In 1924, Gilbert Lewis and Merle Randall published Thermodynamics and the Free Energy of Chemical Substances (1st Edition, McGraw-Hill Book Co., Inc., New York). On page 225 these authors state, “As far as is known, all gases are miscible with one another in all proportions.” In short, they were referencing that no known evidence has been presented to indicate a miscibility gap exists in the vapor phase of mixtures (in other words, vapor–vapor equilibrium).

With the benefit of about a century of research since that time, what is the current knowledge about the existence of vapor–vapor equilibrium for mixtures? Provides evidence (a brief description) plus a citation (if applicable).