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 question whether or not chemical elements unite with one
another always in definite proportions. Berthollet, the great co-worker
with Lavoisier, and now the most authoritative of living chemists,
contended that substances combine in almost indefinitely graded
proportions between fixed extremes. He held that solution is really a
form of chemical combination--a position which, if accepted, left no
room for argument.

But this contention of the master was most actively disputed, in
particular by Louis Joseph Proust, and all chemists of repute were
obliged to take sides with one or the other. For a time the authority of
Berthollet held out against the facts, but at last accumulated evidence
told for Proust and his followers, and towards the close of the first
decade of our century it came to be generally conceded that chemical
elements combine with one another in fixed and definite proportions.

More than that. As the analysts were led to weigh carefully the
quantities of combining elements, it was observed that the proportions
are not only definite, but that they bear a very curious relation to one
another. If element A combines with two different proportions of element
B to form two compounds, it appears that the weight of the larger
quantity of B is an exact multiple of that of the smaller quantity. This
curious relation was noticed by Dr. Wollaston, one of the most