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Democritus (460-340 BC)

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Democritus [1] of Abdera (Thrace, in NE Greece) and his teacher Leucippus [2] (?-480 BC) of Miletus (Anatolia, on the Mediterranean coast of Turkey) are recognized as the founders of the theory of Atomism.

There is no doubt that Democritus' Atomism was a theory of the physical world, but consistent with the times, he was satisfied with a qualitative explanation of the world. No measurements needed. He expressed the view that things such as warmth, color and taste were caused by atoms and were not themselves beings. These were not trivial insights: heat was thought to be a fluid (a being) as recently as the mid 1800's.

Democritus also had a theory of ethics, based on his theory of nature, and it was equally perceptive: the ultimate good was cheerfulness

... a state in which the soul lives peacefully and tranquilly, undisturbed by fear or superstition or any other feeling.

Democritus' ideas gave rise to the Epicurean view of the world (Epicurus, 341-270 BC), a philosophy rigorous enough to count such influential Romans as the orator Cicero and the poet Horace among its disciples. The epicurean insistence on material causes for all aspects of our world eventually ran into conflict with the growing influence of Christianity. By the fifth century AD Epicureanism survived only as a marginal philosophy.

[1] Democritus, University of St Andrews, Scotland.
[2] Leucippus, University of St Andrews, Scotland.

Lucretius (99-55 BC)

It is hard to mention Epicureanism without mentionning Lucretius, a Roman poet and the author of the philosophical epic De Rerum Natura (On the Nature of Things), a comprehensive exposition of the Epicurean world-view. Rediscovered in the 17th century, the De Rerum has been part of the intellectual background of virtually every evolutionary theorist in Europe from Lamarck to Herbert Spencer -- including Darwin himself [1].

[1] Lucretius, The Internet Encyclopedia of Philosophy.

John Dalton (1766-1844)

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The atomic theory of matter was revived in the early 1800's through the work of John Dalton [1]. It also took a decided turn from the philosophical to the physical. In 1780 Gay-Lussac had proposed the Law of Conservation of Mass by carefully weighting all elements going into and out of chemical reactions: total mass was conserved. Building on this result and after a careful analysis of the mass ratios of elements reacting during chemical reactions, Dalton formulated his Law of Simple Multiple Proportions, and his atomic hypothesis (1803-1808).

  1. Matter is composed of indivisible particles called atoms.
  2. There is one type of atoms for each basic chemical element and atoms of one type are all perfectly identical.
  3. Atoms are unchangeable.
  4. When elements combine, the smallest unit of the compound has a definite number of atoms of each element.
  5. Chemical reactions do not create or destroy new atoms, they only rearrange them.
From the new atomic theory it was easy to derive the old conservation laws. As an amusing aside, Dalton's 1808 writings also included his theory of heat as a liquid called the caloric.

Dalton did not prove that atoms existed, he postulated that they did. And the postulate proved fruitful as both an explanatory and predictive tool. Dalton understood well the diversity of atoms and their relative masses, but he could not estimate their sizes and he had couldn't count how many existed in a given mass of an element.

[1] Dalton, Salt Lake Community College.

Loschmidt and the Size of Atoms, 1865

The atomic theory was refined by numerous chemists and physicists. Hardly any scientist of that era did not dabble in atoms and contributions flowed from many quarters. Johann Loschmidt, in 1865, was the first to give a well reasoned and accurate estimate of the size of a molecule of air [1] and thereby of all small molecules and atoms: one millionth of a millimeter.

[1] Loschmidt-1865, On the Size of the Air Molecules, John Park's ChemTeam: Classic Papers from the History of Chemistry, (Cached).

Maxwell and the Number of Atoms, 1873

Knowing the size of a small molecule doesn't immediately tell you how many molecules or atoms exist in a given volume of a the corresponding substance. One needs a way to account for the space between the atoms or molecules. In 1873, James Clerk Maxwell gave a first estimate of Loschmidt's Number [1], the number of molecules in 1 cm3 of air. His experiments used some of Loschmidt's results. He estimated 1.9 x 1019 (nineteen million million million) molecules. Today one prefers Avogadro's Number, NA, the number of molecules in 22.4 liters of a gas, and Maxwell's estimate corresponds to an NA value of 4.25 x 1023. NA is now known to great accuracy and is approximatively 6.02 x 1023.

[1] Molecules, Lecture delivered before the British Association at Bradford, by Prof. Clerk-Maxwell, F.R.S., Le Moyne College, Classic Chemistry compiled by Carmen Giunta (Cached).