Contributions to the knowledge of the Van der Waals ψ-surface II
The part of the transverse plait in the neighborhood of the plaitpoint in Kuenen’s experiments on retrograde condensation
Abstract by Johanna Levelt Sengers

This short note describes an amazing experment that Kamerlingh Onnes performed after he had obtained a few liters of helium gas for the first time, only 1½ years prior to the liquefaction of helium. Kamerlingh Onnes pressed helium gas into a glass container filled with liquefied hydrogen at its boiling point of 20 K, far below the hydrogen critical point of 33 K, but far above that of helium, then unknown. As he increased the pressure, the density of the helium-rich vapor exceeded that of the hydrogen-rich liquid, after which the vapor and liquid exchanged their positions in the tube. The experiment fascinated Van der Waals, who explained this barotropic phenomenon at the Academy a month later. Up to the highest pressures he could reach in the glass tube, Kamerlingh Onnes could not make the two phaeses mix, fore-shadowing the phenomenon of gas-gas phase separation.