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 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 eighty known elements.  An element is the simplest substance conceivable, because it has not been separated into anything simpler.  Water is a compound substance.  It can be separated into oxygen and hydrogen.

Gold, silver, and lead are examples of elements, and water, alcohol, cider, sand, and marble are complex substances, or compounds, as we are apt to call them.  Everything, no matter what its size or shape or character, is formed from the various combinations into molecules of a few simple atoms, of which there exist about eighty known different kinds.  But few of the eighty known elements play an important part in our everyday life.  The elements in which we are most interested are given in the following table, and the symbols by which they are known are placed in columns to the right: