t it is diluted with four times its
own volume of nitrogen.

A solution is a mixture, too; for although substances disappear when
they dissolve, they keep their own properties. Sugar is sweet whether
it is dissolved or not. Salt dissolved in water makes brine; but the
water will act in the way that it did before. It will still help to
make iron rust; and salt will be salty, whether or not it is dissolved
in water. That is why solutions are only mixtures and are not chemical
compounds.

EVERYTHING IN THE WORLD IS MADE OF ATOMS. Everything in the world is
either an element or a compound or a mixture. Most plant and
animal matter is made of very complicated compounds, or mixtures of
compounds. All pure metals are elements; but metals, when they are
melted, can be dissolved in each other to form alloys, which really
are mixtures. Most of the so-called gold and silver and nickel
articles are really made of alloys; that is, the gold, silver, or
nickel has some other elements dissolved in it to make it harder,
or to impart some other quality. Bronze and brass are always alloys;
steel is generally an alloy made chiefly of iron but with other
elements such as tungsten, of which electric lamp filaments are made,
dissolved in it to make it harder. An alloy is a special kind of
solution not quite like an ordinary solution.

You remember that in the opening chapters we often spoke of molecules,
the tiny particles of matter that are always moving rapidly back
and forth. Well, if you were to examine a molecule of water with the
microscope which we imagined could show us molecules, you would find
that the molecule of water was made of three still smaller particles,
called _atoms_. Two of these would be atoms of hydrogen and would
probably be especially small; the third would be larger and would be
an oxygen atom.

In the same way if you looked at a _molecule_ of salt under this
imaginary microscope, you would probably find it made of _two atoms_,
one of sodium (Na) and one of chlorine (Cl), held fast together in
some way which we do not entirely understand.

The smallest particle of an _element_ is called an _atom_.

The smallest particle of a _compound_ is called a _molecule_.

Molecules are usually made of two or more atoms joined together.

_APPLICATION 68._ In the following list tell which things
are elements, which are compounds, and which are mixtures,
remembering that both solutions and alloys are mixtures: