442 PRINCIPLES OF CHEMISTRY 



is much more simple, and therefore its significance in confirmation 

 of Berthollet's doctrine is of particular importance. The only cases 



and NaNO 3 + H 2 SO 4 is equal, as for ethereal salts, to 4. By taking this figure and ad- 

 mitting the above supposition, Thomsen found that for all mixtures of soda with nitric 

 acid, and of sodium nitrate with sulphuric acid, the amounts of heat followed Guldberg and 

 Waage's law ; that is, the limit of decomposition reached was greater the greater the 

 mass of acid added. The relation of hydrochloric to sulphuric acid gave the same results. 

 Therefore the researches of Thomsen fully confirm the hypotheses of Guldberg and 

 Waage and the doctrine of Berthollet. 



Thomsen concludes his investigation with the words: (a) 'When equivalent 

 quantities of NaHO, HNO 3 (or HC1) and H 2 S0 4 react on one another in an aqueous 

 solution, then two-thirds of the soda combines with the nitric and one-third with the 

 sulphuric acid ; (b) this subdivision repeats itself, whether the soda be taken combined 

 with nitric or with sulphuric acid ; (c) and therefore nitric acid has double the tendency 

 to combine with the base that sulphuric acid has, and hence in an aqueous solution 

 it is a stronger acid than the latter.' 



'It is therefore necessary,' Thomsen afterwards remarks, 'to have an expression 

 indicating the tendency of an acid for the saturation of bases. This idea cannot be 

 expressed by the word affinity, because by this term is most often understood that force 

 which it is necessary to overcome in order to decompose a substance into its component 

 parts. This force should therefore be measured by the amount of work or heat employed 

 for the decomposition of the substance. The above-mentioned phenomenon is of an 

 entirely different nature,' and Thomsen introduces the term avidity, by which he desig- 

 nates the tendency of acids for neutralisation. ' Therefore the avidity of nitric acid with 

 respect to soda is twice as great as the avidity of sulphuric acid. An exactly similar 

 result is obtained with hydrochloric acid, so that its avidity with respect to soda is also 

 double the avidity of sulphuric acid. Experiments conducted with other acids showed 

 that not one of the acids investigated had so great an avidity as nitric acid ; some had a 

 greater avidity than sulphuric acid, others less, and in some instances the avidity =QS 

 The reader will naturally see clearly that the path chosen by Thomsen deserves to be. 

 worked out, for his results concern important questions of chemistry, but great faitb 

 cannot be placed in the deductions he has already, arrived at, because great complexity 

 of relations is to be seen in the very method of his investigation. It is especially 

 important to turn attention to the fact that all the reactions investigated are reactions 

 of double decomposition. In them A and B do not combine with C and distribute them- 

 selves according to their affinity ov avidity for combination, but reversible reactions are 

 induced. MX and NY give MY and NX, and conversely ; therefore the affinity or avidity 

 for combination is not here directly determined, but only the difference or relation of the 

 affinities or avidities. The affinity of nitric acid not 'only for the water of constitution, 

 but also for that serving for solution, is much less than that of sulphuric acid. This is 

 seen from thermal data. The reaction No0 5 + H 2 O gives +3,600 heat units, and the 

 solution of the resultant hydrate, 2NHO 3 , in a large excess of water evolves + 14,986 heat 

 units. The formation of S0 3 + H a O evolves +21,308 heat units, and the solution of 

 H 2 SO 4 in an excess of water 17,860 that is, sulphuric acid gives more heat in both cases. 

 The interchange between Na 2 SO 4 and 2HN0 3 is not only accomplished at the expense of 

 the production of NaN0 3 , but also at the expense of the formation of H 2 SO 4 , hence the 

 affinity of sulphuric acid for water plays its part in the phenomena of displacement 

 Therefore in determinations like those made by Thomsen the water does not form a 

 medium which is present without participating in the process ; it also takes part in the 

 reaction. (Compare Chapter IX., Note 14.) 



Whilst retaining essentially the methods of Thomsen, Ostwald (1876) determined the 

 variation of the sp. gr. (and afterwards of volume), proceeding in the same dilute solutions, 

 on the saturation of acids by bases, and in the decomposition of the salts of one acid by 

 the other, and arrived at conclusions o{ just the same nature as Thomson's. Ostwald's 



