ON THE PHENOMENA AND THEOEIES OF SOLUTION. 453 



In this case, common salt, we find in accordance with the general rule 

 (Thomsen, iii. p. 28 et seq.) that the heat of dissolution being negative 

 the heat of dilution is also negative. Supposing we explain the heat- 

 absorption which attends the act of solution by referring to the change of 

 the solid salt to liquid it still remains to be considered to what cause we 

 can ascribe the heat-absorption attending dilution when liquid is mixed 

 with liquid and no change of state is involved. There is contraction upon 

 adding water to a solution of common salt, but this, if it is connected 

 with any thermal change at all, would probably lead to an evolution of 

 heat. It has just been shown that the thermal change attending solution 

 of salt in water cannot be due solely to the change of state from solid to 

 liquid. If it were, the same amount of heat-absorption would be observed 

 in dissolving salt in any proportion of water, and no change would be 

 produced by adding more water. There is therefore apparently some 

 agency which gives a positive thermal change on solution of the solid, 

 and another which occasions a negative change on the dilution with more 

 water, the observed amounts of heat absorbed being the difference be- 

 tween these two. 



The positive thermal change probably corresponds to some kind of 

 union between the salt molecules and the water. The negative thermal 

 change is chiefly connected in the act of solution with the physical change 

 from solid to liquid. The negative change consequent on dilution is not 

 so easily accounted for. But it may be due to double decomposition with 

 the water. The reaction 



NaHO + HCl = NaCl + H.O 



is attended with heat-evolution, and its reversal 



NaCl + H2O = NaHO + HCl 



must lead to heat-absoi'ption. 

 From Thomsen's results 



NaHO + HCl + 2OOH0O = + 13745, 

 and 



NaCl -1- lOHoO = - 525, 

 and 



(NaCllOHjO) + 190H,O = 



- (1230 - 525) = - 705. 



So that for complete decomposition we should require — 13745 cal., 



whilst in diluting a solution almost saturated down to a very weak con- 



705 

 dition, the hypothesis does not require more than — — , or a little 



more than five per cent, of the whole. 



My own experiments on heat of dissolution of salts at different 

 temperatures (' Proc. R. S.' 1885) led me to the conclusion that decom- 

 position of this kind did occur, and that it increases with rise of 

 temperature. Thomsen, on the other hand (iii. p. 20), considers that 

 his results do not point to a decomposition in such cases as common salt ; 

 but he admits it in^the case of bisulphates of sodium and potassium. 



As a rule, a salt which on being dissolved in water gives out heat gives 

 a further amount when its solution is diluted. On the other hand, a 

 salt which in dissolving absorbs heat absorbs a further amount when 

 its solution is diluted. Of thirty-five salts examined by Thomsen, twenty- 



