416 M. J. Thomsen's Thermo -chemical Researches. 



the value of the left-hand member, 



-§ 2072 + i (NaO SO 3 , 2S0 3 ) = -§2072-^1176= -1772 



thermal units, a result which agrees within 20 thermal units 

 with the result of the experiment. 



By some analogous calculations we find that the thermic effect 

 produced in the inverse reaction of sulphuric acid on nitrate of 

 soda would be expressed by the formula 



(1-^)2072+ (l-^)(NaO SO 3 , j^-S0 3> )= 299 thermal units, 



whilst the experiment gave 288 thermal units. Hence it fol- 

 lows from these experiments that : — 



(a) When an equal number of equivalents of soda, sulphuric 

 acid, and nitric acid are placed together in the same solution, 

 two-thirds of the soda enter into combination with the nitric 

 acid and one- third with the sulphuric acid. 



(b) The division of the base between the two acids occurs in 

 the same manner, whether it has been introduced in the state of 

 sulphate or in that of nitrate. 



(c) Hence nitric acid has twice the tendency to combine with 

 the base which sulphuric acid has ; it is therefore remarkably 

 more energetic in the moist way. 



It is necessary for the development of these phenomena to em- 

 ploy a term in order to express this tendency of acids, more or 

 less great, to neutralization. It cannot be designated by the 

 word affinity ; for this term must be reserved, according to the 

 author, to express the force which must be overcome in order to 

 decompose a combination into its elements, and which can be mea- 

 sured by the work necessary for this decomposition (therefore by 

 the thermic effect) . The phenomena in question are altogether of 

 a different nature; and M. Thomsen proposes the word avidity 

 to denote this tendency of acids to neutralization. Thus the 

 avidity of nitric acid for soda is twice that of sulphuric acid. 



Some calculations precisely similar show that it is the same 

 for hydrochloric acid : for soda its avidity is double that of sul- 

 phuric acid. 



In this hypothesis, in fact, we find that the decomposition of 

 an equivalent of sulphate of soda by an equivalent of hydrochloric 

 acid must absorb 1691 thermal units ; experiment gave 1682. 

 The inverse reaction of sulphuric acid on chloride of sodium 

 must disengage heat, 258 thermal units; experiment gave 

 244*. 



* [It might at first sight seem strange that the avidity of acids for bases 

 varies inversely as their affinity (the latter being measured by the heat dis- 

 engaged in neutralization), if we did not remark that the author has here 



