nce it has
been found to hold true in all cases where more than one compound is,
formed from the same elements. Dalton's law of multiple proportion
states these facts as follows: _When any two elements,_ A _and_ B,
_combine to form more than one compound, the amounts of_ B _which unite
with any fixed amount of_ A _bear the ratio of small whole numbers to
each other_.

~Hypothesis necessary to explain the laws of matter.~ These three
generalizations are called _laws_, because they express in concise
language truths which are found by careful experiment to hold good in
all cases. They do not offer any explanation of the facts, but merely
state them. The human mind, however, does not rest content with the mere
bare facts, but seeks ever to learn the explanation of the facts. A
suggestion which is offered to explain such a set of facts is called an
_hypothesis_. The suggestion which Dalton offered to explain the three
laws of matter, called the _atomic hypothesis_, was prompted by his view
of the constitution of matter, and it involves three distinct
assumptions in regard to the nature of matter and chemical action.
Dalton could not prove these assumptions to be true, but he saw that if
they were true the laws of matter become very easy to understand.

~Dalton's atomic hypothesis.~ The three assumptions which Dalton made in
regard to the nature of matter, and which together constitute the atomic
hypothesis, are these:

1. All elements are made up of minute, independent particles which
Dalton designated as _atoms_.

2. All atoms of the same element have equal masses; those of different
elements have different masses; in any change to which an atom is
subjected its mass does not change.

3. When two or more elements unite to form a compound, the action
consists in the union of a definite small number of atoms of each
element to form a small particle of the compound. The smallest particles
of a given compound are therefore exactly alike in the number and kinds
of atoms which they contain, and larger masses of the substances are
simply aggregations of these least particles.

~Molecules and atoms.~ Dalton applied the name atom not only to the minute
particles of the elements but also to the least particles of compounds.
Later Avogadro, an Italian scientist, pointed out the fact that the two
are different, since the smallest particle of an element is a unit,
while that of a compound must have at least two units in it. He