tes have been withdrawn from the liquid.
This physico-chemical process that takes place in such a battery is
called the _polarization of the plates_.

[Illustration: FIG. 1.]

The accompanying figure will help one to understand the actions going on
in a battery cell of the kind mentioned. Let Pt represent a plate of
platinum, and Zn a plate of zinc, both placed in a vessel containing
hydrochloric acid, which is also represented by the symbols HCl. As such
molecules are extremely minute, there will of course be an immense
number of them between the plates. The plates are now to be connected by
a wire running between them through the air. As soon as these conditions
are fulfilled, a hissing sound will be heard coming from the cell, and
bubbles of gas will be seen to rise from the platinum plate: these
bubbles prove upon analysis to be bubbles of hydrogen. At the same time
the zinc will begin to dissolve, forming what proves by analysis to be
the chloride of zinc; and at the same time a current of electricity
travels through the wire from the platinum to the zinc. The quantity of
electricity that is thus generated is strictly proportionate to the
quantity of hydrogen liberated, which is also proportionate to the
weight of zinc dissolved; and this, in turn, is proportionate to the
surface of the metals exposed to the action of the acid. Now, it happens
under such circumstances as the above, that the liberated hydrogen
adheres very strongly to the platinum, as there is nothing for it to
unite with chemically; and therefore the plate will very soon be visibly
covered with bubbles, which may be scraped off with a feather or a swab,
but only to have the same thing repeated.

This coating of bubbles will prevent the acid from touching the plate,
and so practically diminishes the surface of it; but the quantity of
electricity generated being proportionate to the surface exposed to the
chemical action, it will be understood at once how such polarization of
the plates must soon bring the battery to a standstill.

In 1836 Prof. J. F. Daniell of London contrived a battery, which has
been called the Daniell Cell, in which the metal (copper) that had the
hydrogen liberated upon it was separated by a porous cell from the zinc.
The zinc was immersed in dilute sulphuric acid, and the copper in an
acid solution of blue vitriol (copper sulphate). The porous cup did not
prevent the electricity from passing, nor the decomposition from taki