the whole of the sulphur is fully oxidised; but in many cases some
arsenic is apt to escape, and to give rise to a peculiar garlic-like
odour. The sulphates of soda and potash are thus formed, and float as a
watery liquid on the surface of the slag.

~Red lead~ is an oxide of lead. About one-quarter of its oxygen is very
loosely held, and, hence, is available for oxidising purposes, without
any separation of metallic lead. The rest of the oxygen is also
available; but for each part of oxygen given off, about 13 parts of
metallic lead are deposited. In silver assays this power of readily
giving up oxygen is made use of. The residual oxide (litharge) acts as a
flux.

~Hot air~ is the oxidising agent in roasting operations. The sulphur and
arsenic of such minerals as mispickel and pyrites are oxidised by the
hot air and pass off as sulphur dioxide and "white arsenic." The metals
generally remain in the form of oxide, mixed with more or less sulphate
and arsenate. The residue may remain as a powdery substance (a calx), in
which case the process of roasting is termed calcination; or it may be a
pasty mass or liquid. In the calcination of somewhat fusible minerals,
the roasting should be done at a low temperature to avoid clotting;
arsenic and sulphur being with difficulty burnt off from the clotted
mineral. A low temperature, however, favours the formation of sulphates;
and these (if not removed) would reappear in a subsequent reduction as
sulphides. These sulphates may be decomposed by a higher temperature
towards the end of the operation; their removal is rendered more certain
by rubbing up the calx with some culm and re-roasting, or by strongly
heating the calx after the addition of solid ammonic carbonate. In
roasting operations, as large a surface of the substance as possible
should be exposed to the air. If done in a crucible, the crucible should