ds. A
familiar example of its combining action is found in ordinary combustion,
or burning. On account of the part it plays in this process, oxygen is
called the _supporter of combustion_; but it supports combustion by the
simple method of uniting. The ashes that are left and the invisible gases
that escape into the atmosphere are the compounds formed by the uniting
process. It thus appears that oxygen, in common with the other elements,
may exist in either of two forms:

1. That in which it is in a _free_, or uncombined, condition--the form in
which it exists in the atmosphere.

2. That in which it is a part of compounds, such as the compounds formed
in combustion.

Oxygen manifests its activity to the best advantage when it is in a free
state, or, more accurately speaking, when it is passing from the free
state into one of combination. It is separated from its compounds and
brought again into a free state by overcoming with heat, or some other
force, the affinity which causes it to unite.

*How Oxygen unites.*--The chemist believes oxygen, as well as all other
substances, to be made up of exceedingly small particles, called _atoms_.
The atoms do not exist singly in either elements or compounds, but are
united with each other to form groups of atoms that are called
_molecules_. In an element the molecules are made up of one kind of atoms,
but in a compound the molecules are made up of as many kinds of atoms as
there are elements in the compound. Changes in the composition of
substances (called chemical changes) are due to rearrangements of the
atoms and the formation of new molecules. The atoms, therefore, are the
units of chemical combination. In the formation of new compounds they
unite, and in the breaking up of existing compounds they separate.

The uniting of oxygen is no exception to this general law. All of its
combinations are brought about by the uniting of its atoms. In the burning
of carbon, for example, the atoms of oxygen and the atoms of carbon unite,
forming molecules of the compound known as carbon dioxide. The chemical
formula of this compound, which is CO_2, shows the proportion in which the
atoms unite--one atom of carbon uniting with two atoms of oxygen in each of
the molecules. The affinity of oxygen for other elements, and the affinity
of other elements for oxygen, and for each other, resides in their atoms.

*Oxidation.*--The uniting of oxygen with other elements is termed
_oxidation_. This