508 Prof. L. Meyer on the 



tendency towards (tends vers la) the production of such states 

 of combination. - "* But even in order to reconcile this state- 

 ment with reactions distinctly accompanied by a negative 

 heat-change, far-fetched and artificial explanations of an 

 unsatisfactory nature are necessary. Nevertheless, the funda- 

 mental hypothesis — that the heat of combination is, in reality, 

 affinity transformed into kinetic energy — might have passed 

 as true for a much longer time, had not the progress of 

 thermo-chemical research shown it to be thoroughly un- 

 tenable. 



It will be remembered that Julius Thomsenf made use of 

 the positive or negative heat-change accompanying a chemical 

 reaction to determine the extent to which the reaction had 

 proceeded ; and this was legitimate, owing to the fact that the 

 heat-change is proportional to the quantity of matter which 

 has altered its form of combination. While investigating 

 the expulsion of acids from their salts in dilute aqueous solution 

 by other acids, the very remarkable observation was made that 

 that acid is by no means always the stronger which evolves 

 the greatest heat on neutralization. For example, although 

 sulphuric acid when neutralized in dilute aqueous solution 

 gives rise to an evolution of heat surpassing by three thousand 

 units that furnished by an equivalent amount of nitric or 

 hydrochloric acid, yet it is only half as strong an acid as the 

 latter ; that is, if equivalent amounts of nitric and sulphuric 

 acids be mixed with an amount of caustic soda equivalent to 

 one of them, the sulphuric acid enters into combination with 

 only half as much soda as the nitric acid ; so that one third of 

 the nitric acid remains in the free state, while two thirds of 

 the sulphuric acid is free. There can be absolutely no doubt 

 that nitric acid is by far the stronger acid, although, judging 

 from the thermal theory of affinity, sulphuric acid should be 

 the stronger. 



While Thomsen was prosecuting his researches, it was 

 generally held that the evolution of heat was an absolute mea- 

 sure of affinity ; hence Thomsen devised the term "avidity " 

 to express the " tendency of an acid towards neutralization." 

 But this is nothing else than the real affinity of the acid 

 towards the base, labelled with a special name to avoid con- 

 fusion. Ostwald %, who confirmed and extended Thomson's 

 researches by wholly different methods, named this quantity 

 " relative affinity." 



It appeared, from the investigation of a great number of 



* M. Berthelot, Essai de mScanique chimique, i. p. 421. 



f Thermoch. Unters. i. p. 97 et seq. 



\ T. pr. Chem. 1877, xvi. p. 385 • xviii. p. 328. 



