Evolution of the Doctrine of Affinity. 509 



acids, that there was absolutely no connexion between avidity 

 or relative affinity and heat of neutralization. Even the order 

 of magnitude of the two, when a number of instances is com- 

 pared, is entirely different. The strongest of all acids — nitric 

 acid — occupies only the nineteenth place among forty acids, 

 when they are arranged in the order of their heats of neu- 

 tralization ; while hydrofluoric acid evolves most heat on 

 neutralization, although its avidity is only one twentieth of 

 that of nitric acid ; and so with other instances. As it would 

 be absurd to ascribe the greatest affinity to a base to an acid 

 which is in great part expelled by another, it must be acknow- 

 ledged that the fundamental hypothesis of the thermal doctrine 

 of affinity is not justified by fact. 



But a further conclusion follows from Thomsen's investi- 

 gations, namely, that, while the heat of formation of compounds 

 depends on the nature of their constituents, it does not, at 

 least in many cases, depend on any special change, attraction, 

 or affinity in which both constituents are concerned. This is 

 most easily seen when the heat of formation of salts from strong 

 bases and acids is considered. If, as Thomsen has experimen- 

 tally shown, the total amount of heat evolved during the process 

 of formation of a salt from the elements which it contains, and 

 its solution in a large quantity of water be measured, the 

 extremely remarkable fact is to be noticed that a definite dif- 

 ference in composition involves a similarly definite difference 

 in heat of formation, varying only within very narrow limits*. 



The heat of formation of a salt of lithium is, for example, in 

 round numbers, always 11,400 calories greater than that of a 

 salt of sodium with the same acid, and about 2000 calories 

 greater than that of a salt of potassium ; and so for other 

 metals. This has been proved for the chlorides, bromides, 

 iodides, hydrates, hydrosulphides, sulphates, dithionates, and 

 nitrates of nineteen metals. If, on the other hand, the metal 

 remain the same, but the acid radical be varied, a definite 

 difference in the heat of formation is again to be observed for 

 each acid radical. That of the bromides is always about 

 21,800 calories less than that of the chlorides, and that of the 

 iodides 52,300 less; while the chlorides invariably evolve 

 during their formation about 200,000 calories less than the 

 corresponding sulphates. The heat of formation of every salt 

 may therefore be represented as the sum of certain numbers, 

 each of which numbers is peculiar to one constituent group or 

 element, and remains constant into whatever form of combi- 

 nation that element or group enters. It is therefore possible, 



* Thomsen, Thermoch. Untersuch. iii. pp. 290, 456, 545; see also 

 Lothar Meyer's Moderne Theorien der Che?nie, 5th edition, p. 448. 



