erent kinds of atoms and they all have names.
Some of them are more familiar than hydrogen and helium. For example,
there is the iron atom, the copper atom, the sulphur atom and so on.
Some of these atoms you ought to know and so, before telling you more of
how atoms are formed by protons and electrons, I am going to write down
the names of some of the atoms which we have in the earth and rocks of
our world, in the water of the oceans, and in the air above.

Start first with air. It is a mixture of several kinds of gases. Each
gas is a different kind of atom. There is just a slight trace of
hydrogen and a very small amount of helium and of some other gases which
I won't bother you with learning. Most of the air, however, is nitrogen,
about 78 percent in fact and almost all the rest is oxygen. About 20.8
percent is oxygen so that all the gases other than these two make up
only about 1.2 percent of the atmosphere in which we live.

[Illustration: Pl. II.--Bird's-eye View of Radio Central
(Courtesy of Radio Corporation of America).]

The earth and rocks also contain a great deal of oxygen; about 47.3
percent of the atoms which form earth and rocks are oxygen atoms. About
half of the rest of the atoms are of a kind called silicon. Sand is made
up of atoms of silicon and oxygen and you know how much sand there is.
About 27.7 percent of the earth and its rocks is silicon. The next most
important kind of atom in the earth is aluminum and after that iron and
then calcium. Here is the way they run in percentages: Aluminum 7.8
percent; iron 4.5 percent; calcium 3.5 percent; sodium 2.4 percent;
potassium 2.4 percent; magnesium 2.2 percent. Besides these which are
most important there is about 0.2 percent of hydrogen and the same
amount of carbon. Then there is a little phosphorus, a little sulphur, a
little fluorine, and small amounts of all of the rest of the different
kinds of atoms.

Sea water is mostly oxygen and hydrogen, about 85.8 percent of oxygen
and 10.7 percent of hydrogen. That is what you would expect for water is
made up of molecules which in turn are formed by two atoms of hydrogen
and one atom of oxygen. The oxygen atom is about sixteen times as heavy
as the hydrogen atom. However, for every oxygen atom there are two
hydrogen atoms so that for every pound of hydrogen in water there are
about eight pounds of oxygen. That is why there is about eight times as
high a percentage of oxygen in sea water as there is of