AFFINITY, CHEN' ICAL. The force that bolds in combination the constituent elements of chemical compounds and eaus•s the reactions taking place between material substances. The nature of chemical affinity is as little understood as the nature of gravitation, and the hypotheses on the subject, which have been advanced since the earliest times, arc still confined nl ithin the domain of pure speculation. Borelli and Lein cry imagined that the ultimate particles of matter were supplied with minute hooks, the shape of which determined the capacity of a particle for combining with certain other par ticles. Bergman, Berthollet, and others. thought that chemical affinity might be identical with the energy of gravitation. Berzelins sought to explain all eheinical phenomena on the hy pothesis that chemical el mbination was caused by the mutual attraction of electrically different substances. All these hypotheses. hoNvever. go no further in explaining the transformations of matter than Jul the ancient idea, according to which those transformations were due to the mutual love or hatred of the different kinds of atoms. tiurli ideas are ineapalde of either theo retical development or practieal application, and science must, at least for the present, discard them :is useless hypotheses and confine itself solely to the experimental study of the mode of action of the chemical forces, without reference to their ultimate nature. In this direction the
science of chemistry has, in recent years, made considerable progress. The principles of thermo dynamics have been successfully applied to many transformations, and certain general laws have been established, according to which all chemical reactions seem to take place. The second prin ciple of thermodynamics proves Ihat when a transformation takes place in a material system while no energy is being supplied to it. from without, the system is capable of doing a certain amount of external work. The maximum ex ternal work which may be obtained through a transformation taking place under ideal condi tions (that is to say, through a reversible iso thermal process), may be taken as a measure of the tendency according to which the transforma tion takes place. In the case of a chemical trans formation, that tendency is obviously the "affin ity of the reaction." This maximum external work, done during a chemical reaction, or, as it is usually expressed, the change of free energy involved in a reaction, is ascertained either by studying reacting mixtures after they have reached the state of equilibrium, or, in the case of galvanic combinations, by determining the eleetro-motive force. See articles REACTION,