38 QUANTITATIVE ASPECTS OF CHEMISTRYof multiple proportions, and he found for them a reasonable ex-
planation in the atomic theory (1808). According to this theory,
the elements consist of atoms which were thought by Dalton
to be indivisible. The atoms of the same element are all alike
in weight and in all their other properties; the atoms of different
elements differ in properties. When elements combine to form
compounds, it is the individual atoms which are concerned.
Suppose for example one atom of element A combines with
one atom of element B, to form the compound AB; then, since
the weights of atoms of the same element are always alike and
since whatever amount of the compound is taken it always contains
an equal number of atoms of each element, the proportion by
weight of the two elements must always be the same in this
compound. Thus the law of definite proportions is a necessary
deduction from the atomic theory. Let it be understood, how-
ever, that the law is a fact established by careful measurements.
The theory is simply the best effort of the human mind to furnish
an explanation of the facts.
The reasoning is similar for the law of multiple proportions.
Suppose that one atom of A can combine with two atoms of B in
forming an entirely different, but none the less definite, compound.
Let us designate this compound AB2. The law of definite propor-
tions would hold for this compound as well as for the first. Fur-
thermore, if we should take such amounts of each compound that
each contained the same number of atoms of A then the second
would contain twice as many atoms of B as the first. The weight
of B in the second would, therefore, be exactly twice the weight of
B in the first compound. Thus the law of multiple proportions is
also a necessary deduction from the atomic theory.
Atomic Weights. We have found that the combining ratio of
oxygen and zinc is 1:4.09. The atomic theory stipulates that
the ratio of the numbers of atoms is simple, but it may be 1:1,
1:2, 1:3, 2:1, 3:1, 3:2, 2:3 or any reasonably simple ratio.
Having at hand no way of telling what the real ratio is we pro-
ceed to assume one to be the correct ratio — in other words,
we make a guess. Where we have nothing else to guide us we
make the simplest possible guess and assume the ratio to be
1:1. If this is correct then the atom of zinc weighs 4.09 times
as much as the atom of oxygen.
The weight of a single atom is so very small that it would mean