suggested the name _molecule_ for the least particle of a compound which
can exist, retaining the name _atom_ for the smallest particle of an
element. In accordance with this distinction, we may define the atom and
the molecule as follows: _An atom is the smallest particle of an element
which can exist. A molecule is the smallest particle of a compound which
can exist._ It will be shown in a subsequent chapter that sometimes two
or more atoms of the same element unite with each other to form
molecules of the element. While the term atom, therefore, is applicable
only to elements, the term molecule is applicable both to elements and
compounds.

~The atomic hypothesis and the laws of matter.~ Supposing the atomic
hypothesis to be true, let us now see if it is in harmony with the laws
of matter.

1. _The atomic hypothesis and the law of conservation of matter._ It is
evident that if the atoms never change their masses in any change which
they undergo, the total quantity of matter can never change and the law
of conservation of matter must follow.

2. _The atomic hypothesis and the law of definite composition._
According to the third supposition, when iron combines with sulphur the
union is between definite numbers of the two kinds of atoms. In the
simplest case one atom of the one element combines with one atom of the
other. If the sulphur and the iron atoms never change their respective
masses when they unite to form a molecule of iron sulphide, all iron
sulphide molecules will have equal amounts of iron in them and also of
sulphur. Consequently any mass made up of iron sulphide molecules will
have the same fraction of iron by weight as do the individual iron
sulphide molecules. Iron sulphide, from whatever source, will have the
same composition, which is in accordance with the law of definite
composition.

3. _The atomic hypothesis and the law of multiple proportion._ But this
simplest case may not always be the only one. Under other conditions one
atom of iron might combine with two of sulphur to form a molecule of a
second compound. In such a case the one atom of iron would be in
combination with twice the mass of sulphur that is in the first
compound, since the sulphur atoms all have equal masses. What is true
for one molecule will be true for any number of them; consequently when
such quantities of these two compounds are selected as are found to
contain the same amount of iron, the one will contain twice as much