In our days, it seems natural using a system of hard bodies perturbed by intermolecular potentials for the modeling of the thermodynamic properties of fluids. In fact, it seems so natural that one hardly notices the misuse of the term ‘intermolecular potential’ in the previous phrase to refer to the ‘interhard- body-potential’ purported to exist for the modeling of fluids. The reason for this tacit acceptance is that, in our mental models for fluids, we identify molecules with hard bodies, almost as a primitive concept. The question then naturally arises: Was it always this way? Or, perhaps, was there a time when scientists did not recognize the existence of molecules? Could it be possible that there was a time when scientists would not accept the use of a system formed by hard bodies as the supporting structure for the modeling of fluids? It is not easy to imagine that less than one hundred years ago, the existence of atoms and molecules was still under discussion.