e being simultaneously liberated from the carbonate.
In the original form of the Douglas-Hunt process, ferrous chloride was
formed by the interaction of sodium chloride (common salt) with ferrous
sulphate (green vitriol), the sodium sulphate formed at the same time
being removed by crystallization. The ground ore was stirred with this
solution at 70 deg. C. in wooden tubs until all the copper was dissolved.
The liquor was then filtered from the iron oxides, and the filtrate
treated with scrap iron, which precipitated the copper and reformed
ferrous chloride, which could be used in the first stage of the process.
The advantage of this method rests chiefly on the small amount of iron
required; but its disadvantages are that any silver present in the ores
goes into solution, the formation of basic salts, and the difficulty of
filtering from the iron oxides. A modification of the method was
designed to remedy these defects. The ore is first treated with dilute
sulphuric acid, and then ferrous or calcium chloride added, thus forming
copper chlorides. If calcium chloride be used the precipitated calcium
sulphate must be removed by filtration. Sulphur dioxide is then blown
in, and the precipitate is treated with iron, which produces metallic
copper, or milk of lime, which produces cuprous oxide. Hot air is blown
into the filtrate, which contains ferrous or calcium chlorides, to expel
the excess of sulphur dioxide, and the liquid can then be used again. In
this process ("new Douglas-Hunt") there are no iron oxides formed, the
silver is not dissolved, and the quantity of iron necessary is
relatively small, since all the copper is in the cuprous condition. It
is not used in the treatment of ores, but finds application in the case
of calcined argentiferous lead and copper mattes.

The precipitation of the copper from the solution, in which it is
present as sulphate, or as cuprous and cupric chlorides, is generally
effected by metallic iron. Either wrought, pig, iron sponge or iron bars
are employed, and it is important to notice that the form in which the
copper is precipitated, and also the time taken for the separation,
largely depend upon the condition in which the iron is applied. Spongy
iron acts most rapidly, and after this follow iron turnings and then
sheet clippings. Other precipitants such as sulphuretted hydrogen and
solutions of sulphides, which precipitate the copper as sulphides, and
milk of lime, which gives copper o