of 

 the lamp till the acid collects as crystals, or as a white powder. 



