, in fact,
between the apparatus, fig. 90, and the trough represented fig. 91, is that
twice the quantity of surface of contact between the metal and acid is
allowed in the first to what would occur in the second.

1009. When the extreme plates of the arrangement just described, fig. 90,
are connected metallically through the galvanometer _g_, then the whole
represents a battery consisting of two pairs of zinc and platina plates
urging a current forward, which has, however, to decompose water unassisted
by any direct chemical affinity before it can be transmitted across the
cell iii, and therefore before it can circulate. This decomposition of
water, which is opposed to the passage of the current, may, as a matter of
convenience, be considered as taking place either against the surfaces of
the two platina plates which constitute the electrodes in the cell in, or
against the two surfaces of that platina plate which separates the cells ii
and iii, fig. 91, from each other. It is evident that if that plate were
away, the battery would consist of two pairs of plates and two cells,
arranged in the most favourable position for the production of a current.
The platina plate therefore, which being introduced as at _x_, has oxygen
evolved at one surface and hydrogen at the other (that is, if the
decomposing current passes), may be considered as the cause of any
obstruction arising from the decomposition of water by the electrolytic
action of the current; and I have usually called it the interposed plate.

1010. In order to simplify the conditions, dilute sulphuric acid was first
used in all the cells, and platina for the interposed plates; for then the
initial intensity of the current which tends to be formed is constant,
being due to the power which zinc has of decomposing water; and the
opposing force of decomposition is also constant, the elements of the water
being unassisted in their separation at the interposed plates by any
affinity or secondary action at the electrodes (744.), arising either from
the nature of the plate itself or the surrounding fluid.

1011. When only one voltaic pair of zinc and platina plates was used, the
current of electricity was entirely stopped to all practical purposes by
interposing one platina plate, fig. 92, i.e. by requiring of the current
that it should decompose water, and evolve both its elements, before it
should pass. This consequence is in perfect accordance with the views
before give