60 
On the Microscopic Characters of the Crystals 0/ Arseni- 
ous Acid. By William A. Guy, M.B. Cantab., Pro- 
fessor of Forensic Medicine, King^s College, London. 
(Read May 8th, 1861.) 
In submitting to your society this paper on the miscro- 
scopic characters of the crystals of arsenious acid, I have two 
principal objects in view. I wish, in the first place, to 
illustrate, by a striking instance, the great value of the 
binocular microscope as a means of diagnosis ; and, in the 
second place, to give a more exact account than any at 
present in existence of the crystalline forms assumed by 
a very important poison. That to render such an account 
is not a work of mere supererogation, a reference to the 
descriptions of the crystals given in works of authority would 
readily prove. 
Most authors describe the crystals as regular octahedra, 
without recognising any other crystalline forms. Some 
writers, however, speak of the regular octahedron and its 
modifications, or of forms traceable to the octahedron ; and 
acicular crystals, long prismatic needles, triangular and 
hexagonal plates, and even tetrahedra, are to be met with in 
the descriptions of authors."^ 
I may add that, in illustrated works, the octahedral 
crystals are usually figured in the form in which they are 
most readily identified ; the less usual positions of the octa- 
hedra and the rarer forms and modifications of the crystal 
being omitted. 
The imperfect and somewhat conflicting accounts thus 
given of the crystals of arsenious acid are, doubtless, to be 
explained, partly by the difficulty of examining them, whether 
by lens or microscope, when sublimed, as they were for- 
merly, in thick reduction-tubes of narrow bore ; partly to 
the great variety of lights and shadows presented by the 
crystals, especially when viewed by transmitted light; and 
partly to the imperfect relief given to the crystals when 
examined by the monocular microscope. 
* Consult Pereira's 'Materia Medica/ 4th edition, p. 685, in which 
the tetrahedron is mentioned as one form of the crystal; Miller's 'Elements 
of Chemistry/ part ii, p. 961, in which mention is made of long prismatic 
needles, isomorphous with tliose of oxide of antimony; and Taylor, on 
'Poisons,* 2d edition, p. 385, in which equilateral triangular plates are 
specified. Pereira cites a foreign authority (Wohler) who found in a 
cobalt roasting- furnace arsenious acid crystallised in hexahedral plates 
derived from a right rhombic prism. 
