OF ARTS AND SCIENCES. 59 



Crystals are stout, and have a section in general triangular. A per- 

 fect and easy cleavage parallel to {lOT} can be obtained, the lustre of 

 which is pearly. Prism faces, except sometimes those of {213}, are 

 remarkable for the absence of striations. The color of the mineral 

 varies slightly : some crystals are almost pure white ; others very light 

 amber, and these are transparent. The mean specific gravity of two 

 determinations gave the figures 3.01589. Fuses easily to a white 

 glass, and when decomposed with the mixture of bisulphate of potas- 

 sium and fluor-spar, imparts to the flame a green color, far more 

 intense than I have previously obtained with Tourmaline. 



IODIDE OF ARSENIC. 



In 1880 I undertook the study of the physical and chemical proper- 

 ties of iodide of arsenic. Professor J. P. Cooke had previously shown 

 (Proceedings of the American Academy, Vol. XIII.) that solutions of 

 iodide of antimony in bisulphide of carbon were oxidized when exposed 

 to sunlight, provided that air had free access. From the analogy of 

 the properties of arsenic and antimony, it was thought that the iodides 

 of these elements would exhibit the same behavior. My observations 

 in this investigation I have recorded below. 



Iodide of arsenic is very easily prepared by shaking up in a flask a 

 solution of iodine in bisulphide of carbon with pulverized arsenic. By 

 repeated crystallizations from bisulphide of carbon, the resulting iodide 

 is purified, and is finally deposited in yellow-red hexagonal tables. 

 Since iodide of arsenic is more soluble in bisulphide of carbon than 

 iodide of antimony, the conditions are more favorable for obtaining 

 large and stout crystals than in the case of Sbl^ Solutions of AsT^ in 

 bisulphide of carbon are light red, but rapidly change in sunlight, the 

 oxidation closely resembling that of Sbl^ Free iodine is given off", but 

 remains dissolved in the bisulphide of carbon, coloring the solution dark 

 purple. Where the solution of iodide of arsenic has moistened the 



