chemistry is founded, was the logical outgrowth, in the mind of John
Dalton, of those early studies in meteorology.

The way it happened was this: From studying the rainfall, Dalton turned
naturally to the complementary process of evaporation. He was soon led
to believe that vapor exists, in the atmosphere as an independent gas.
But since two bodies cannot occupy the same space at the same time,
this implies that the various atmospheric gases are really composed of
discrete particles. These ultimate particles are so small that we cannot
see them--cannot, indeed, more than vaguely imagine them--yet each
particle of vapor, for example, is just as much a portion of water as if
it were a drop out of the ocean, or, for that matter, the ocean itself.
But, again, water is a compound substance, for it may be separated, as
Cavendish has shown, into the two elementary substances hydrogen and
oxygen. Hence the atom of water must be composed of two lesser atoms
joined together. Imagine an atom of hydrogen and one of oxygen. Unite
them, and we have an atom of water; sever them, and the water no longer
exists; but whether united or separate the atoms of hydrogen and of
oxygen remain hydrogen and oxygen and nothing else. Differently mixed
together or united, atoms produce different gross substances; but the
elementary atoms never change their chemical nature--their distinct
personality.

It was about the year 1803 that Dalton first gained a full grasp of the
conception of the chemical atom. At once he saw that the hypothesis,
if true, furnished a marvellous key to secrets of matter hitherto
insoluble--questions relating to the relative proportions of the atoms
themselves. It is known, for example, that a certain bulk of hydrogen
gas unites with a certain bulk of oxygen gas to form water. If it be
true that this combination consists essentially of the union of atoms
one with another (each single atom of hydrogen united to a single atom
of oxygen), then the relative weights of the original masses of hydrogen
and of oxygen must be also the relative weights of each of their
respective atoms. If one pound of hydrogen unites with five and one-half
pounds of oxygen (as, according to Dalton's experiments, it did), then
the weight of the oxygen atom must be five and one-half times that of
the hydrogen atom. Other compounds may plainly be tested in the same
way. Dalton made numerous tests before he published his theory. He found
that hydrogen