collects at the negative foil under the tube H. These facts can be
proved by dipping a red-hot wick or taper into the gas of the tube
O and seeing it blaze in presence of the oxygen which feeds the
combustion, then dipping the lighted taper into the gas of the
tube H and watching it burn with the blue flame of hydrogen. The
volume of gas at the CATHODE or negative electrode is always twice
that at the ANODE or positive electrode, as it should be according
to the known composition of water.

Now, if we disconnect the battery and join the two platinum
electrodes of the voltameter by a wire, we shall find a current
flowing out of the voltameter as though it were a battery, but in
the reverse direction to the original current which decomposed the
water. This "secondary" or reacting current is evidently due to
the polarisation of the foils--that is to say, the electro-
positive and electro-negative gases collected on them.

Professor Groves constructed a gas battery on this principle, the
plates being of platinum and the two gases surrounding them oxygen
and hydrogen, but the most useful development of it is the
accumulator or storage battery.

The first practicable secondary battery of Gaston Plante was made
of sheet lead plates or electrodes, kept apart by linen cloth
soaked in dilute sulphuric acid, after the manner of Volta's pile.
It was "charged" by connecting the plates to a primary battery,
and peroxide of lead (PbO2) was formed on one plate and spongy
lead (Pb) on the other. When the charging current was cut off the
peroxide plate became the positive and the spongy plate the
negative pole of the secondary cell.