sulphate of copper, soon becomes covered with a red powdery crust, which
is metallic copper; and ere long the solution is changed in colour from
blue to greenish-yellow. The action is simple; the iron merely displaces
the copper in the solution, in which a sulphate of iron is consequently
formed. This test is characteristic, and even of considerable delicacy.
At the same time other substances may cause a reddish encrustation on
iron by simply rusting it, so that the test cannot be relied on alone.
The four preceding reagents taken together are amply sufficient to prove
the existence of copper in a solution. Three other tests, however, may
be here briefly alluded to.
Caustic potass in a solution not too diluted causes a fine azure-blue
precipitate, the hydrated peroxide of copper.
Oxide of arsenic, with the previous addition of a few drops of ammonia,
causes a fine apple-green or grass-green precipitate, the arsenite of
copper. This test, which is both delicate and characteristic, has been
already fully considered under the head of Arsenic.
The process by fluid reagents, as hitherto laid down, merely proves the
presence of copper, but does not indicate the acid with which the oxide
is combined. In order to determine whether it is sulphuric acid, the
fluid must also be tested with nitrate of baryta followed by nitric
acid: a heavy white precipitate is thus produced, which the excess of
nitric acid does not redissolve.