of electrons and protons who
play and in the way they arrange themselves. Larger games can be formed
if a number of atoms join together. Then there is a "molecule." Of
molecules there are as many kinds as there are different substances in
the world. It takes a lot of molecules together to form something big
enough to see, for even the largest molecule, that of starch, is much
too small to be seen by itself with the best possible microscope.

What sort of a molecule is formed will depend upon how many and what
kinds of atoms group together to play the larger game. Whenever there is
a big game it doesn't mean that the little atomic groups which enter
into it are all changed around. They keep together like a troop of boy
scouts in a grand picnic in which lots of troops are present. At any
rate they keep together enough so that we can still call them a group,
that is an atom, even though they do adapt their game somewhat so as to
fit in with other groups--that is with other atoms.

What will the kind of atom depend upon? It will depend upon how many
electrons and protons are grouped together in it to play their little
game. How any atom behaves so far as associating with other groups or
atoms will depend upon what sort of a game its own electrons and protons
are playing.

Now the simplest kind of a game that can be played, and the one with the
smallest number of electrons and protons, is that played by a single
proton and a single electron. I don't know just how it is played but I
should guess that they sort of chase each other around in circles. At
any rate I do know that the atom called "hydrogen" is formed by just one
proton and one electron. Suppose they were magnified until they were as
large as the moon and the earth. Then they would be just about as far
apart but the smaller one would be the proton.

That hydrogen atom is responsible for lots of interesting things for it
is a great one to join with other atoms. We don't often find it by
itself although we can make it change its partners and go from one
molecule to another very easily. That is what happens every time you
stain anything with acid. A hydrogen atom leaves a molecule of the acid
and then it isn't acid any more. What remains isn't a happy group either
for it has lost some of its playfellows. The hydrogen goes and joins
with the stuff which gets stained. But it doesn't join with the whole
molecule; it picks out part of it to associate with and that leave