he air because in time the vessel becomes empty. The only way in
which this could happen would be for the molecules of the liquid to
pass from the liquid into the surrounding medium; but this is really
saying that the molecules are in motion.

From these phenomena and others it is reasonably clear that substances
are composed of molecules, and that molecules are not inert, quiet
particles, but that they are in incessant motion, moving rapidly
hither and thither, sometimes traveling far, sometimes near. Even the
log of wood which lies heavy and motionless on our woodpile is made
up of countless billions of molecules each in rapid incessant motion.
The molecules of solid bodies cannot escape so readily as those of
liquids and gases, and do not travel far. The log lies year after year
in an apparently motionless condition, but if one's eyes were keen
enough, the molecules would be seen moving among themselves, even
though they cannot escape into the surrounding medium and make long
journeys as do the molecules of liquids and gases.

96. The Companions of Molecules. Common sense tells us that a
molecule of water is not the same as a molecule of vinegar; the
molecules of each are extremely small and in rapid motion, but they
differ essentially, otherwise one substance would be like every other
substance. What is it that makes a molecule of water differ from a
molecule of vinegar, and each differ from all other molecules? Strange
to say, a molecule is not a simple object, but is quite complex, being
composed of one or more smaller particles, called atoms, and the
number and kind of atoms in a molecule determine the type of the
molecule, and the type of the molecule determines the substance. For
example, a glass of water is composed of untold millions of molecules,
and each molecule is a company of three still smaller particles, one
of which is called the oxygen atom and two of which are alike in every
particular and are called hydrogen atoms.

97. Simple Molecules. Generally molecules are composed of atoms
which are different in kind. For example, the molecule of water has
two different atoms, the oxygen atom and the hydrogen atoms; alcohol
has three different kinds of atoms, oxygen, hydrogen, and carbon.
Sometimes, however, molecules are composed of a group of atoms all of
which are alike. Now there are but seventy or eighty different kinds
of atoms, and hence there can be but seventy or eighty different
substances whose