ELECTRON).
It was formerly thought possible to arrange the elements in the order of their chemical affinity, so that each element in the array would expel all those that preceded it from their com binations, and tables of this sort were published two centuries ago (in 1718) by Geoffroy. Later, it was found that no such definite law exists; and Claude Louis Berthollet, in his fa mous de Statique Chimique' (Paris 1803), pointed out that the combination of sub stances depends not only upon the affinity that such substances have for each other, but also upon the relative quantities in which they are present. For example, if barium sulphate and potassium carbonate are melted together, the former is always partially converted into car bonate; but in order to effect its total conversion into carbonate, it is necessary that the quan tity of carbonate of potassium present shall be at least six or seven times as great as the equation BaSO. -I- IC:SO.
would appear to require. In recent times the necessity of taking account of the relative masses of the combining substances has been fully recognized, and a good idea of the im portant results that have followed may be had from Nernst's 'Theoretical Chemistry from the Standpoint of Avogadro's Rule and Thermo dynamics' (translated into English by Prof. C. S. Palmer). Numerical estimates of the "affinity" of various substances have been ob tained by many methods, but the results are not always in as good accord with one another as could be wished, and a discussion of the differences would require a volume. For a brief but excellent review of the different methods consult W. C. D. Whetham, 'Solution and Elec trolysis.' See DISSOCIATION; ELECTRON; EQUI LIBRIUM (Chemical) ; and SOLUTIONS.