to its atoms. The sulphate ions had a great deal rather play the
game called "zinc sulphate" than the game called "hydrogen sulphate." So
the zinc atoms leave their places to join with the sulphate ions. But
wait a minute! The sulphate ions have two extra electrons which they
kept from the hydrogen atoms. They don't need the two lonely electrons
which each zinc atom could bring and so the zinc atom leaves behind it
these unnecessary electrons.

Every time a zinc atom leaves the plate it fails to take all its
electrons with it. What leaves the zinc plate, therefore, to go into
solution is really not a zinc atom but is a zinc ion; that is, it is the
nucleus of a zinc atom and all except two of the planetary electrons.

Every time a zinc ion leaves the plate there are left behind two
electrons. The plate doesn't want them for all the rest of its atoms
have just the same number of protons as of electrons. Where are they to
go? We shall see in a minute.

Sometimes the zinc ions which have got into solution meet with sulphate
ions and form zinc sulphate molecules. But if they do these molecules
split up sooner or later into ions again. In the upper part of the
liquid in the jar, therefore, there are sulphate ions which are negative
and two kinds of positive ions, namely, the hydrogen ions and the zinc
ions.

Before the zinc ions began to crowd in there were just enough hydrogen
ions to go with the sulphate ions. As it is, the entrance of the zinc
ions has increased the number of positive ions and now there are too
many. Some of the positive ions, therefore, and particularly the
hydrogen ions, because the sulphate prefers to associate with the zinc
ions, can't find enough playfellows and so go down in the jar.

Down in the bottom of the jar the hydrogen ions find more sulphate ions
to play with, but that leaves the copper ions which used to play with
these sulphate ions without any playmates. So the copper ions go still
further down and join with the copper atoms of the copper plate. They
haven't much right to do so, for you remember that they haven't their
proper number of electrons. Each copper ion lacks two electrons of being
a copper atom. Nevertheless they join the copper plate. The result is a
plate of copper which has too few electrons. It needs two electrons for
every copper ion which joins it.

How about the zinc plate? You remember that it has two electrons more
than it needs for every zinc ion which has left