60 



On the Microscopic Characters of the Crystals o/Arseni- 

 ous Acid. By William A. Guy, M.B. Cantab., Pro- 

 fessor of Forensic Medicine, King's College, London. 



(Read May 8th, ISGl.) 



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 the}' 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 rorm of the crystal; Miller's 'Elements 

 of Cliemistry,' part ii, p. 961, in which mention is made of long prismatic 

 jieenles, isomorphous with tiiose 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. 



