178 Process for procuring pure Platinum, &c. 
cine it with nitrate of potash. With this view, we triturate the 
black powder with twice its weight of a mixture of three parts 
of nitre and one of caustic potash, and introduce the whole into 
a silver crucible, which is to be kept at a cherry-red heat for 
half an hour. In consequence of the affinity of the potash for 
the oxides of osmium and iridium, the nitric acid of the nitre is 
decomposed, and oxidizes these metals. ‘The crucible is to be 
withdrawn from the fire, allowed to cool, and cold water is then 
poured on the materials. This dissolves the potash, the whole 
oxide of osmium, and a little of the oxide of iridium. The whole 
being thrown on a filter, the oxide of iridium remains above, 
which is to be washed and dried. 
22. The filtered liquor which contains the combination of pot- 
ash and oxide of osmium, as well as a little oxide of iridium, is 
put into a flask, and saturated with nitric acid. ‘The liquid is 
then put into a retort, to which is fitted a tubulated globe, sur- 
rounded with moistened cloths. On distilling, the water which 
rises in vapour carries with it all the oxide of osmium. When 
the liquid is two-thirds drawn over, the whole osmium is usually 
volatilized. The liquid remaining in the retort contains the ni- 
trate of potash, and a trace of iridium. The aqueous solution 
of osmium is as colourless and limpid as distilled water. It has 
a strong and peculiar odour, extremely irritating to the nostrils, 
and which it is dangerous to inhale for any length of time. In 
order to obtain the osmium from this solution, it is put into a 
matrass, and we add a little muriatic acid to acidulate it slightly, 
and then insert a plate of pure zinc. The oxide of osmium is 
decomposed by the zinc, which is dissolved in the muriatic acid, 
and the osmium is precipitated to the bottom of the liquor in 
the form of a blackish-blue powder. When the oxide of osmium 
is completely decomposed, which may be recognised by the li- 
quid losing its odour, we decant the fluid, pour the powder of os- 
mium on a filter, wash it copiously with water, dry it, and put it 
immediately up in a well-stopped phial. 
23. The oxide of iridium, proceeding from the calcination of 
the black powder with nitre and potash, which remained on the 
filter, is by no means pure. It is a mixture of oxide of iridium, 
of a certain quantity of the black powder, or alloy of osmium and 
iridium, which has not been affected by the nitre, and a little 
oxide of silver, derived from the crucible. This mixture is to be 
treated with nitro-muriatic acid, which dissolves only the oxide 
of iridium, converts the oxide of silver into a ehloride, and does 
not act on the alloy. We next filter and wash. The unattacked 
alloy, and the chloride of silver, remain on the filter. | This re- 
siduum is to be washed with water containing a little saat 
which 
