ABSTRACT
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 . . . [Ilts 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.]
By the end of the nineteenth century most physicists and chemists believed in the reality of atoms and, like Maxwell, thought that they were indestructible and indivisible, though not necessarily structureless, as some internal constitution was required to explain their observed spectra. That atoms indeed have an internal
constitution became the accepted view when in 1899, two years after his discovery of the electron, Joseph John Thomson announced 'the splitting of the atom,' for he had established that in the process of ionization part of the mass of the atom got free and became detached from the original atom. Similarly, soon after its discovery in 1896, it became apparent that the explanation of radioactivity in terms of the expulsion of subatomic particles implied a 'divisible atom.' Thus Marie Curie noted in 1900, that the atoms of radioactive elements are 'indivisible' from the chemical point of view but 'divisible' when undergoing a radioactive decay. The picture which emerged in the first decade of the twentieth century, was the following: when the energy available in the laboratory setting is insufficient for ionization, (non-radioactive) atoms could be considered 'elementary' constituents of matter. But in other environments (such as ones where high voltages or high temperature exist) they may split into their constituent parts.
