snlUT.M CHLORIDE IIKUTHOLLET'S LAWS 433 



nations. Therefore the researches of Thomseu fully confirm the 

 hypotheses of (iuldberg and Waage and the doctrine of Berthollet. 27 



-" ThoiMsen concludes his investigation with the words: (a) 'When equivalent 

 quantities <>f XaHO,HXO 5 (or HC1) and ^H. 2 SO 4 react on one another in an aqueous solu- 

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

 arid ; ifn this snl)division repeats itself, whether the soda be taken combined with nitric 

 or with sulphuric acid ; ('<) and therefore nitric acid has double the tendency to combine 

 with the base that sulphuric acid has, and therefore in the wet way 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 = 0.' 

 The reader will naturally clearly see that the path chosen by Thomsen deserves being 

 worked out, because his results concern important questions of chemistry, but great faith 

 cannot be placed in the deductions as yet arrived at by Thomsen, because great com- 

 plexity 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 or 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 N 2 O 5 + H.>O gives +3600 heat units, and the 

 solution of the resultant hydrate, 2NHO 5 , in a large excess of water evolves +14986 heat 

 units. The formation of SO 3 + H.,O evolves +21308 heat units, and the solution of 

 H.,,SO 4 in an excess of water 178(50 that is, sulphuric acid gives more heat in both cases. 

 The interchange between Na.,SO 4 and 2HNO 5 is not only accomplished at the expense of 

 the production of NaNO 3 , but also at the expense of the formation of H.jSO.,, 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, but surely plays its own 

 part. No less essential is the circumstance that sodium sulphate is able to combine with 

 An excess of sulphuric acid to form the acid salt, whilst the salts of nitric and hydrochloric 

 acids do not have this property. But with that method of interpretation of Guldberg 

 and Waage's deduction which is given in the text, the adaptability of their formula to 

 the phenomena observed by Thomsen may be expected, notwithstanding these incidental 

 -actions, although the idea of the relative tendencies of acids for combining with alkalis 

 cannot be deduced from the coincidence of the results of experiment with those of calcu- 

 lation. If sodium oxide (Na^O) were brought into contact with sulphuric anhydride and 

 nitric anhydride (SO 3 and N.^Os) the distribution would probably be different, and even if 

 stronger solutions than those employed by Thomsen were taken perhaps the result would 

 be different, more especially judging from what is said in the following note. (Compare 

 Also Chapter IX. Note 14.) 



VOL. I. F F 



