to the slimes. If much
carbonaceous matter be present (and this is generally so when iron
sponge is used as the precipitant) the crude product is heated to
redness in the air; this burns out the carbon, and, at the same time,
oxidizes a little of the copper, which must be subsequently reduced. A
similar operation is conducted when arsenic is present; basic-lined
reverberatory furnaces have been used for the same purpose.

_Electrolytic Refining._--The principles have long been known on which
is based the electrolytic separation of copper from the certain elements
which generally accompany it, whether these, like silver and gold, are
valuable, or, like arsenic, antimony, bismuth, selenium and tellurium,
are merely impurities. But it was not until the dynamo was improved as a
machine for generating large quantities of electricity at a very low
cost that the electrolysis of copper could be practised on a commercial
scale. To-day, by reason of other uses to which electricity is applied,
electrically deposited copper of high conductivity is in ever-increasing
demand, and commands a higher price than copper refined by fusion. This
increase in value permits of copper with not over L2 or $10 worth of the
precious metals being profitably subjected to electrolytic treatment.
Thus many million ounces of silver and a great deal of gold are
recovered which formerly were lost.

The earliest serious attempt to refine copper industrially was made by
G. R. Elkington, whose first patent is dated 1865. He cast crude copper,
as obtained from the ore, into plates which were used as anodes, sheets
of electro-deposited copper forming the cathodes. Six anodes were
suspended, alternately with four cathodes, in a saturated solution of
copper sulphate in a cylindrical fire-clay trough, all the anodes being
connected in one parallel group, and all the cathodes in another. A
hundred or more jars were coupled in series, the cathodes of one to the
anodes of the next, and were so arranged that with the aid of side-pipes
with leaden connexions and india-rubber joints the electrolyte could,
once daily, be made to circulate through them all from the top of one
jar to the bottom of the next. The current from a Wilde's dynamo was
passed, apparently with a current density of 5 or 6 amperes per sq. ft.,
until the anodes were too crippled for further use. The cathodes, when
thick enough, were either cast and rolled or sent into the market
direct. Silver and