58 
Guy, 071 Crystals 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 
OA I 1 /\ A /7 found. The principal 
/ \ I I \} M /_/ 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). 
J 3. 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 (PI. 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. 
