Johan Kuenen (1866-1922) did experimental research in the Leiden Physics Laboratory into mixtures of gases. He studied among other things the transition from the gaseous to the liquid phase. Plaster models were made in the laboratory specially for this research. At different temperatures Kuenen measured the volume, the proportions of the mixture and the free energy of a gas. The relationship between these components was represented in a plaster model. The transitions from gas to liquid were represented in such models by metal wires.
