VI. On chemical dynamics and statics wider the influence of light

The nature of the forces which come into play when substances react one upon another chemically, is a problem which has specially engaged scientific minds during the last century. During the second half of that period chemical statics and dynamics have developed into a veritable science. The general law governing the velocity of chemical reaction and chemical equilibrium in homogeneous systems is now known as the law of mass action, and was to a great extent foreseen by Berthollet. In heterogeneous systems the law concerning the velocity of physical or molecular transformation also proves to be of a general and simple nature; the velocity being directly proportional to the surface of contact of the reacting parts of the heterogeneous systems and to the remoteness of the system from the point of equilibrium. The velocity of chemical reaction and chemical equilibrium in heterogeneous systems represent no phenomena sui generis , the laws, concerning them being only combinations of the above two laws. The laws relating to equilibrium found their rational explanation and foundation in the thermodynamic researches of Horstmann, and more fully in those of W. Gibbs and van’t Hoff, whilst the laws applying to the velocity of reaction in homogeneous systems are the result of van’t Hoff’s thermodynamic considerations. In all the above researches the phenomena of the velocity of chemical reaction and of chemical equilibrium are the outcome of those intrinsic properties of matter, always existent in and inseparable from it, which we usually call chemical affinity or chemical potential. It is known, however, that a system can be brought into a state of reaction, and that new systems and new equilibria can be formed, when energy from an external source, such as light or electricity, is introduced into it. The effect of an electric current upon a chemical system, e.g ., is determined by Faraday’s law of electrolysis, whilst the thermodynamic connexion between chemical and electrical (and gravitation) energy has been developed by W. Gibbs.