Guy, on Crystals of Arsemous 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 plau 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 sufficed 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 Avas 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 insufficient. 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 eai'ly 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 different attitudes or postures of the regular octahe- 

 dron, or of the truncated octahedron, or of the lengthened 



