The true explanation of Gay-Lussac's law of combination by
volumes was thought out almost immediately by an Italian savant,
Amadeo, Avogadro, and expressed in terms of the atomic theory.
The fact must be, said Avogadro, that under similar physical
conditions every form of gas contains exactly the same number of
ultimate particles in a given volume. Each of these ultimate
physical particles may be composed of two or more atoms (as in
the case of water vapor), but such a compound atom conducts
itself as if it were a simple and indivisible atom, as regards
the amount of space that separates it from its fellows under
given conditions of pressure and temperature. The compound atom,
composed of two or more elementary atoms, Avogadro proposed to
distinguish, for purposes of convenience, by the name molecule.
It is to the molecule, considered as the unit of physical
structure, that Avogadro's law applies.

This vastly important distinction between atoms and molecules,
implied in the law just expressed, was published in 1811. Four
years later, the famous French physicist Ampere outlined a
similar theory, and utilized the law in his mathematical
calculations. And with that the law of Avogadro dropped out of
sight for a full generation. Little suspecting that it was the
very key to the inner mysteries of the atoms for which they were
seeking, the chemists of the time cast it aside, and let it fade
from the memory of their science.

This, however, was not strange, for of course the law of Avogadro
is based on the atomic theory, and in 1811 the atomic theory was
itself still being weighed in the balance. The law of multiple