436 On Iridium and Osmium. [Dxc. 
The osmium. being collected at the, bottom of the vessel, the 
liquid must be carefully drawn off. Water is repeatedly poured 
upon the metal to wash it thoroughly. It is even proper in the 
first washings to employ water acidulated with a little sulphuric 
acid in order to dissolve any particles of zine which it may contain. 
Thus washed, the osmium is pure. It has only to be dried in a 
gentle heat, and then preserved in close vessels. 
§ VL. Separation of the Chromic Acid from the Residue from which 
the Osmium was distilled. 
The chromium and osmium being dissolved in the same liquid, 
but the one being fixed and the other volatile, we had recourse to 
heat to separate them from each other. The chromium of course 
remains in the residue combined with potash. To obtain this metal, 
the liquid is poured out of the retort, evaporated to dryness in a 
capsule, redissolved in water, and filtered, to separate any silica 
which it may contain. 7 
Then pronitrate of mercury is poured into the solution.” A red 
precipitate falls, which is prochlorate of mercury. This precipitate 
is washed with a great deal of water. It is then dried, and calcined 
in acrucible, to obtain the green oxide of chromium. . 
When in our first experiments we employed potash to decompose 
the black powder from platinum, we were obliged to recommence 
the operation four or five times, in order to dissolve all the chro- 
mium and the osmium. But when nitrate of potash, in the pro- 
portion of two parts of the salt to one of the powder, is employed 
for the purpose, two operations are sufficient, because the oxygen 
contained in this salt is: suflicient to oxidate the two metals; and 
the potash which becomes free is capable of dissolving them. 1 
therefore recommend nitre instead of potash in this operation. 
§ VII. Treatment of the Black Powder from which the Chromium 
and Osmium have been separated by Muriatic Acid. 
After having well washed the black powder from platinum treated 
with nitre, a little of it is put into muriatic acid diluted with its 
bulk of water. If this mixture, when heated a little, gives out the 
smell of osmium, the powder must be treated again with half its 
weight of nitre, following the same process as before. 
The residue, which has a bluish-black colour, is now to be 
digested in moderately concentrated muriatic acid. An action 
immediately takes place, which is announced by the elevation of the 
temperature of the mixture, and by the green colour which the 
Tiguid acquires. oe 
Sometimes during this operation the smell of oxymuriatic acid is 
perceived, and a small quantity of it seems to be formed ;, for 
having heated the mixture in a retort furnished with ‘a receiver, } 
Obtained a liquid, which speedily destroyed the tincture of litmus. 
When the acid in excess has remained for some days on the 
