MadSci Network: Science History

Re: How did early knowledge of atom, molecules, etc. come about?

Date: Wed Jan 26 03:42:49 2005
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Science History
ID: 1106625775.Sh

Lindsey, this is a very broad question, and one that I find particularly fascinating. It opens 
up a lot of interesting history of science. I am going to write quite a long answer. I am not 
going to provide detailed references: you will need to do your own research on some of the 
names that I mention. Carmen Giunta's "Classic Chemistry" web pages are a good place to start.

"Atom" comes from a Greek word meaning "indivisible". The idea of atoms (as far as we 
know) started with two Greeks of about 400-500 BC: Democritus and Leucippus. We do not 
have much of their actual writings. We do have an extensive collection of the writings of 
Aristotle, who did not like their ideas, and quoted several parts of their works quite 
extensively in trying to rebut them.

The Greeks in the classical period were struggling with the concept of infinity. The argument 
about atoms went like this: if you have a rock, you can break it into gravel; gravel you can 
smash into sand; sand you can grind into dust; and so on. But sooner or later you must 
come to a particle that is so small that it cannot be broken up any further. This smallest-
possible particle is an atom.

A modern philosopher would simply say "Why? Why couldn't you go on for ever? Why 
couldn't matter be continuous, or infinitely divisible?" But the understanding of the Greeks in 
the classical period was different to ours, and this argument carried a lot of weight.

Leucippus thought that different materials had different types of atom: he supposed that the 
atoms of vinegar were sharp and prickly, while those of milk were like smooth spheres.

The Romans  were fairly methodical in taking over and developing any Greek science or 
philosophy that was of immediate use, and ignoring the rest (like atoms) which had no 
direct application. In mediaeval times, the ideas of Plato and Aristotle were hugely influential 
in Europe, and those of Leucippus and Democritus were therefore in disfavour. 

The modern atomic theory started at the very beginning of the 19th century, with some 
work by J.L. Proust (1799 & 1802) showing that pure chemical compounds -- or at least 
many of them -- have constant composition. Water, for example, always has just 7.94 times 
as much oxygen as hydrogen (by mass). If matter is continuous, it is hard to find an 
explanation of why this should be so: it would seem more likely that you should be able to 
make oxygen-rich water or hydrogen-rich water, just like you can bake an egg-rich or a 
sugar-rich cake. But IF water is made up of compound atoms, each consisting of an oxygen 
atom and a hydrogen atom, and an oxygen atom is just 7.94 times as heavy as a hydrogen 
atom, then it is easy to understand why water must have this constant composition.

(We now know that the molecule -- modern replacement for the 'compound atom' concept 
-- of water contains two hydrogen atoms and an oxygen atom, and that an oxygen atom 
weighs just 15.88 times as much as a hydrogen atom).

John Dalton collected a little extra evidence and published a book in which he described the 
first modern atomic theory in 1807. The decisive evidence came from the discovery of the 
law of multiple proportions: copper reacts with oxygen in the proportions 3.97:1 to produce 
black copper oxide, and 7.94:1 to produce red copper oxide, and 7.94 is just twice 3.97. 
Easily explained if the oxides contain one and two copper atoms respectively along with a 
single oxygen atom. Wollaston showed a similar result for the relative proportions of oxalic 
acid and potash in two potassium oxalate salts -- one and two compound atoms of potash 
respectively for each compound atom of oxalic acid. It is not too hard to see how this works 
with atoms, but if we regard matter as continuous, this sort of result can only be seen as a 
bizarre co-incidence.

Throughout the 19th century, the evidence for atoms was this sort of indirect evidence, and 
many prominent chemists did not 'believe in' atoms, or were agnostic about their existence. 
Furthermore, there was absolutely no way of determining how large or small atoms were -- 
except that they were much too small to be seen in the best available microscopes.

I cannot resist including one of my favourite quotations. Thomas Thompson had been very 
critical of the prominent English chemist Humphry Davy, for rejecting Dalton's atomic 
theory. John Davy, his brother and biographer defends Sir Humphry's point of view.

"Dr Thompson has mistaken it, not distinguishing between his sentiments on the 
hypothetical parts of Mr Dalton's views, the atomic doctrines relating to the weight, size, 
and number of atoms, and those which he held derived from the results of analysis, the 
facts and true foundations of the theory. The former are still, as they were at first, and as 
they ever must be, open to objection, being beyond the scrutiny of the senses and the test 
of experiment." (John Davy, 1836, Memoirs of the life of Sir Humphry Davy, Vol II, p. 439.)

A Chemistry textbook by Alexander Smith, an American chemist, published in 1906, still 
maintained an agnostic view about the actual existence of atoms, and even suggested that 
an atom should be thought of as a measure of quantity, like a bushel or a pint.

A series of investigations by physicists in the late 19th and early 20th centuries provided the 
first direct evidence of atoms, subatomic particles, and the internal structure of atoms. 
These investigations also provided the first opportunity for measuring the actual size of an 
individual atom. Some of the prominent names in various aspects of this work are Rutherford, 
J.J. Thompson, Millikan, Moseley, Niels Bohr, and Chadwick. By the 1920s enough more direct 
evidence had accumulated that no scientist could seriously doubt the physical existence of 
atoms. In the 1960s new microscopic techniques were developed that eventually produced 
the first images of individual atoms. (Enter "field ion microscopy" in the MadSci search 
engine). Other forms of microscopy that could distinguish features down to the atomic size 
level were developed at the end of the twentieth century.

A lot of the material in this answer is drawn from the historical and philosophical researches 
of Maureen Christie, whose help I acknowledge.

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