The discovery of Type-IV binary fluid phase behavior

The computer analysis by Scott and Van Konynenburg of the Van der Waals binary mixture for constant excluded volume uncovered the Type-IV phase diagram as a transition between Type-II and Type-III phase diagrams. The common boundary of the regions of Type-II and Type-III is a locus of tricritical points. The tricritical locus meets the common boundary of regions of Type-II and Type-IV at what Meijer coined the Van Laar point in 1989. The present paper shows, however, that as early as 1905 the Dutch chemist Van Laar produced Type-II and Type-III phase diagrams for the geometric-mean Van der Waals binary mixture, and found the exact coordinates of what we call the Van Laar point, but he did not notice tricriticality explicitly. He postulated and proved the existence of the Type-IV phase diagram. Aspects of his proof are discussed.