240 
Selected Articles. 
and iridium, part in the form of a deep purple powder, and 
part in regular octahsedrons. 
The other washings are evaporated to dryness in porcelain 
capsules: the residue is heated to redness, and then treated 
with aqua regia. The platina obtained from these washings 
is rarely pure, and must be re-dissolved, as otherwise it would 
not be malleable. When this spongy platina is dissolved a se- 
cond time, a little iridium is generally found in the state of an 
insoluble residue. 
The differences between this process and that of Wollas- 
ton are, that this chemist uses aqua fortis, and dilutes the hy^ 
drochloric acid with an equal proportion of water; by a di- 
gestion of the crude platina in this menstruum for three or four 
days, he avoids the solution of the iridium. It is possible, says 
M. Sobolewskoy, that this method may attain the desired end, 
on a small scale, but in large operations, it is indifferent whe- 
ther the first solutions contain less iridium, as the remainder 
of the crude platina will retain a much larger proportion, and 
its quantity will go on progressively increasing in each suc- 
ceeding solution, for the solubility of the iridium is in a direct 
ratio to the quantity of it, mixed with the crude platina* 
Moreover, experience has shown that the presence of iridium 
in solutions of platina does not injure the purity of this metal, 
if care be taken to keep the acid in excess, and if the preci- 
pitate be well washed. The employment of weak acids oc- 
casions a great loss of time. The concentrated acids, on the 
contrary, although dissolving a portion of iridium with the 
platina, offer the most simple and effectual means of extract- 
ing the latter from its combinations. 
The solution of one part of platina requires ten to fifteen 
parts of aqua regia, of the indicated strength. The propor- 
tion of acid to be employed must depend on the size of the 
grains of platina, and their texture. That of Tagil which is 
in large fragments requires more acid than the fine grains. 
The purified platina is forged in the following manner: the 
spongy mass, is reduced to powder in a brass mortar, by means 
of a pestle of the same, then passed through a fine sieve ; with 
this powder a cylindrical iron tube is filled, and subjected to 
