60 PROCEEDINGS OF THE AMERICAN ACADEMY 



walls of the flask or containing vessel during the process of oxidation, 

 minute octahedrons of arsenious oxide are deposited. A white deposit 

 is also formed at the botton of the flask, but in quantities too small to 

 test. This deposit is probably -As.^O^. Now, this decomposition, un- 

 like that of Sbl^, is complete, and the chemical reaction may be expressed 

 by the formula, — 



4 Asl, -\- 0^ = 2 As^O, + 67-7. 



The reaction for the oxidation of Sbl^ in a similar way, as given in 

 the paper above alluded to, is as follows : — 



SbT^ 4- 0=SbOI-{- I-I. 



The oxidation of iodide of arsenic, therefore, goes a step farther, so 

 that instead of oxi-iodide, thei'e is formed an oxide of arsenic. 



After the solution has been filtered from the oxide, distilled several 

 times with fresh bisulphide of carbon, and then allowed to crystallize, 

 no new modification of Aslg could be detected. In like manner, there 

 is no variety corresponding to the yellow trimetric iodide of antimony. 

 Iodide of arsenic sublimes easily in yellow-red leaves, with an hexago- 

 nal outline, which, when examined with the polariscope, exhibit the phe- 

 nomena of optically uniaxial crystals, with a negative double refraction. 



The melting point of iodide of arsenic is 138i^°-139°, about 28° 

 lower than that of Sbl^ (hexagonal variety), which is given 167°. 



Iodide of arsenic crystallizes in the hexagonal system, and is isomor- 

 phous with SbL (red variety). The crystals of both substances consist 

 of a rhombohedron modified by the first acute rhombohedron and basal 

 plane, and parallel to this latter form perfect cleavages are very easily 

 obtained. The crystals which I measured were prepared by the method 

 above described, and also by crystallizing a German preparation of the 

 same iodide from bisulphide of carbon. 



Forms {100} {lOTl} + i? ^ 



{110^ {IT02} _|7? ( Figs. 5 and 6. 



{Ill} {0001} oP ) 



Faces of the form {110} were more perfectly developed than those of 

 1 100}; in consequence of this the fundamental angle (110) A (HI) ^^'^'^^ 

 taken for calculation. Fundamental angle 110 A 1 H = 59° 48' 27". 

 The axes of Miller's system make with each other the angle 51° 38' 

 (54° 40' for iodide of antimony) ; the vertical axis of Naumann's sys- 

 tem c = 2.9796 (c = 2.769 for Sbl^). In the following summary of 



