Guy, on Crystals of Arsenious Acid. 
53 
almost every specimen I examined. Hence^ it was clear that 
half- crystals were among the possibilities of arsenious acid 
sublimed. Well, this half-crystal which I was soon en- 
couraged to have made in glass, when placed in a certain 
position, gave me the precise figure which had perplexed me j 
it gave also the equilateral triangle with the half adjoining 
triangle for its base (one of the commonest crystalline forms) ; 
also, the half-triangle itself ; also the hexagon, and the 
hexagon tipped with three small, dark, triangular facettes. 
Now this appearance of a triangular prism, terminated at 
each end with an equilateral triangle, is given by the tilting 
forward of the half-crystal ; and just as the whole crystal 
adhering by a solid angle becomes by transmitted light a dark 
square, so this half-crystal appears as a dark oblong. 
But the long dark lines which I had taken for needles or 
prisms, what were they ? Possibly not distinct and separate 
crystals, but only deceptive appearances like the dark squares 
and oblongs. Could they be the forward edges of large deep 
plates, owing their dark appearance to the same depth of 
crystalline mass? It was reserved for the binocular microscope 
to demonstrate this. On examining with this instrument a 
vast number of specimens, and passing under review thousands 
and thousands of crystals, I find many large hexagonal plates 
with their edges thrown forward, but very few prismatic 
crystals. I also find triangular plates of various thickness, 
square plates also of Varying substance, and a few rhombic 
and rhomboidal plates. But my catalogue is not yet exhausted. 
Before I made use of the binocular microscope, I thought that 
I had encountered one or two cubes ; but as the assertion that 
I had met with cubes was received somewhat incredulously, 
I looked for them in the field of the binocular with great 
interest. I found several figures which approached very 
closely to the cube, and in one instance encountered a perfect 
cubical crystal. I say this without any sort of hesitation. I 
have also more frequently met with the rhombic dodecahedron, 
and its mdcle, or twin- crystal. I have not yet seen a tetra- 
hedrorij though in one specimen obtained from Scheele^s green, 
and abounding in triangles less symmetrically formed than 
usual, I thought that I discerned the marks of the tetrahedron. 
Be this as it m ay, I am quite sure that this form of crystal should 
be set down among mere possibilities : I have not seen it in any 
one of many hundreds of specimens of crystalline deposit ob- 
tained from arsenious acid itself, or from the metal arsenic. 
It is probable that the deep triangular plates, which abound 
in some specimens, have been taken for tetrahedra. 
I have now briefly^ sketched the course of experiments, ob- 
