er, and that the nature of elasticity, and the reason for caloric being elastic, remains still unexplained. (Lavoisier 1789, 22) Duhem thought that ascribing atoms combining powers in the form of atomicities (valencies) amounted to simply reading properties of elements in compounds apparent at the macroscopic level into atoms, providing, to paraphrase Lavoisier, an explanation of the combining power of elements by an assumption of Paul Needham – Was Duhem Justified in not Distinguishing Between Physical and Chemical Atomism? 110 the combining power of elements. The atomic theory of the hydrogen molecule proposed by Heitler and London is not at all like this, but provides a substantial theory of the quantum nature of hydrogen atoms from which the stability of the hydrogen molecule is derived. What was on offer at the time Duhem was writing which went beyond empty ascriptions to atoms of the properties that were to be explained seems to have been restricted to ideas about the shape of microentities or the vortex theories. These may not have incited a great deal of interest on the part of chemists (although Jones (1902, 38-9) is an example of a chemist who found some interest in vortex theories) and Duhem's dismissing them without difficulty may have been equally uninteresting to chemists. But it served to make the point that there was nothing on offer beyond tautology that looked like a promising start to an atomic theory of chemical combination. Whatever the explanatory merit of a substantial atomic theory, how could it be anything but a physical theory of atoms? To say otherwise is surely to court the occult, bringing to mind the pre-enlightenment division of the universe into suband superlunary regions with modes of explanation peculiar to each. Certainly Duhem thought it was anathema to modern science to bifurcate phenomena into separate realms with laws appropriate to each. This was the philosophical basis of his opposition to Berthelot's defence of the principle of maximum work, which presupposed that a dis