LEAD FROM LEAD OXIDE. 15
atmosphere have been isolated, and sulphur is obtained from the associated
minerals. In other cases, the elements are dissolved out from mixtures by
means of suitable solvents, e.g., gold and silver by mercury.
OXIDES ABB REDUCED. The usual reducing agents are hydrogen, car-
bon or carbon monoxide (No. 1), aluminium (Nos. 2 to 5), magnesium,
sodium, potassium cyanide (Nos. 6 and 7), substances in solution, such as
sulphurous acid (No. 8) and finally the cathodic action of the electric current,
an agent of very general application. Sulphides are either changed to oxides
by roasting, or they are smelted with iron whereby the metal is formed in
the presence of a slag consisting chiefly of ferrous sulphide (Nos. 10, 11).
Halides may be decomposed by metallic sodium or the fused salts may be
electrolyzed (No. 14).
From salts containing oxygen, the elements are usually obtained by an
indirect method, as for example by first forming the oxides. Examples
of direct reduction, however, are the deposition of copper from copper sul-
phate solution by means of iron, the corresponding precipitation of silver
by copper, and the preparation of phosphorus by the reduction of acid cal-
cium phosphate with carbon at a high temperature.
REDUCTION OF OXIDES BY CABBON.
Technically, carbon is the most important reducing agent; its first
product of combustion, carbon monoxide, also has a reducing power since
it is readily oxidized further to carbon dioxide; in fact this latter action is
often the most important, as for example in the blast-furnace process for the
reduction of iron from its ores. An example of the action of carbon mon-
oxide at lower temperatures is the reduction of gold chloride solutions
(cf. No. 25). The dissociation of carbon dioxide at high temperatures,
2 CO2 = 2 CO + O2, takes place in opposition to the combustion of carbon
monoxide, 2 CO + O2 = 2 CO2. Accurate experiments have recently shown
that this dissociation becomes appreciable at above 1500° and increases
rapidly with further rise of temperature, with the result that the combustion
of carbon monoxide, and consequently its reducing effect, becomes dimin-
ished. This contradicts the opinion which formerly prevailed to the effect
that increase of temperature always favors such technical reduction processes.
- Lead from Lead Oxide.
Place a mixture of 50 g. litharge and 3 g. of very fine, sifted,
wood-charcoal in a porcelain crucible, which is from two-thirds
to three-fourths filled thereby, cover the mixture with 3 g. of
powdered borax glass and heat strongly over the blast lamp.
When, after about half an hour, the reduction is complete, pour
the reduced lead upon an inverted porcelain crucible cover which
has been previously heated so that the hot lead will not crack it.