accurate
of observers, and a little later it was confirmed by Johan Jakob
Berzelius, the great Swedish chemist, who was to be a dominating
influence in the chemical world for a generation to come. But this
combination of elements in numerical proportions was exactly what Dalton
had noticed as early as 1802, and what bad led him directly to the
atomic weights. So the confirmation of this essential point by chemists
of such authority gave the strongest confirmation to the atomic theory.

During these same years the rising authority of the French chemical
world, Joseph Louis Gay-Lussac, was conducting experiments with gases,
which he had undertaken at first in conjunction with Humboldt, but which
later on were conducted independently. In 1809, the next year after
the publication of the first volume of Dalton's New System of Chemical
Philosophy, Gay-Lussac published the results of his observations, and
among other things brought out the remarkable fact that gases, under
the same conditions as to temperature and pressure, combine always in
definite numerical proportions as to volume. Exactly two volumes of
hydrogen, for example, combine with one volume of oxygen to form water.
Moreover, the resulting compound gas always bears a simple relation to
the combining volumes. In the case just cited, the union of two volumes
of hydrogen and one of oxygen results in precisely two volumes of water
vapor.

Naturally enough, the champions of the atomic theory seized upon
these observations of Gay-Lussac as lending strong support to their
hypothesis--all of them, that is, but the curiously self-reliant and
self-sufficient author of the atomic theory himself, who declined
to accept the observations of the French chemist as valid. Yet the
observations of Gay-Lussac were correct, as countless chemists since
then have demonstrated anew, and his theory of combination by volumes
became one of the foundation-stones of the atomic theory, despite the
opposition of the author of that theory.

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 vapo