42 TEE POPULAR SCIENCE MONTHLY 



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 percus- 

 sion, 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 fur- 

 nace, the occasional changes of food in cold storage, are further exam- 

 ples 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 sub- 

 stance to a phase of dissipated energy without limitation as to their 

 relative proportions — is characteristic of the mathematician. A chem- 

 ical 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 



