it is to be noticed that the thermal effect may be in- fluenced by the degree of dilution of the reacting solutions, and that in certain cases (notably for carbonic acid and ammonia) the solution of the neutral salt may give an evolution of heat on the addition either of excess of acid or excess of base.
If we examine the table a little more closely, we see that there is an almost constant difference between the corresponding numbers of two vertical columns, and the same applies to two horizontal series. In other words, the difference between the heats of neutralisation of the acids a, 6, c, . . . by the same base remains almost constant whatever be the base employed. We find the same thing for the heats of neutralisation of several different bases a', &', c', . . . by the same acid.
We conclude from this that the heat of neutralisation is composed additively of two factors, one of which depends entirely on the nature of the base and the other only on the nature of the acid. 1 These factors preserve a fixed and invariable value, and it seems, therefore, that it would be quite incorrect to try to deduce from the heats of neutrali- sation a measure of the affinities which operate between
Hess's law of thebmoneutrality gives us the most general expression for this state of things. This law states that no thermal effect is produced by mixing two dilute salt solutions, whatever be the state of equilibrium resulting from the reaction. The law is in agreement with ob- served facts, provided that we are dealing only with salts
��1 A similar rule applies to the changes of volume which accom- pany neutralisation phenomena. This will be pointed out and demonstrated later (in the fourth part).
- Let us compare this with the law of gravitation. The attraction
which is exercised between two bodies is proportional to the product of their masses. The heat of neutralisation, which corresponds to the formation of a salt, is composed, on the contrary, of the sum of two factors, and therefore does not appear to express the mutual attrac- tion between the base and the acid.
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