parting a dark colour
to the copper. Electrolytic copper should contain at least 99.92% of
metallic copper, the balance consisting mainly of oxygen with not more
than 0.01% in all of lead, arsenic, antimony, bismuth and silver. Such
a degree of purity is, however, unattainable unless the conditions of
electrolysis are rigidly adhered to. It should be observed that the
free acid is gradually neutralized, partly by chemical action on
certain constituents of the slime, partly by local action between
different metals of the anode, both of which effect solution
independently of the current, and partly by the peroxidation (or
aeration) of ferrous sulphate formed from the iron in the anode. At
the same time there is a gradual substitution of other metals for
copper in the solution, because although copper _plus_ other (more
electro-positive) metals are constantly dissolving at the anode, only
copper is deposited at the cathode. Hence the composition and acidity
of the solution, on which so much depends, must be constantly watched.

The dependence of the mechanical qualities of the copper upon the
current-density employed is well known. A very weak current gives a
pale and brittle deposit, but as the current-density is increased up
to a certain point, the properties of the metal improve; beyond this
point they deteriorate, the colour becoming darker and the deposit
less coherent, until at last it is dark brown and spongy or
pulverulent. The presence of even a small proportion of hydrochloric
acid imparts a brown tint to the deposit. Baron H. v. Hubl (_Mittheil.
des k. k. militar-geograph. Inst._, 1886, vol. vi. p. 51) has found
that with neutral solutions a 5% solution of copper sulphate gave no
good result, while with a 20% solution the best deposit was obtained
with a current-density of 28 amperes per sq. ft.; with solutions
containing 2% of sulphuric acid, the 5% solution gave good deposits
with current-densities of 4 to 7.5 amperes, and the 20% solution with
11.5 to 37 amperes, per sq. ft. The maximum current-densities for a
_pure_ acid solution at rest were: for 15% pure copper sulphate
solutions 14 to 21 amperes, and for 20% solutions 18.5 to 28 amperes,
per sq. ft.; but when the solutions were kept in gentle motion these
maxima could be increased to 21-28 and 28-37 amperes per sq. ft.
respectively. The necessity for adjusting the current-density to the