Page:Popular Science Monthly Volume 74.djvu/485

Rh of diagrams, immediately recognized the importance of this paper, part of which he incorporated as a chapter in his "Theory of Heat," and, shortly before his death, he sent Gibbs a copy of a model of the thermodynamic surface constructed to scale with his own hands. These solid diagrams have played a great part in the elaborate studies of the continuity of gaseous and liquid states by Van der Waals and his pupils, of which we have recently witnessed the final triumph in the liquefaction of helium.

During the years 1875-8, Gibbs published the work which is his chief title to fame, his memoir "On the Equilibrium of Heterogeneous Substances." This treatise deals, as the title implies, with the statics of chemical substances which, as gases, vapors, liquids or solids, are in actual physical contact with each other, whether influenced or modified by gravity, osmosis, catalysis, capillarity or electromotive force or existing under such varied aspects as gaseous mixtures, liquid films, "solid solutions," or crystals. For the first time chemical substances are treated as continuous or contiguous "phases" of "matter in mass" acted upon, like mechanical systems, by forces having "potentials"—a new way of looking at things which has since become the definite view-point of physical chemistry. As Larmor says, "Gibbs made a clean sweep of the subject, and workers in the modern experimental science of physical chemistry have returned to it again and again to find their empirical principles forecasted in the light of pure theory, and to derive fresh inspiration for new departures." Some of its theorems, as the Helmholtz doctrine of free energy, Konowalow's theorem of indifferent points, Curie's theory of "crystal habit, were rediscovered by later investigators in ignorance of the earlier work. Indeed the primary intention of Gibbs's memoir, to treat chemical changes as a branch of mechanics, was not, at first, clearly understood, the long deferred review in the "Fortschritte der Physik" merely listing its contents. Maxwell, however, with the same cordial recognition which he had shown to Rowland, grasped its significance at once, incorporated some of its results in his memoir on "Diffusion," and in an appreciative discourse before the Cambridge