De Morgan evidently did not keep up with the latest developments in
science, however, because two years before his birth, John Dalton, an
English schoolteacher, had changed the atomic theory of matter from a
philosophical speculation into a firmly established principle. The
evidence that convinced Dalton and many other contemporary scientists of
the reality of atoms came from quantitative chemical analysis.
Dalton knew that many chemical substances could be separated into two or
more simpler substances. Chemicals that could be separated further were
called compounds; those that could not were called elements. Careful
experiments by Dalton and others showed that whenever two or more
elements combined chemically to make a compound the relative amounts of
the elements had to be carefully adjusted to fit a definite proportion
in order to have no elements left over after the reaction was finished.
For example, if hydrogen and oxygen were combined to form water, the
weight of oxygen had to be eight times the weight of hydrogen;
otherwise, either some hydrogen or some oxygen would be left over.
This fundamental truth is now called the Law of Definite Proportions.
Another important principle, called the Law of Multiple Proportions, is
illustrated by hydrogen peroxide, which is made up of the same two
elements that are found in water. The weight of oxygen in hydrogen
peroxide, however, is 16 times the weight of hydrogen or exactly twice
the relative weight found in water.
These principles of chemical combination convinced Dalton that each
chemical element consists of small, indivisible units, all just alike,
called atoms, and that each chemical compound also has basic units,
called molecules, which cannot be divided without reducing the compound
into its elements—that is, destroying it as a compound. He visualized a
molecule of a compound as formed by the uniting of individual atoms of
two or more elements. It was obvious to him that in any molecule of a
compound, the weight of each atom of a component element bore a
proportionate relationship to the weight of the entire molecule which
was equal to the proportion, by weight, of all that element in the
compound. And although Dalton had no idea how heavy any individual atom
really was, he could tell how many _times_ heavier or lighter it was
than an atom of another element.
Incidentally, Dalton mistakenly thought that one atom of oxygen was
eight times as heavy as one atom of hydrogen instead of 16 times as
heavy. He assumed a water molecule to be HO instead of H₂O.