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 molecules are composed of atoms which are alike. When
the atoms comprising a molecule are all alike, the substance is called
an element, and is said to be a simple substance. Throughout the
length and breadth of this vast world of ours there are only about