The Reality of Molecules
1. Chemistry . In 1771 work was completed on the first edition of the Encyclopedia Britannica, a Dictionary of Arts and Sciences compiled upon a new plan . . . by a Society of Gentlemen in Scotland.' The entry atom, written by William Smellie, a man renowned for his devotion to scholarship and whisky [K1], reads as follows. 'Atom. In philosophy, a particle of matter, so minute as to admit no division. Atoms are the minima naturae [smallest bodies] and are conceived as the first principles or component parts of all physical magnitude.' Democritus might have disagreed, since his atoms were not necessarily minute. Epicurus might have objected that the atom has structure--though it cannot be divided into smaller parts by physical means. Both men might have found the definition incomplete since it did not mention that atoms--as they believed--exist in an infinite variety of sizes and shapes, any one variety being forever incapable of transforming itself into any other. They might have wondered why no reference was made to the πρώτη λη, the prime matter of which all atoms are made. It is likely, however, that an imaginary dialogue between the Greek and the late eighteenth century philosophers might rapidly have led to a common understanding that in the two thousand years which separated them very little had changed regarding the understanding of the basic structure of matter.
The period of rapid change began in 1808, when John Dalton commenced the publication of his New System of Chemical Philosophy [D1]. This event marks the birth of modern chemistry, according to which all modes of matter are reducible to a finite number of atomic species (eighteen elements were known at that time). Dalton's early assessment (in 1810) of the youngest of the sciences sounds very modern: 'I should apprehend there are a considerable number of what may be properly called elementary principles, which can never be metamorphosed, one into another, by any power we can control. We ought, however, to avail ourselves of every means to reduce the number of bodies or principles of this appearance as much as possible; and after all we may not know what elements are absolutely indecomposable, and what are refractory, because we do not know the proper means for their reduction. We have already observed that all atoms of the same kind, whether simple or compound, must necessarily be conceived to be