  From the time Newton introduced the notion of forces of affinity acting between atoms and responsible for their chemical behaviour there had been a problem forging a link between those forces and experimentally measurable effects. Daltonian atomists simply assumed that atoms combine in the way required to account for the measurable proportions of elements in compounds. The theory gave no account of the relative strengths of chemical bonding or hints of what would replace what in a chemical reaction. Chemical thermodynamics was able to make headway with this problem. Considerations based on entropy changes and heats of reaction made it possible to predict in which direction a particular chemical reaction will proceed and to provide an experimental measure of the affinities involved, where the affinities are not forces between atoms but provide a measure of the facility with which one macroscopic chemical substance combines with another. In the late nineteenth century leading scientists such as Ostwald, Duhem and Planck were inclined to take thermodynamics as the model of how science should proceed, maintaining a secure and productive relationship with experiment whilst avoiding hypotheses of the kind involved in atomism.