ABSTRACT
In 1867, in his inaugural lecture as the first Cavendish Professor, Maxwell remarked that “Two theories of the constitution of bodies have struggled for victory with various fortunes since the earliest ages of speculation: one is the theory of a universal plenum, the other is that of atom and void.” Atomism had gained ascendancy ever since Newton and by the beginning of the nineteenth century John Dalton could base his New System of Chemical Philosophy on the belief that there existed a considerable number of what he called “elementary principles,” which can never “be metamorphosed, one into another, by any power we can control” and that “all atoms of the same kind, whether simple or compound, must necessarily be conceived to be alike in shape, weight, and every other particular.” By the late 1860s some 70 chemical elements were known, and in his Cavendish lecture Maxwell, echoing Dalton, noted that “the molecule … is a very different body from any of those with which experience has hitherto made us acquainted … [I] ts the mass, and the other constants which define its properties, are absolutely invariable … [Furthermore] there are innumerable other molecules, whose constants are not approximately, but absolutely identical with those of the first molecule, and this whether they are found on earth, in the sun, or in the fixed stars.” But Maxwell concluded his address with the question: “But what if these molecules, indestructible as they are, turn out not to be substances themselves, but mere affections of some other substance … a uniformly dense plenum?” The tension between what kind of substance, whether material particles or a continuous plenum has persisted, as has the question of what is primary, substance or law. [If an underlying plenum is assumed, ‘particles’ are then considered localized, stable concentrations of energy, spin, charge and other constant attributes.]
