that in the Daniell cell the current decomposes a solution of
sulphate of copper and deposits the pure metal on the copper
plate. If we simply make a solution of blue vitriol in a glass
beaker and dip the wires from a voltaic cell into it, we shall
find the wire from the negative pole become freshly coated with
particles of new copper. The sulphate has been broken up, and the
liberated metal, being positive, gathers on the negative
electrode. Moreover, if we examine the positive electrode we shall
find it slightly eaten away, because the sulphuric acid set free
from the sulphate has combined with the particles of that wire to
make new sulphate. Thus the copper is deposited on one electrode,
namely, the cathode, by which the current leaves the bath, and at
the expense of the other electrode, that is to say, the anode, by
which the current enters the bath.

The fact that the weight of metal deposited in this way from its
salts is proportional to the current, has been utilised for
measuring the strength of currents with a fine degree of accuracy.
If, for example, the tubes of the voltameter described on page 38
were graduated, the volume of gas evolved would be a measure of
the current. Usually, however, it is the weight of silver or
copper deposited from their salts in a certain time which gives
the current in amperes.

Electro-plating is the principal application of this chemical
process. In 1805 Brugnatelli took a silver medal and coated it
with gold by making it the cathode in a solution of a salt of
gold, and using a plate of gold for the anode. The shops of our
jewellers are now bright with teapots, salt cellars, spoons, and
other articles of the table made of inferior metals, but