of Affinity, as far as any person acquainted with chemical operations could contend for it. For it must be allowed by all, that solubility, and other collateral circumstances, influence the course of chemical combinations, since they determine whether or not there shall take place that contact of elements without which affinity cannot possibly operate. [Note 23\6: _Hist. Ind. Sc._ b. xiv. c. iii.] [Note 24\6: _Chemistry_, p. 199. 6th edition.] [Note 25\6: _Leçons de Philosophie Chimique_, p. 386.] 8. _Affinity is Definite as to quantity._--In proportion as chemists obtained a clearer view of the products of the laboratory as results of the composition of elements, they saw more and more clearly that these results were definite; that one element not only preferred to combine with another of a certain kind, but also would combine with it to a certain extent and no further, thus giving to the result not an accidental and variable, but a fixed and constant character. Thus salts being considered as the result of the combination of two opposite principles, acid and alkali, and being termed _neutral_ when these principles exactly balanced each other, Rouelle (who was Royal Professor at Paris in 1742) admits of neutral salts with excess of acid, neutral salts with excess of base, and perfect neutral salts. Beaume maintained[26\6] against him that there were no salts except those perfectly neutral, the other classes being the results of mixture and imperfect {25} combination. But this question was not adequately treated till chemists made every experiment with the balance in their hands. When this was done, they soon discovered that, in each neutral salt, the proportional weights of the ingredients which composed it were always the same. This was ascertained by Wenzel, whose _Doctrine of the Affinities of Bodies_ appeared in 1777. He not only ascertained that the proportions of elements in neutral chemical compounds are definite, but also that they are reciprocal; that is, (to express his results in a manner now employed by chemists), that if A, a certain weight of a certain acid, neutralize _m_, a certain weight of a certain base, and B, a certain weight of a certain other acid, neutralize _n_, a certain weight of a certain other base; the compound of A and _n_ will also be neutral; as also that of B and _m_. The same views were again presented by Richter in 1792, in his _Principles of the Measure of Chemical Elements_. And along with these facts, that of the combination of elements in multiple proportions being also taken into account, the foundations of the Atomic Theory were laid; and that Theory was propounded in 1803 by Mr. Dalton. That theory, however, rests upon the Idea of Substance, as well as upon that Idea of Chemical Affinity which we are here considering; and the discussion of its evidence and truth must be for the present deferred. [Note 26\6: Dumas, _Phil. Chim._ p. 198.]