(1) DEFINITIONS.--An element is a substance that _cannot_ be decomposed.
A compound is a substance that _can_ be decomposed into other different
substances; and if the decomposition goes far enough, these substances
will be elements.
A mixture is made up of two or more components (elements and compounds
or both), _physically_ put together. It differs from a compound whose
compounds are _chemically_ united.
(2) LAWS.--_Law of Definite Proportions_: All specimens of a compound
contain the same elements in the same proportions.
_Law of Multiple Proportions_: When two compounds consist of the same
elements, the proportion of one is a simple multiple of the proportion
of the other.
_Law of Combining Proportions_: Each element enters into all its
compounds by a fixed proportional weight.
The fundamental laws of chemistry are proved by experiment.
(3) THE ATOMIC THEORY.--The atomic theory teaches that matter is
composed of minute particles which themselves cannot be divided, but
which unite to form molecules which can be divided.
A _molecule_, then, is the smallest amount of a substance that can exist
in a free state.