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