Parachor and Critical Co-ordinates

Abstract

IN 1923, Macleod discovered an empirical formula¹ which was valid near the critical point of a pure substance where d_l and d_g are the densities of the liquid and gas, respectively, and σ is the surface tension of the interface separating the liquid and gas. This formula was expressed by Sugden as² where M is the molecular weight of the substance, and P is a constant (characteristic of the substance) called the parachor. As is well known³, equation (1) is only approximately true; that is, the parachor varies slightly with temperature near the critical point. In recent years the parachor has fallen into desuetude, undoubtedly because it was linked² to an empirical view of the structure of atoms and molecules, which is no longer acceptable. Nevertheless, equation (2), considered only as an approximate experimental law, can still be useful as will be seen later.