Liquidons and gasons; controversies about the continuity of states

Two basic ideas about the nature of the gas-liquid transition were voiced around 1870: that in the supercritical state, vapor and liquid are indistinguishable (Andrews), and that condensation and critical behavior can be understood on the basis of a simple assumption about molecular interaction (Van der Waals). For many scientists at that time, however, the notion of continuity of states was almost inconceivable. The older ideas, that liquid and gas molecules were different from each other, and that the liquid “dissolves” in the vapor at the critical point, survived for a long time. Unfamiliarity with the implications of Gibbs' thermodynamics in Europe helped to keep the older ideas alive. This paper describes the controversies that raged between proponents and opponents of the Andrews-Van der Waals view between 1880 and 1907, and how they were fanned by erroneous or incorrectly interpreted experiments carried out all over Europe. Kamerlingh Onnes and his staff in Leiden repeated a number of the controversial experiments in the period 1892–1907 and discovered that impurity was by far the largest source of error. By 1907 the controversy was unequivocally decided by the Leiden group in favor of the Andrews-Van der Waals view. A replay of the old controversy took place in the period 1933–1952.