ON THE PHENOMENA AND THEOEIES OP SOLUTION. 



455 



Fig. 3. 



nearly equal amount of heat evolved per molecule of NaHO added. 

 Thomsen gives (i. p. 297) — ■ 



4NaH0Aq + H2S04Aq= + 31368 



The second molecule of NaHO therefore seems to give more heat than 



the first. 



In the case of acids and alkalies we do not hesitate to accept such 



results as indicating the formation of definite chemical compounds, and 



that they do correspond with the 

 known basicity of the majority of the 

 acids there can be no doubt. But 

 there are very few cases in which the 

 thermal value of the action of the 

 second or third molecule of alkali is 

 exactly equal to that of the first. 



There can be little doubt that in 

 every one of these cases of dilution 

 and of neutralisation the observed 

 thermal efiect is due partly to chemi- 

 cal combinations, partly to changes 

 which are commonly distinguished 

 as physical.' 



Specific Heats and Vapour 

 Peessuees. 



The question we are now con- 

 sidering as to whether in a solution 



I isl ooo 



the solvent and the substance dis- 

 solved in it or any portion thereof 

 exist independently of each other is 

 in some degree answered by the facts 

 known as to the specific heats and 

 vapour pressures. In the case of 

 salts dissolved in water the value of 

 the molecular heat (that is, the pro- 

 duct of the specific heat into mole- 

 cular weight) in moderately dilute 

 solutions diSers very little from that 

 of the water alone contained in the 

 solution. In strong solutions of salts 

 (5 to 20 mols. of water to 1 mol. 

 salt) it is sometimes greater, some- 

 times less than that of the water 

 alone. In weaker solutions (50 to 



200 molecules water) it is generally less than the water alone. For 



example : — 



' Vide Correlation of Physical Properties of Solvtion with Concentration. — Mende- 

 lejeff, Ber. xix. 370 and 400, discusses the relations of contraction and thermal 

 change. 



(4 000 l":'''04 =14329 



IS OOP I _ iei/Ri/E I 



2 = «3 

 10000 3 = H' 



JH,'AsO^=73eZ 

 HilPO^ =7329 



/ MOL-.ACID + nMOLS ALKALI 



