McEvoy: “Or take John Dalton's conjecture about the fact that every compound
(e.g. water) always breaks down into elements that were in fixed proportion by
ratio and of identical weight. Explanation? Every element is present in the
form of atoms and each of these is uniform in weight with any other of that
element, and the combination of such atoms to make a specific compound is
always based on a uniform ratio between each element. That may seem obvious
enough as an idea worth exploring but that doesn't mean it is so obvious
everyone had it.”
Yes, it seems obvious Dalton’s father did NOT have it. For Dalton would not
have called it a ‘conjecture’ otherwise. But it may do to revise the details.
What John Dalton did was to assimilate the known experimental work of many
people to summarize the empirical evidence on the composition of matter. He
noticed that distilled water everywhere analyzed to the same elements, hydrogen
and oxygen. Similarly, other purified substances decomposed to the same
elements in the same proportions by weight.
“Therefore we may conclude that the ultimate particles of all homogeneous
bodies are perfectly alike in weight, figure, etc. In other words, every
particle of water is like every other particle of water; every particle of
hydrogen is like every other particle of hydrogen, etc.” Furthermore, Dalton
concluded that there was a unique atom for each element, using Lavoisier's
definition of an element as a substance that could not be analyzed into
something simpler. Thus, Dalton concluded the following: “Chemical analysis and
synthesis go no farther than to the separation of particles one from another,
and to their reunion. No new creation or destruction of matter is within the
reach of chemical agency. We might as well attempt to introduce a new planet
into the solar system, or to annihilate one already in existence, as to create
or destroy a particle of hydrogen. All the changes we can produce, consist in
separating particles that are in a state of cohesion or combination, and
joining those that were previously at a distance.”
Then he proceeds to give a list of relative weights in the compositions of
several common compounds, summarizing:
1st. That water is a binary compound of hydrogen and oxygen, and the relative
weights of the two elementary atoms are as 1:7, nearly;
2nd. That ammonia is a binary compound of hydrogen and azote nitrogen, and the
relative weights of the two atoms are as 1:5, nearly.
Dalton concludes that the fixed proportions of elements by weight suggest that
the atoms of one element combined with only a limited number of atoms of the
other elements to form the substances that he listed.
Alas, Dalton's atomic theory remained controversial (and unobvious) throughout
the whole of the 19th century. Whilst the Law of definite proportion WAS
accepted (and regarded even as ‘obvious’ by a few), the hypothesis that this
was due to atoms was not so widely accepted (read: ‘unobvious’). When Sir
Humphry Davy presented Dalton the Royal Medal from the Royal Society, Davy
explicitly says (rather than implicates) that the theory only becomes useful
when the atomic conjecture “IS IGNORED!” The fact that Sir Benjamin Collins
Brodie published the first part of his Calculus of Chemical Operations as a
non-atomic alternative to the Atomic Theory did not help. Sir Benjamin
describes Dalton’s atomic theory as a 'thoroughly materialistic bit of joiners
work,’ which seems to implicate “obviously FALSE.” It did not help Dalton’s
reputation, either, that Alexander Williamson used his Presidential Address to
the London Chemical Society to defend the Atomic Theory against its critics and
doubters. Why? Well, because this influential address led to further symposia
held by the London Chemical Society at which the positivists again attacked the
supposition that there even were atoms!
The matter was finally resolved in Dalton's favour in the early 20th century
with the rise of atomic physics – when everything suddenly became obvious to
all!
Cheers,
Speranza
