This is a function of the _density_, of the current,
i.e., the proportion of its intensity to the surface of the cathode. If
the density is too great there is an escape of hydrogen, and the metal
is deposited in a spongy condition. If the density of the current falls
below a certain minimum, an oxide is deposited in place of metal. The
electrolytic treatment of ores often renders it possible to separate
the different metals which may be present. These are deposited in
succession, and are sharply separated if the electromotive power is not
too great.
1. _Zinc_.--The zinciferous compounds--calamine, blende, and zinc
ash--are all poor conductors. They are first dissolved, and the salts
obtained are electrolyzed, employing anodes of coke. Blende should be
roasted before it is dissolved. The electrolytic bath should be as
concentrated as possible to avoid sponginess of the metal and an escape
of hydrogen. In a saturated solution the formation of hydrogen decreases
as the density of the current augments.
2.
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