The precipitate must be washed with caution upon a filter, and then
dried. It is employed in blowpipe analysis to detect salts of potassa
in the presence of sodium and lithium.


16. _Oxide of Copper_ (CuO).--Pure metallic copper is dissolved in
nitric acid. The solution is evaporated in a porcelain dish to
dryness, and gradually heated over a spirit-lamp, until the blue color
of the salt has disappeared and the mass presents a uniform black
color. The oxide of copper so prepared must be powdered, and preserved
in a vial. It serves to detect, in complicated compounds, minute
traces of chlorine.


17. _Antimoniate of Potassa_ (KO, SbO^{6}).--Mix four parts of the
bruised metal of antimony, with nine parts of saltpetre. Throw this
mixture, in small portions, into a red-hot Hessian crucible, and keep
it at a glowing heat for awhile after all the mixture is added. Boil
the cooled mass with water, and dry the residue. Take two parts of
this, and mix it with one part of dry carbonate of potassa, and expose
this to a red heat for about half an hour. Then wash the mass in cold
water, and boil the residue in water; filter, evaporate the filtrate
to dryness, and then, with a strong heat, render it free of water.
Powder it while it is warm, and preserve it in closed vials. It is
used for the detection of small quantities of charcoal in compound
substances, as it shares its oxygen with the carbonaceous matter, the
antimony becomes separated, and carbonate of potassa is produced,
which restores red litmus paper to blue, and effervesces with acids.