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Modern Atomism

Most Physicists (and all Chemists) will probably agree that the crucial empirical observations that set modern science on the track of atoms (as we now know them) occurred around the transition between the Eighteenth and Nineteenth Centuries when a number of scientists including Antoine Laurent Lavoisier, Bryan and William Higgins, Joseph Louis Proust, John Dalton and Joseph Louis Gay-Lussac^23.1 discovered that certain chemical agents combined in simple integer ratios of their " MOLECULAR WEIGHTS" with other agents, a phenomenon most easily explained by assuming that these agents were the true chemical elements sought by the Alchemists^23.2 and furthermore that one MOLECULAR WEIGHT of any ELEMENT contained the same number of ATOMS of that element! This specific hypothesis is credited to Lorenzo Romano Amadeo Avogardo who in 1811 made a clear distinction between ATOMS (irreducible chemical units) and MOLECULES, which are clumps of atoms. For his trouble he got AVOGADRO'S NUMBER  N₀  named after him. The actual number of atoms (or, for that matter, molecules) in one MOLECULAR WEIGHT (or MOLE) of the corresponding element is

$$N_0 \equiv 6.02205 \times 10^{23} \hbox{\rm molecules per mole.}$$ (23.1)

You may recognize this number from the Chapter on THERMAL PHYSICS, in particular the Section on the KINETIC THEORY OF GASES, the qualitative assumptions of which dated back as far as Robert Boyle, Robert Hooke and Isaac Newton himself in the late Seventeenth Century. The work of Daniel Bernoulli in 1738 foreshadowed the use of kinetic theory by Joseph Loschmidt in 1865 to make the first determination of the value of  N₀  from measurements of the actual behaviour of gases. STATISTICAL MECHANICS actually played a major rôle in the development of modern Atomic theory, but its rôle is often downplayed in historical accounts simply because its is harder to understand. I will probably do likewise - but at least I admit it!