H₂ and He laboratory data and van der Waals vapour curve derived from Maxwell construct. Adiabatic compression curves of an initial sphere to sheet-, filament- and point-like geometries for interstellar conditions (T = 10 K, P = 10^-12 Pa) are shown, as explained in Appendix B. Sheet-like collapses are allowed to reach the phase transition regime even without cooling. Cooling would displaces the curves to lower temperature.