catalytic agencies such physical forces as light, electricity, extremes of heat or cold or the action of living tissues, and from this point of view the explosion of a cartridge or a charge of dynamite by percussion, the decomposition of water by electrolysis and its synthesis by the electric spark, the effects of light in photography and in healing disease, the wonderful thermodynamic effects of Henri Moissan's electric furnace, the occasional changes of food in cold storage, are further examples or analogues of catalytic action, and this is all we know of its physical nature. As to a dynamic explanation of how catalysis takes place, we have not got beyond the familiar jest of the laboratories: "Q. What is catalysis? A. Action by contact. Q. What is action by contact? A. Catalytic action." Gibbs's treatment of the subject is interesting as affording a mathematical criterion of what catalysis is and what it is not. It will be remembered that when the entropy of an isolated chemical system, say a bar of steel, has attained a maximum or its free energy a minimum value, the final state of the substance in question has been called by Gibbs a "phase of dissipated energy," implying that it has become physically and chemically inert, so that its equilibrium will not be sensibly disturbed by the presence of other substances or by such small physical agencies as an electric spark. But when the proportion of the proximate components of the substance in connection with its pressure and temperature is such that it does not constitute a phase of dissipated energy, the contact of a very small body or physical agency may produce energetic changes in its mass which do not stop short of complete dissipation. This is catalysis, and Gibbs's definition of a catalytic agent—one capable of reducing a substance to a phase of dissipated energy without limitation as to their relative proportions—is characteristic of the mathematician. A chemical system at constant temperature has several states of equilibrium corresponding to different minima of its isothermal potentials, and on the solid diagrams of Gibbs these minima are valleys at the bottoms of sloping curves. The effect of a catalytic agent on the diagram is to obliterate the ridge between two depressions representing different states of equilibrium on the free energy surface. This means that a system disturbed by a catalytic agent may pass from a higher to a lower minimum of free energy, but never from a lower to a higher unless acted upon by external forces of considerable magnitude. When the lowest minimum of free energy, indicated by the lowest depression on the diagram, has been attained, the substance can no more leave the final phase of dissipated energy than an inert body can be made to go up a hill without the intervention of external forces. On Gibbs's showing, the phase of dissipated energy is the criterion of catalytic action, the condition for which is that the substance acted upon should not have attained such a phase, while the forces operating flow, as in other mechanical, thermal, chemical or electric happenings, from higher to lower potentials. The accuracy of this reasoning is
