[53] In order to give an idea of the quantity of heat evolved in the formation of salts I append a table of data for _very dilute aqueous solutions_ of acids and alkalis, according to the determinations of Berthelot and Thomsen. The figures are given in major calories--that is, in thousands of units of heat. For example, 49 grams of sulphuric acid, H_{2}SO_{4}, taken in a dilute aqueous solution, when mixed with such an amount of a weak solution of caustic soda, NaHO, that a neutral salt is formed (when all the hydrogen of the acid is replaced by the sodium), evolves 15,800 units of heat. 49 parts of 63 parts of H_{2}SO_{4} HNO_{3} NaHO 15·8 13·7 KHO 15·7 13·8 NH_{3} 14·5 12·5 CaO 15·6 13·9 BaO 18·4 13·9 MgO 15·6 13·8 FeO 12·5 10·7 (?) ZnO 11·7 9·8 Fe_{2}O_{3} 5·7 5·9 These figures cannot be considered as the heat of neutralisation, because the water here plays an important part. Thus, for instance, sulphuric acid and caustic soda in dissolving in water evolve very much heat, and the resultant sodium sulphate very little; consequently, the amount of heat evolved in an anhydrous combination will be different from that evolved in a hydrated combination. Those acids which are not energetic in combining with the same quantity of alkalis required for the formation of normal salts of sulphuric or nitric acids always, however, give less heat. For instance, with caustic soda: carbonic acid gives 10·2, hydrocyanic, 2·9, hydrogen sulphide, 3·9 major calories. And as feeble bases (for example, Fe_{2}O_{3}) also evolve less heat than those which are more powerful, so a certain general correlation between thermochemical data and the degree of affinity shows itself here, as in other cases (_see_ Chapter II., Note 7); this does not, however, give any reason for measuring the affinity which binds the elements of salts by the heat of their formation in dilute solutions. This is very clearly demonstrated by the fact that water is able to decompose many salts, and is separated in their formation.