other insoluble impurities collected at the bottom of
the trough up to the level of the lower side-tube, and were then run off
through a plug in the bottom into settling tanks, from which they were
removed for metallurgical treatment. The electrolyte was used until the
accumulation of iron in it was too great, but was mixed from time to
time with a little water acidulated by sulphuric acid. This process is
of historic interest, and in principle it is identical with that now
used. The modifications introduced have been chiefly in details, in
order to economize materials and labour, to ensure purity of product,
and to increase the rate of deposition.

The chemistry of the process has been studied by Martin Kiliani
(_Berg- und Huttenmannische Zeitung_, 1885, p. 249), who found that,
using the (low) current-density of 1.8 ampere per sq. ft. of cathode,
and an electrolyte containing 1-1/2 lb. of copper sulphate and 1/2 lb.
of sulphuric acid per gallon, all the gold, platinum and silver
present in the crude copper anode remain as metals, undissolved, in
the anode slime or mud, and all the lead remains there as sulphate,
formed by the action of the sulphuric acid (or SO4 ions); he found
also that arsenic forms arsenious oxide, which dissolves until the
solution is saturated, and then remains in the slime, from which on
long standing it gradually dissolves, after conversion by secondary
reactions into arsenic oxide; antimony forms a basic sulphate which in
part dissolves; bismuth partly dissolves and partly remains, but the
dissolved portion tends slowly to separate out as a basic salt which
becomes added to the slime; cuprous oxide, sulphide and selenides
remain in the slime, and very slowly pass into solution by simple
chemical action; tin partly dissolves (but in part separates again as
basic salt) and partly remains as basic sulphate and stannic oxide;
zinc, iron, nickel and cobalt pass into solution--more readily indeed
than does the copper. Of the metals which dissolve, none (except
bismuth, which is rarely present in any quantity) deposits at the
anode so long as the solution retains its proper proportion of copper
and acid, and the current-density is not too great. Neutral solutions
are to be avoided because in them silver dissolves from the anode and,
being more electro-negative than copper, is deposited at the cathode,
while antimony and arsenic are also deposited, im