set the metal free, it must be remembered that sulphur itself will
reduce metallic lead from fused litharge, and that oxygen will similarly
set free the metal in fused lead sulphide. There is no impropriety in
describing sulphur as a reducing agent; but it is absurd to call oxygen
one. Some confusion will be avoided if these substances and those which
are opposite to them in property be classed as oxidising and
de-oxidising, sulphurising, and de-sulphurising agents. Most oxidising
agents also act as de-sulphurisers.

_The de-oxidising agents_ most in use are the following:--

~Charcoal.~--Powdered wood charcoal; it contains more or less
hygroscopic moisture and about 3 or 4 per cent. of ash. The rest may be
considered carbon. Carbon heated with metallic oxides takes the oxygen;
at low temperatures it forms carbon dioxide, and at higher ones, carbon
monoxide. Other conditions besides that of temperature have an influence
in producing these results; and as the quantity of charcoal required to
complete a definite reaction varies with these, it should be calculated
from the results of immediate experience rather than from theoretical
considerations.

~Flour.~--Ordinary wheat flour is convenient in use. On being heated it
gives off inflammable gases which have a certain reducing effect, and a
residue of finely divided carbon is left. It is likely to vary in the
quantity of moisture it contains. Two parts of flour should be used
where one part of charcoal would be otherwise required.

~Tartar.~--This is crude hydric potassic tartrate; the purified salt,
cream of tartar, may be used. On being heated it gives off inflammable
gases, and leaves a residue formed of potassic carbonate mixed with