Guy, 071 Crystals of Arsenious Acid. 
51 
The substitution of the modern form of reduction-tube, in 
which the vapours of arsenious acid are made to pass through 
a narrow glass tube with thin sides, has made the examination 
by the microscope more easy but the simple plan which 
I suggested about three years since, for obtaining the crystals 
on a flat surface, has offered still greater facilities, of which 
it is but natural that I should have largely availed myself. 
The knowledge of the subject thus obtained may be said 
to have been completed by the use of the binocular micro- 
scope. 
The most superficial and cursory examination of the 
first specimens obtained upon a flat surface sufiiced to con- 
vince me that very much remained to be done before our 
knowledge of the true crystalline characters of arsenious 
acid could be placed on a level with the practical importance 
of such knowledge. In the first place, it was quite clear that 
those descriptions which spoke only of the regular octahedron 
as the one proper form of the crystal were wholly inade- 
quate; and that even those which recognised, not only the 
perfect crystal, but all the forms traceable to the octa- 
hedron were still insufiicient. We ought to know what 
particular forms to look for. Again, it must be interesting, 
and might be practically important to know something more 
of the alleged acicular or prismatic crystals, of the triangular 
and hexagonal plates, and of the tetrahedra, described and 
figured in Pereira^s work. The crystallographer, too, could 
scarcely abstain from speculating on the possible occurrence 
among these octahedra of those other members of the re- 
gular system, the cube and the rhombic dodecahedron. 
Some, if not all, of these questions I hope to be able to 
answer, without proving tedious to those who have not the 
special interest in this subject which I have myself. Re- 
verting to my early examinations of the crystalline deposits 
of arsenious acid as obtained on a flat surface, I may state 
that I encountered many forms and 'appearances which I 
was not able to explain to my own satisfaction. When 
viewed by transmitted light, a large proportion of the 
crystals wore the appearance of dark squares, a smaller 
number of dark oblong figures, a still smaller number of 
long, thick, black lines. These latter, the long lines, I took 
to be the acicular or prismatic crystals described in books. 
The dark squares and oblongs were not so readily explained. 
Then, again, I encountered among the crystals transmitting 
or reflecting light, in addition to forms which might be 
merely difierent attitudes or postures of the regular octahe- 
dron, or of the truncated octahedron, or of the lengthened 
