It was not until M.Van der Waals exhumed the memoir of Gibbs, when numerous physicists or chemists, most of them Dutch--Professor Van t'Hoff, Bakhius Roozeboom, and others--utilized the rules set forth in this memoir for the discussion of the most complicated chemical reactions, that the extent of the new laws was fully understood.
The chief rule of Gibbs is the one so celebrated at the present day under the name of the Phase Law.

We know that by phases are designated the homogeneous substances into which a system is divided; thus carbonate of lime, lime, and carbonic acid gas are the three phases of a system which comprises Iceland spar partially dissociated into lime and carbonic acid gas.

The number of phases added to the number of independent components--that is to say, bodies whose mass is left arbitrary by the chemical formulas of the substances entering into the reaction--fixes the general form of the law of equilibrium of the system; that is to say, the number of quantities which, by their variations (temperature and pressure), would be of a nature to modify its equilibrium by modifying the constitution of the phases.
Several authors, M.Raveau in particular, have indeed given very simple demonstrations of this law which are not based on thermodynamics; but thermodynamics, which led to its discovery, continues to give it its true scope.

Moreover, it would not suffice merely to determine quantitatively those laws of which it makes known the general form.

We must, if we wish to penetrate deeper into details, particularize the hypothesis, and admit, for instance, with Gibbs that we are dealing with perfect gases; while, thanks to thermodynamics, we can constitute a complete theory of dissociation which leads to formulas in complete accord with the numerical results of the experiment.