454 REPORT— 1886. 



nine conform to this rule. The exceptions most difficult to explain are 

 sodium sulphate and carbonate, both of which exhibit anomalies in their 

 solubility. 



However we may ultimately explain the peculiarities of these two 

 salts, the fact remains that heat evolved or absorbed during the admixture 

 of any substance with water is in every case a continuous function of 

 the quantity of water added, and a thermal change gradually diminishing 

 in amount is observed on the addition of successive quantities of water, 

 till an indefinitely large volume has been added, and the change becomes 

 too small to be traced by the thermometer. 



Similarly the contraction which ensues on diluting an aqueous 

 solution proceeds continuously, and, as already shown, the molecular 

 volume of a salt in solutions of different strengths is continuously 

 greater the larger the amount of salt present, so that in none of these 

 thermal or volumetric phenomena is any discontinuity observed nor any 

 indications of the formation of compounds of definite composition dis- 

 tinguishable by characteristic properties. 



If, however, we admit that no definite chemical compounds are 

 formed in such a case as the admixture of sulphuric acid II2SO4 with 

 water, what are we to say to the parallel case of the neutralisation of 

 some polybasic acids with alkali ? 



Take orthophosphoric acid. Difference per 



Molecule of NaHO. 

 H3P04Aq + iNaHOAq = + 7329 C. 

 + i]SraHOAq= + 14829 

 + 2N'aH0Aq = + 27078 12249 



+ 3NaH0Aq = + 34029 6951 



+ 6NaH0Aq = + 36280 ^ V' ' =417. 



The addition of 



The first mol. H2O to H2SO4 gives + 6379 



The second mol. HjO to the preceding mixture + 3039 

 The third mol. H2O to the preceding mixture + 1719 



So the addition of 



The first mol. of NaHO to H3PO4 gives + 14829 



The second mol. of NaHO to the preceding mixture + 12249 

 The third mol. of NaHO to the preceding mixture + 6951 



Arsenic acid gives similar results, the several values being in 

 each case a little higher than those obtained with phosphoric acid 

 (Thomsen, i. 204). 



The case of periodic acid (Thomsen, i. 244) is still more noteworthy. 



H5l06Aq+ KHOAq = + 5150C. 

 + 2KH0Aq= 26590 

 + 3KH0Aq= 29740 

 + 5KH0Aq= 32040 



Here we have great inequality in the amount of heat evolved on 

 addition of successive molecules of alkali and no sign of approaching a 

 maximum. These values are indicated on the accompanying diagram 

 (fig. 3):- 



In the case of sulphai'ic acid, neutralisation by soda shows a more 



