ered with points, the bubbles
collect more freely at the points and are more quickly carried
away to the surface of the liquid. These means are, however,
hardly practical except in cells for laboratory use.

Second--_chemical means_. If a highly oxidizing substance be
added to the electrolyte, it will destroy the hydrogen bubbles by
combining with them while they are in a nascent state, and this
will prevent the increase in internal resistance and the opposing
electromotive force. Such substances are bichromate of potash,
nitric acid, and chlorine, and are largely used.

Third--_electro-chemical means_. Double cells, arranged to
separate the elements and liquids by means of porous partitions
or by gravity, may be so arranged that solid copper is liberated
instead of hydrogen at a point where the current leaves the
liquid, thereby entirely obviating polarization. This method also
is largely used.

_Local Action._ When a simple cell stands idle, _i.e._, with its
circuit open, small hydrogen bubbles may be noticed rising from the
zinc electrode instead of from copper, as is the case where the
circuit is closed. This is due to impurities in the zinc plate, such
as particles of iron, tin, arsenic, carbon, etc. Each of these
particles acts with the surrounding zinc just as might be expected of
any pair of dissimilar elements opposed to each other in an
electrolyte; in other words, they constitute small voltaic cells.
Local currents, therefore, are generated, circulating between the two
adjacent metals, and, as a result, the zinc plate and the electrolyte
are needlessly wasted and the general condition of the cell is
impaired. This is called _local action_.

_Amalgamated Zincs._ Local action might be prevented by the use of
chemically pure zinc, but this, on account of its expense, cannot be
employed commercially. Local action, however, may be overcome to a
great extent by amalgamating the zinc, _i.e._, coating it with
mercury. The iron particles or other impurities do not dissolve in the
mercury, as does the zinc, but they float to the surface, whence the
hydrogen bubbles which may form speedily carry them off, and, in other
cases, the impurities fall to the bottom of the cell. As the zinc in
the pasty amalgam dissolves in the acid, the film of mercury unites
with fresh zinc, and so always presents a clear, bright, homogeneous
surface to the actio