complicated compounds.

15. _Oxalate of Nickel_ (NiO, [=]O).--It is prepared by dissolving the
pure oxide of nickel in diluted hydrochloric acid. Evaporate to
dryness, dissolve in water, and precipitate with oxalate of ammonia.
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.

18. _Silver Foil._--A small piece of silver foil is used for the
purpose of detecting sulphur and the sulphides of the metals, which
impart a dark stain to it. If no silver foil is at hand, strips of
filtering paper, impregnated with acetate of lead, will answer in many
cases.

19. _Nitroprusside of Sodium_ (Fe^{2}Cy^{5}, NO^{5}, 2Na).--This is a
very delicate test for sulphur, and was discovered by Dr. Playfair.
This test has lately been examined with considerable ability by Prof.
J.W. Bailey, of West Point. If any sulphate or sulphide is heated by
the blowpipe upon charcoal with the carbonate of soda, and the fused
mass is placed on a watch-glass,