1813.) On Iridium and Osmium. 447 
if it contain either iron, or titanium, or silica, or alumina, the 
blue oxide precipitates in a proportion relative to that of the sub- 
stance which is the determining cause of the precipitation: 
If it is oxide of iron or of titanium which is mixed with the 
solution of iridium, the precipitate formed by the alkalies is 
green ; but if it is merely silica or alumina, the precipitate is blue, 
with a shade of violet. That which is obtained by ‘barytes: is 
green. There can be no doubt that the precipitation is brought 
about by the action of these bodies on the oxide of iridium. The 
following experiment seems to me very good, as a demonstration of 
this. 1 mixed with a blue solution of iridium a small quantity of 
sulphate of alumina, and then added an excess of ammonia. A 
deep coloured precipitate fell; but the liquid continued still more 
intensely coloured. ‘The addition of a greater quantity of sulphate 
of alumina entirely discoloured it. ‘These precipitates of alumina 
and oxide of iridium cannot be deprived of their colour by repeated 
washings in boiling water. 
This great affinity of alumina for blue oxide of iridium, and the 
violet blue colour of the compound, gave me a strong suspicion 
that iridium is the colouring matter of the Oriental sapphire. | This 
metal might have escaped so much the more easily from the che- 
mists who analyzed that stone, as not more than a thousandth part of 
it would be wanting to form the deepest shade known in the sap- 
phire. If this substance were common, it would perhaps be pos- 
sible to make a beauiful blue colour of it for painters. 
Oxymuriatic acid decomposes ammoniaco-muriate of iridium. 
We have only to pass the acid gas into a vessel containing the salt 
mixed with water. ‘The salt disappears, and a gas is disengaged in 
bubbles, in proportion as the solution takes place. When the 
solution is complete, and when no more gas is given out even by 
the assistance of heat, ammonia is no longer found in the solution. 
At least no triple salt is obtained by condensing the liquid, and no 
ammonia is disengaged when the liquid is mixed with potash and 
distilled, We obtain merely a triple salt of iridium and potash. 
We may then by this operation obtain pure muriate of iridium. Ie 
has a yellowish red colour. 
Muriate of Iridium and Potash, 
This salt is always formed when a solution of muriate of potash 
is mixed with a solution of iridium; or when after exposing a 
mixture of iridium and potash toa red heat, the compound is dis- 
solved in muriatic acid. 
This salt has a purple colour so intense, that it appears black; 
but we may satisfy ourselves that it is really purple, by rubbing: it 
upon a sheet of white paper. Though this salt, which is very 
little soluble in water, gives but very small crystals, yet I have 
ascertained that they have a very distinct octoliedral shape. 
A hundred parts of these erystals being long exposed to a red 
heat, decrepitated, and were reduced to fifty parts. ‘They had: the 
