ON THE H YDR AUG YRO-BIN IODIDES OP POTASSIUM, ETC. 271 
but a very dilute solution of it will scarcely dissolve a larger 
proportion of the biniodide, and as the iodide of potassium of 
commerce, is seldom quite pure and dry, it not unfrequently 
happens that a little more is required. 
The double salt, may also be prepared by dissolving one 
equiv. bi-chloride, or per-nitrate of mercury, in a solution of 
four equiv. of iodide of potassium, evaporating to dryness and 
dissolving the double iodide from the hydro-chlorate or ni- 
trate of potassa, by means of alcohol. The process previously 
indicated is most eligible. When made from the per-nitrate 
of mercury, it is liable to contain nitre, and as this salt is 
nearly isomorphous with the double iodide, a considerable 
portion may crystallize with it without altering materially the 
appearance of the crystals. It may, however, be detected by 
its insolubility in strong alcohol. 
Composition of the yellow iodide of mercury, — When 
dissolving biniodide of mercury in a solution of iodide of 
potassium, a portion of the yellow or the green iodide of mer- 
cury, was found to precipitate. This is probably owing to the 
biniodide gradually loosing iodine on keeping — 5 to 10 per 
cent, was precipitated from some which had been kept some 
time, while from a portion which had been recently prepared 
little more than one per cent, was separated. 
If the Hgl 2 be in excess, the precipitate is the yellow io- 
dide, but if there be a slight excess of KI, it is the prot- 
iodide. 
The yellow iodide contains 3 atoms I, and 2 atoms Hg. 
It is stated in the chemical authorities which I have consulted, 
to be the sesqui-iodideof mercury. The readiness with which 
it is converted into Hgl 2 and Hgl, led to some further experi- 
ments with it. 
When sublimed, I have found it to be invariably converted 
into Hgl 2 and Hgl. 
Heated in a solution of chloride of sodium it is decomposed, 
the solution dissolving Hgl 2 , and leaving Hgl. 
When treated with repeated portions of strong boiling alco- 
