ve copper ions and of negative
sulphate ions.

On top of this solution of copper sulphate I poured very carefully a
weak solution of sulphuric acid. As I told you, an acid always has
hydrogen in its molecules. Sulphuric acid has molecules formed by two
hydrogen atoms and one of the groups which we decided to call sulphate.
A better name for this acid would be hydrogen sulphate for that would
imply that its molecule is the same as one of copper sulphate, except
that the place of the copper is taken by two atoms of hydrogen. It takes
two atoms of hydrogen because the copper atom has two lonely electrons
while a hydrogen atom only has one. It takes two electrons to fill up
the game which the electrons of the sulphate group are playing. If it
can get these from a single atom, all right; but if it has to get one
from each of two atoms, it will do it that way.

I remember when I mixed the sulphuric acid with water that I learned to
pour the acid into the water and not the other way around. Spatterings
of sulphuric acid are not good for hands or clothes. With this solution
I filled the jar almost to the top and then hung over the edge a sort of
a crow's foot shape of cast zinc. The zinc reached down into the
sulphuric acid solution. There was a binding post on it to which a wire
could be connected. This wire and the one which came from the plate of
copper at the bottom were the two terminals of the battery. We called
the wire from the copper "positive" and the one from the zinc
"negative."

Now we shall see why and how the battery worked. The molecules of
sulphuric acid dissociate in solution just as do those of copper
sulphate. When sulphuric acid molecules split, the sulphate part goes
away with two electrons which don't belong to it and each of the
hydrogen atoms goes away by itself but without its electron. We call
each a "hydrogen ion" but you can see that each is a single proton.

In the two solutions are pieces of zinc and copper. Zinc is like all the
rest of the metals in one way. Atoms of metals always have lonely
electrons for which there doesn't seem to be room in the game which is
going on around their nuclei. Copper as we saw has two lonely electrons
in each atom. Zinc also has two. Some metals have one and some two and
some even more lonely electrons in each atom.

What happens then is this. The sulphate ions wandering around in the
weak solution of sulphuric acid come along beside the zinc plate and
beckon