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_.