Residual Affinity

Abstract

I WAS greatly interested in Prof. Armstrong's recent articles on “Residual Affinity,” as it is a subject I brought before the Royal Society of Edinburgh fully nine years ago, as one of the main causes of solution, molecular compounds, &c. I was, however, somewhat disappointed with the conclusions he came to, and was tempted to exclaim in Scriptural language, “Ye did run well; what did hinder you that you are again entangled in the yoke of bondage?” Prof. Armstrong comes to the conclusion that HCl and NH₃ combine owing to the residual affinity of Cl for N. Now how can this be? If we regard it from a thermal point of view, we find that, in the combination of HCl with NH₃, 41,900 units of heat are given out, while the combinations H with Cl and N with H₃ give out 22,000 and 11,890 units respectively; that is, the residual affinity of N for Cl, as measured by heat, exceeds by about one-third the sum of the affinities of H for Cl and H₃ for N; and yet, under ordinary circumstances, Cl has very little affinity for N. Is it not more rational to conclude that the residual affinity is not confined to the negative elements, but extends to both, and that the combination of HCl and NH₃ is due mainly to the residual affinity of Cl and N for H? It is easy to understand that this residual affinity is so lowered in intensity that neither Cl nor N can retain unassisted more than one and three atoms; but when the energy of the H is reduced by combination with another body, each of them can then act upon it. That residual affinity exists in both positive and negative elements seems to roe evident from the fact that the heats of solution of salts in water vary directly as the affinity of the metal for the O of the water and also directly as the affinity of the negative element for the H, as I have pointed out in my letter on “Laws of Solution,” in NATURE, vol. xxxiv. p. 263. It seems strange to me that chemists will search out for occult causes of phenomena which can much more easily be explained by what is already known of the actions of one element on another rather than abandon the assumption that chemical affinity acts in definite units.