ON HYDRARGYRI IODIDUM VIRIDE, B. P. 
505 
Calomel.41*7 
Mercurous Iodide.35 3 
Mercury.23 0 
100*0 
Gmelin states that the ingredients must be used in exactly equivalent pro¬ 
portions, as any excess of iodide of potassium would separate metallic mercury. 
This would probably occur in accordance with the equation :— 
2HgI + KI=HgKI 3 +Hg. 
Soluble salt. 
But although I repeated the experiment several times, weighing the ingre¬ 
dients accurately, and digesting them together for different periods, in no case 
could I obtain anything like complete decomposition. The first portion of mer-^ 
curous iodide formed was apparently decomposed by the remainder of the iodide' 
of potassium in preference to the residual calomel. 
The results ot these experiments were consequently confirmatory of those 
previously obtained, and contributed to show that the green colour was really 
due to a mixture of mercurous iodide and metallic mercury. 
I then resorted to the last process given by Gmelin, namely the reaction of 
iodide of potassium on mercurous acetate. Equivalent quantities of crystallized 
mercurous acetate and iodides of potassium were triturated with water. The 
first effect was the production of a nearly black precipitate, which gradually 
became green, and, after digesting a day or two, passed to a light yellowish- 
green. The appearance of this precipitate was again different to that of either 
of the other iodides I had prepared. Upon testing it, I found it to contain a 
considerable quantity of undecomposed mercurous acetate ; consequently, free 
metallic mercury was present in it. 
The imperfect decomposition of the calomel and mercurous acetate I thought 
possibly arose from employing them in the undissolved condition. Of course 
calomel could not be dissolved, but mercurous acetate is, to a certain extent, 
soluble. We cannot employ hot water for the purpose without decomposing a 
portion of the acetate, and converting it into a mercuric salt. I therefore pre¬ 
pared a cold aqueous solution of the mercurous acetate, and tried the effect of 
mixing it with a very dilute solution of iodide of potassium. Taking care to 
keep the acetate in slight excess, I succeeded in obtaining a pure yellow preci¬ 
pitate, but, owing to the very slight solubility of mercurous acetate, the quan¬ 
tity of precipitate I obtained was so small that I did not submit it to analysis, 
and am therefore unable to say whether it was pure mercurous iodide or the 
mercuroso-mercuric iodide of Gmelin. 
This latter compound I have not yet studied in comparison with the yellow 
mercurous iodide, but I am making a few experiments for that purpose. 
It results, then, from what I have stated, that by none of the processes indi¬ 
cated could I succeed in obtaining pure mercurous iodide, possessed of a green 
colour ; that by any of the reactions 1 could obtain a green product, but that 
then it always contained free metallic mercury. Moreover, that these green 
products were never alike in colour, but varied from a dull dark green to light 
yellowish-green. 
It also results that to obtain pure mercurous iodide by the process given in 
the Pharmacopoeia, the trituration should be continued until the green mass 
which forms becomes dry, and assumes a dull yellow colour. So obtained, the 
product is permanent. It still contains small quantities of mercuric iodide and 
free mercury ; the former may, and perhaps ought, to be removed by washing 
with alcohol or ether. 
