, if we apply to the two fluids a
pressure depending on temperature alone. Thermodynamics will furnish
us, in addition, with the expression of the heat of vaporization and
of, the specific heats of the two saturated fluids.

This new study has given us also most valuable information on
compressible fluids and on the theory of elastic equilibrium. Added to
certain hypotheses on electric or magnetic phenomena, it gives a
coherent whole from which can be deduced the conditions of electric or
magnetic equilibrium; and it illuminates with a brilliant light the
calorific laws of electrolytic phenomena.

But the most indisputable triumph of this thermodynamic statics is the
discovery of the laws which regulate the changes of physical state or
of chemical constitution. J.W. Gibbs was the author of this immense
progress. His memoir, now celebrated, on "the equilibrium of
heterogeneous substances," concealed in 1876 in a review at that time
of limited circulation, and rather heavy to read, seemed only to
contain algebraic theorems applicable with difficulty to reality. It
is known that Helmholtz independently succeeded, a few years later, in
introducing thermodynamics into the domain of chemistry by his
conception of the division of energy into free and into bound energy:
the first, capable of undergoing all transformations, and particularly
of transforming itself into external action; the second, on the other
hand, bound, and only manifesting itself by giving out heat. When we
measure chemical energy, we ordinarily let it fall wholly into the
calorific form; but, in reality, it itself includes both parts, and it
is the variation of the free energy and not that of the total energy
measured by the integral disengagement of heat, the sign of which
determines the direction in which the reactions are effected.

But if the principle thus enunciated by Helmholtz as a consequence of
the laws of thermodynamics is at bottom identical with that discovered
by Gibbs, it is more difficult of application and is presented under a
more mysterious aspect. 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 pr