s the
other part to take the place of the hydrogen in the original molecule of
acid from which it came. Many of the actions which we call chemistry are
merely the result of such changes of atoms from one molecule to another.

Not only does the hydrogen atom like to associate in a larger game with
other kinds of atoms but it likes to do so with one of its own kind.
When it does we have a molecule of hydrogen gas, the same gas as is used
in balloons.

We haven't seemed to get very far yet toward radio but you can see how
we shall when I tell you that next time I shall write of more
complicated games such as are played in the atoms of copper which form
the wires of radio sets and of how these wires can do what we call
"carrying an electric current."

LETTER 2

WHY A COPPER WIRE WILL CONDUCT ELECTRICITY

MY DEAR YOUNG ATOMIST:

You have learned that the simplest group which can be formed by protons
and electrons is one proton and one electron chasing each other around
in a fast game. This group is called an atom of hydrogen. A molecule of
hydrogen is two of these groups together.

All the other possible kinds of groups are more complicated. The next
simplest is that of the atom of helium. Helium is a gas of which small
quantities are obtained from certain oil wells and there isn't very much
of it to be obtained. It is an inert gas, as we call it, because it
won't burn or combine with anything else. It doesn't care to enter into
the larger games of molecular groups. It is satisfied to be as it is, so
that it isn't much use in chemistry because you can't make anything else
out of it. That's the reason why it is so highly recommended for filling
balloons or airships, because it cannot burn or explode. It is not as
light as hydrogen but it serves quite well for making balloons buoyant
in air.

This helium atom is made up of four electrons and four protons. Right at
the center there is a small closely crowded group which contains all the
protons and two of the electrons. The other two electrons play around
quite a little way from this inner group. It will make our explanations
easier if we learn to call this inner group "the nucleus" of the atom.
It is the center of the atom and the other two electrons play around
about it just as the earth and Mars and the other planets play or
revolve about the sun as a center. That is why we shall call these two
electrons "planetary electrons."

There are about ninety diff