  Gibbs (1876–8) developed a theory to account for what, from the point of view of the atomic theory, had been regarded as ‘anomalous’ vapour densities by regarding them as consisting of a mixture of vapours of different chemical constitution in thermal equilibrium. The theory was able to predict relative densities of the component vapours as a function of temperature in a way that was supported by experiment. It is true that atomists could not only accommodate this result by interpreting it in atomic terms but also welcomed it as a way of removing the problems the phenomena had caused for the determination of molecular weights from vapour densities. But it remains the fact that the thermodynamic predictions are independent of atomic considerations once it is recognised that the chemical formulae needed for them can be, and were, obtained and interpreted in a way independent of atomism. As a matter of historical fact, Deville, the major participant in the experimental confirmation, was opposed to atomism, as Duhem (2002, 96–7) stressed.