1815.) On Iridium and Osmium. 435 
§ IV. Saturation and Distillation of the Alkaline Liquid. 
When the black powder communicates no longer. to. the alkali, 
either osmium or chromium, the alkaline liquids mixed together are 
treated in the following way :— 
1. The excess of alkali is gradually saturated with. nitric acid, 
taking great care not to add an excess of acid, least.a portion of the 
precipitate should be redissolved. A flocky precipitate of a bottle- 
green colour appears, the composition of which has been already 
mentioned ; and the liquid, which before had an orange-red colour, 
assumes that of a pure yellow. 
2. The yellow liquid, being filtered, contains only chromate of 
potash and oxide of osmium. To separate them, a little nitric acid 
is added to the liquid, to destroy the combination of the potash with 
the oxide of osmium. It is then distilled till no more osmium 
comes over. This is easily ascertained, by separating the product 
from time to time, and observing when it has no smell. ‘To collect 
the oxide of osmium, which is very volatile, it is necessary to sur- 
round the receiver with ice, or at least with cold water frequently 
renewed. 
The liquid obtained is colourless, like water, but easily distin- 
guished by the strong odour which it exhales, and the metallic taste 
which it possesses. 
Sometimes during this distillation there forms at the neck of the 
retort, and even within the receiver, a black matter, having in cer- 
tain aspects a coppery lustre. By means of water it may be de- 
tached in small brilliant scales. We shall return afterwards to this 
matter. 
§ V. Precipitation of the Osmium. 
When we wish to separate the osmium from the water, a little 
muriatic acid is added, and a plate of zinc introduced. If the 
quantity of muriatic acid is considerable, we observe a blue sub- 
stance, which detaches itself from the surface of the zinc in the 
form of clouds, and which, by dissolving as it spreads through the 
liquid, communicates a purple tint to the whole. But when the 
muriatic acid is diminished to a certain point, the liquid assumes a 
fine indigo-blue colour. The blue matter at last separates from the 
liquid in the form of flocks, which, when they are united together, 
appear black, and the liquid becomes colourless as water, 
It may happen that the quantity of muriatic acid added is not 
sufficient to precipitate the whole of the osmium. This will be 
easily perceived by the effervescence ceasing before the liquid has 
Jost its colour and its smell. In such a case a small additional quan- 
tity of acid must be poured in, to keep up a slight effervescence, 
and to prevent the osmium from attaching itself to the zinc, from 
which it would be very difficult to separate it afterwards by ‘rnecha- 
nical means. 
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