ON THE IODIDES OP ARSENIC. 
99 
It should be preserved in glass-stopt vials. The same pro- 
cess, with the addition of another equivalent of iodine, insures 
the production of the duo-semi, (alias per,) iodide of arse- 
nic. The fusion of this latter iodide requires nicer manage- 
ment of heat, and, on account of its greater capacity for va- 
porisation, this should be still more rapidly conducted. The 
same phenomena attend its progress, and it diners in its phy- 
sical aspect from the sesqu'iodide only in color, which is two 
or three shades darker. Perhaps a slight excess of iodine is 
preferable during combination, to avoid any risk of free arse- 
nic being retained in the solution. 
Although M. Plisson has, (as far as my knowledge ex- 
tends,) announced but one iodide of arsenic, there can be no 
doubt of the existence of another, as I have proved by both 
synthesis and analysis. 
These iodides are positive elementary compounds, con- 
taining no equivalent of water. When exposed to the ac- 
tion of light and air, they gradually attract moisture, but I 
have not noticed any sensible change of color. They are va- 
porisable at the ordinary temperature to some extent ; and if 
the exposure is sufficiently prolonged, they entirely disap- 
pear. 
When treated with water in quantities too small to effect 
solution, they are decomposed: — Iodo-hydric, and arsenious 
acids are formed, mingled with undecomposed portions of the 
salt. 
In water sufficient to effect the solution, the sesqu'iodide is, 
according to M. Plisson, converted into a neutral hydriodate; 
" although," says he, "the liquor reddens violently the color 
of tournesol;" — an action which he attributes to the weak sa- 
turating properties of the arsenious acid. 
These iodides may be prepared by bringing the dry ele- 
ments into contact, in a suitable flask, or tube, and applying 
heat. They are easily sublimed during the process, but there 
is invariably a greater or less degree of decomposition attend- 
ing this mode. The moist way is not only the safest, but 
decidedly the best mode; and its use is preferred by M. Plis- 
