ence over the quantity of the matters evolved during electrolytic
action, one would expect them to be present when electrodes of different
substances, and possessing very different chemical affinities for such
matters, were used. Platina has no power in dilute sulphuric acid of
combining with the oxygen at the _anode_, though the latter be evolved in
the nascent state against it. Copper, on the other hand, immediately unites
with the oxygen, as the electric current sets it free from the hydrogen;
and zinc is not only able to combine with it, but can, without any help
from the electricity, abstract it directly from the water, at the same time
setting torrents of hydrogen free. Yet in cases where these three
substances were used as the positive electrodes in three similar portions
of the same dilute sulphuric acid, specific gravity 1.336, precisely the
same quantity of water was decomposed by the electric current, and
precisely the same quantity of hydrogen set free at the _cathodes_ of the
three solutions.

808. The experiment was made thus. Portions of the dilute sulphuric acid
were put into three basins. Three volta-electrometer tubes, of the form
figg. 60. 62. were filled with the same acid, and one inverted in each
basin (707.). A zinc plate, connected with the positive end of a voltaic
battery, was dipped into the first basin, forming the positive electrode
there, the hydrogen, which was abundantly evolved from it by the direct
action of the acid, being allowed to escape. A copper plate, which dipped
into the acid of the second basin, was connected with the negative
electrode of the _first_ basin; and a platina plate, which dipped into the
acid of the third basin, was connected with the negative electrode of the
_second_ basin. The negative electrode of the third basin was connected
with a volta-electrometer (711.), and that with the negative end of the
voltaic battery.

809. Immediately that the circuit was complete, the _electro-chemical
action_ commenced in all the vessels. The hydrogen still rose in,
apparently, undiminished quantities from the positive zinc electrode in the
first basin. No oxygen was evolved at the positive copper electrode in the
second basin, but a sulphate of copper was formed there; whilst in the
third basin the positive platina electrode evolved pure oxygen gas, and was
itself unaffected. But in _all_ the basins the hydrogen liberated at the
_negative_ platina electrodes was the _same in q