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 enters
into compounds in smaller proportions than any other element
known to him, and so, for convenience, determined to take the
weight of the hydrogen atom as unity. The atomic weight of
oxygen then becomes (as given in Dalton's first table of 1803)
5.5; that of water (hydrogen plus oxygen) being of course 6.5.
The atomic weights of about a score of substances are given in
Dalton's first paper, which was read before the Literary and
Philosophical Society of Manchester, October 21, 1803. I wonder
if Dalton himself, great and acute intellect though he had,
suspected, when he read that paper, that he was inaugurating one
of the most fertile movements ever entered on in the whole
history of science?

Be that as it may, it is certain enough that Dalton's
contemporaries were at first little impressed with the novel
atomic theory. Just at this time, as it chanced, a dispute was
waging in the field of chemistry regarding a matter of empirical
fact which must necessarily be settled before such a theory as
that of Dalton could even hope for a bearing. This was the