he most efficient agents for the reduction of
metallic oxides. It separates not only the metals from their oxygen
compounds, but likewise from their sulphur compounds, while it is
converted through the action of the oxygen into carbonate of potash,
or, in the latter case, combines with the sulphur and forms the
sulphureted cyanide of potassium. This separation is facilitated by
its easy fusibility. But in many cases it melts too freely, and
therefore it is better to mix it, for blowpipe analysis, with an equal
quantity of soda. This mixture has great powers of reduction, and it
is easily absorbed by the charcoal, while the globules of reduced
metal are visible in the greatest purity.

PREPARATION.--Deprive the ferrocyanide of potassium (2KCy + FeCy) of
its water by heating it over the spirit-lamp in a porcelain dish. Mix
eight parts of this anhydrous salt with three parts of dry carbonate
of potash, and fuse the mixture by a low red heat in a Hessian, or
still better, in an iron crucible with a cover, until the mass flows
quiet and clear, and a sample taken up with an iron spatula appears
perfectly white. Pour the clear mass out into a china or porcelain
dish or an iron plate, but with caution that the fine iron particles
which have settled to the bottom, do not mix with it. The white fused
mass must be powdered, and kept from the air. The cyanide of potassium
thus prepared, contains some of the cyanate of potassa, but the
admixture does not deteriorate it for blowpipe use. It must be
perfectly white, free from iron, charcoal, and sulphide of potassium.
The solution of it in water must give a white precipitate with a
solution of lead, and when neutralized with hydrochloric acid, and
evaporated to dryness, it must not give an insoluble residue by
dissolving it again in water.

6. _Nitrate of Potassa, Saltpetre_ (KO, NO^{5}).--Saturate boiling
water with commercial saltpetre, filter while hot in a beaker glass,
which is to be placed in cold water, and stir while the solution is
cooling. The greater part of the saltpetre will crystallize in very
fine crystals. Place these crystals upon a filter, and wash them with
a little cold water, until a solution of nitrate of silver ceases to
exhibit any reaction upon the filtrate. These crystals must be dried
and powdered.

Saltpetre, when heated with substances easy of oxidation, yields its
oxygen quite readily, and is, therefore, a powerful means of
oxidation. In blowpipe an