POST-CLASSICAL ATOMISM
The two principal links in the chain which binds the world of the
ancient Greek (and _one_ Roman) Atomists with those of our own
times are Descartes (1596-1650) and Gassendi (1592-1655). Gassendi
revived and ably supported the atomic theory of Epicurus; while
Descartes resuscitated the mechanistic conceptions of the Greek
Atomists on the basis of a corpuscular theory of matter, corresponding
in many respects to the doctrine of Democritus.
The work of Leucippus connects through Descartes and Gassendi, Huygens,
Boyle and Newton, directly with that of Higgins and Dalton, founders of
the modern chemical atomic theory.
Dalton had already adopted a physical atomic theory, based no doubt on
the work of his illustrious predecessors, and upon his own thorough
study of the gases of the atmosphere, before he formulated his chemical
atomic hypothesis. He opposed the prevailing belief that the various
gases exist in the atmosphere in a state of chemical combination,
asserting that the atmosphere is a physical mixture.
In 1805, he extended his atomic hypothesis to the explanation of
chemical phenomena. His first postulate was, of course, that all
matter is made up of small particles; these he found possessed the
power of attracting (and holding) other particles. He therefore
concluded that these invisible particles never subdivide in taking part
in chemical changes, and that all atoms of any one element must be
alike. But the atoms of the different elements vary in weight, form and
combining power. He established the rule, already assumed by William
Higgins (1789), that different atoms tend to combine in the proportion
of atom to atom.
When a compound was composed of two elements only, it was presumed to
be binary; that is, since water, for example, was known to be composed
of hydrogen and oxygen, it was supposed by Dalton that it _must_
consist of one atom of each of these elements; as was also assumed in
the case of ammonia, which he knew to be a combination of hydrogen and
nitrogen. Now we know that one atom of hydrogen combined with one atom
of oxygen forms, not water, but oxide of hydrogen; while the combining
of _two_ atoms of hydrogen with one atom of oxygen forms water.
Dalton established the fact that elements combine only in _definite
proportions_--e.g., that oxygen and hydrogen will combine only in
the _proportion_ of 8 to 1. Had Dalton known that a molecule
of water contains _two_ atoms of hydrogen, and _one_ of
oxygen, he would have known that _one_ atom of oxygen must weigh
_sixteen_ times--instead of eight times--as much as one atom of
hydrogen. _One part_ of hydrogen _by weight_ does combine
with _eight parts by weight_ of oxygen, but this does not prove
that the portion of hydrogen in water contains only one atom. Since
it contains two atoms, instead of one, then, in order to preserve the
relative combining (or “equivalent”) weights of the two substances,
we must assume that the _atomic_ weight of oxygen is 16, for
the _combining_ weights of elements represent the relative (not
actual) weights of the atoms. And each element has its own fixed
combining weight, ascertained by experiment.
Water, then, is not a mere mixture or combination of hydrogen and
oxygen, in the sense that a pound of hydrogen and eight _and a
half_ pounds of oxygen will, when exploded, produce 9½ pounds of
water. Combination would indeed be effected, but there would be a
residue in the container of just one-half pound of oxygen. The mass of
oxygen that has combined will weigh exactly eight times as much as the
hydrogen; and now we know that the water formed will contain exactly
twice as many atoms of hydrogen as there are oxygen atoms present in
the combination, uniting not only “one by one,” but, in this case, two
to one. What the law of definite proportions proves is that chemical
combinations always take place _between atoms_.
But there is also a law of _multiple proportions_. It is
found that when one element forms more than one compound with a
second element, the quantities of the first element which combine
with the combining weight of the second element are always simple
multiples--never fractions--of the combining weight of the first
element.
We have seen that the _atomic_ weight of oxygen is 16--i.e., 16
times heavier than the lightest element, hydrogen, which is therefore
(on the relative scale of atomic weights) 1. But the _combining_
weight of oxygen is 8--i.e., eight _parts_ of oxygen will combine
with so many parts (combining weight) of some other element. Suppose
we take for example nitrogen. Now the _combining_ weight of
nitrogen is 14, and its _atomic_ weight is also 14. Fourteen parts
of nitrogen will combine therefore with eight parts of oxygen. But
fourteen parts of nitrogen will also combine with four simple multiples
of eight parts of oxygen, forming in all five different compound
substances. For example:
14 parts of nitrogen combine with 16 or 2x8 parts of oxygen.
14 parts of nitrogen combine with 24 or 3x8 parts of oxygen.
14 parts of nitrogen combine with 32 or 4x8 parts of oxygen.
14 parts of nitrogen combine with 40 or 5x8 parts of oxygen.
John Dalton advanced a theory satisfactorily to explain why in
successive compounds the amount of oxygen goes up in jumps, and why
this jump in each case is equivalent to the combining weight of oxygen.
The regular jumps in the case of the five nitrogen compounds simply
mean that one more atom has entered, in each successive case, into
combination. These combining weights, as we have seen, simply represent
the relative weights of the various elements. “Atomic weight” is merely
the name given by Dalton to the relative combining weights of the
atoms. He had no information as to how many atoms actually combine in
making a compound, much less of the actual weight of the single atoms
of the elements.
Nor did he know _why_ atoms combine at all, in any proportions,
but, being a thorough-going Newtonian, both his physical theory and
his chemical theory have a common basis in Newton’s doctrine of
mutually repulsive atoms. If an element A unites with an element B, the
repulsion of the atoms of B from one another must tend, he thought, to
the formation of a binary compound. “Binary compounds must first be
formed, then ternary, and so on, till the repulsion of the atoms of B
refuses to admit any more.”[28]
While we have traveled quite a distance in atomic and chemical theory
since the days of John Dalton (1766-1844), it is to the everlasting
credit of this (at the time) obscure and poverty-stricken teacher
of mathematics and physics that he gave us, in 1808, “A new System
of Chemical Philosophy” which unites the atomic theory of modern
science, first advanced by his admirers and contemporaries, Baron J. J.
Berzelius (1779-1848) and Sir Humphrey Davy (1778-1829).
Little did Dalton dream that the “atomic weight” of the elements is
equivalent to the number of _electrons_ in each atom of a given
substance![29] Or that the atomic _number_ of an element, arranged
(successively) in the order of their atomic _weights_, from 1
(hydrogen) to 92 (uranium), is an index to the number of positive
electrical charges on the atomic nucleus, around which revolve
in “planetary orbits” an equivalent number of negatively charged
_electrons_--the real building-stones of the Universe.
FOOTNOTES:
[Footnote 28: _Nicholson’s Journal_, 29, 147, 1811.]
[Footnote 29: On this point, see Lodge, Sir Oliver, “Within the Atom,”
_Scientific American_, November, 1923.]
=TRANSCRIBER’S NOTES=
Simple typographical errors have been silently corrected; unbalanced
quotation marks were remedied when the change was obvious, and
otherwise left unbalanced.
Punctuation, hyphenation, spelling and ligatures were made consistent
when a predominant preference was found in the original book; otherwise
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The verses of the Invictus poem have been left as they are in the
printed version. A Table of Contents has been added.