Page:Popular Science Monthly Volume 74.djvu/483

Rh vanish, and nothing would remain but the motions of material systems and the laws of mechanics. Hence the second law must either be obtained from our actual experience "with real bodies of sensible magnitude," or else derived a posteriori, as shown by Boltzmann, Helmholtz and Gibbs, from averages of the hypothetical motions of mechanical systems. In aid of this conception of the problem, Maxwell introduced his whimsical notion of the "sorting demon," a being endowed with molecular vision, who would be able through intelligence alone to sort or direct the molecular movements at will and so reverse the action of the second law on occasion.

The second stage of thermodynamics begins in 1872-3 with August Friedrich Horstmann's application of the entropy principle to problems of chemical dissociation. In October, 1873, Horstmann announced the condition for chemical equilibrium to be that of maximum entropy, and in December of the same year Gibbs, in a modest footnote, stated that the condition for thermodynamic equilibrium in a chemical system at constant temperature and pressure is that the function now universally known as the thermodynamic potential should be a minimum. In 1875 Lord Rayleigh stated that dissipation of energy is a sufficient if not a necessary condition for chemical change, and in October, 1875, appeared the first installment of Gibbs's memoir of three hundred pages on chemical equilibrium, which, by its applications of the entropy principle to all physico-chemical or energetic phenomena, has become a true scientific classic doing for the second law what Helmholtz, in his treatise on the conservation of energy, had previously done for the first.

Gibbs began his work in thermodynamics in 1873, with two important papers on diagrams and surfaces. In the first of these he made a careful and thoroughgoing study of all the diagrams that might be of use or value in thermodynamics, the best known being that upon which volume and pressure are erected to scale as coordinates, derived from the familiar "Watts' indicator diagram found upon every steam engine. Of the new diagrams which Gibbs introduced, he attached most importance to the volume-entropy diagram, because it tells more about the physical properties of a working substance than about the heat employed or the work done. But the most important of Gibbs's innovations for practical engineering purposes is the