naces, if the sulphur is to be recovered. Heap, stall or
shaft furnace roasting is not very satisfactory, as it is very difficult
to transform all the sulphide into oxide.

The conversion of copper sulphide into the chlorides may be accomplished
by calcining with common salt, or by treating the ores with ferrous
chloride and hydrochloric acid or with ferric chloride. The dry way is
best; the wet way is only employed when fuel is very dear, or when it is
absolutely necessary that no noxious vapours should escape into the
atmosphere. The dry method consists in an oxidizing roasting of the
ores, and a subsequent chloridizing roasting with either common salt or
_Abraumsalz_ in reverberatory or muffle furnaces. The bulk of the copper
is thus transformed into cupric chloride, little cuprous chloride being
obtained. This method had been long proposed by William Longmaid, Max
Schaffner, Becchi and Haupt, but was only introduced into England by the
labours of William Henderson, J. A. Phillips and others. The wet method
is employed at Rio Tinto, the particular variant being known as the
"Dotsch" process. This consists in stacking the broken ore in heaps and
adding a mixture of sodium sulphate and ferric chloride in the
proportions necessary for the entire conversion of the iron into ferric
sulphate. The heaps are moistened with ferric chloride solution, and the
reaction is maintained by the liquid percolating through the heap. The
liquid is run off at the base of the heaps into the precipitating tanks,
where the copper is thrown down by means of metallic iron. The ferrous
chloride formed at the same time is converted into ferric chloride which
can be used to moisten the heaps. This conversion is effected by
allowing the ferrous chloride liquors slowly to descend a tower, filled
with pieces of wood, coke or quartz, where it meets an ascending current
of chlorine.

The sulphate, oxide or chlorides, which are obtained from the
sulphuretted ores, are lixiviated and the metal precipitated in the same
manner as we have previously described.

The metal so obtained is known as "cement" copper. If it contains more
than 55% of copper it is directly refined, while if it contains a lower
percentage it is smelted with matte or calcined copper pyrites. The
chief impurities are basic salts of iron, free iron, graphite, and
sometimes silica, antimony and iron arsenates. Washing removes some of
these impurities, but some copper always passes in