Guy, on Crystals of Arsenious Acid. 53 



almost every specimen I examined. Hence, it was clear tliat 

 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 ; 

 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 made, or twin-crystal. I have not yet seen a tetra- 

 hedron; 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 may, 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- 



