  Acceptance of the kinetic theory implied acceptance of the existence of atoms and molecules with a well-defined mass. However, it was perfectly clear that they must have further properties. For example, they needed properties that would explain chemical combination, and, specifically, the notion of valency. They also needed properties that would account for spectra. Answers to these challenges were forthcoming in the form of the electron structure of the atom and the quantum mechanics that governs it. There is a sense in which contemporary physics, with its account of the properties of atoms and molecules in terms of their electron structure and the explanation of many macroscopic phenomena in terms of the atomic and molecular structures underlying them, comes close to the ideal of Democritus. A general account of the properties of the material world is offered in terms of underlying particles with a few well-defined properties governed by well-defined laws. The difference between the contemporary situation and the ideals of Democritus or the mechanical philosophers lies in the epistemological access to the general atomistic theory. The contemporary theory became possible only as a result of centuries of scientific development. The quantum mechanical laws governing the atomic world were responses to quite specific problems revealed by experiment in areas such as black-body radiation, emission and absorption spectra, the specific heats of gases and radioactivity. The properties ascribed to electrons, for instance, such as their charge and half-integral spin, were themselves responses to quite specific experimental findings involving discharge tube phenomena and spectra.  Atomism, which began its life as speculative metaphysics, has become a securely established part of experimental science.