236 MAGIE— THERMAL RELATIONS OF SOLUTIONS. [April 20. 



water associated with the molecules of the solute is increased, while 

 that of the water associated with the ions is diminished. A similar 

 relation holds for the volumes of solutions of electrolytes. Since the 

 heat capacity and the volume of a body are both quantities in their 

 nature additive of the similar properties, of the parts of the body, it 

 is to be expected that there will be a similarity in their behavior in 

 any complex and changing body like a solution. The relations 

 described confirm in a striking way the correctness of the dissociation 

 hypothesis and the general accuracy of the dissociation factors 

 obtained from observations on electrical conductivity, while they also 

 indicate as a necessary conclusion from the facts, that there is inter- 

 action between the solute and solvent. This general theory which 

 was formerly based on the facts which have been described, receives, 

 I believe, strong confirmation from the study of the heats of dilution. 



When a solution containing one gram-molecule of solute is 

 diluted by the addition of a unit volume of water, heat is either 

 evolved or absorbed. This amount of heat is the heat of dilution. 

 It is of course a function of the concentration. It also shows a 

 remarkable dependence on the temperature. For example, if a solu- 

 tion of barium chloride containing one half gram-molecule in 50 

 gram-molecules of water is diluted to double its volume at 7° C. 49 

 gram-calories of heat are absorbed. The same dilution at 24.5° C. is 

 accompanied by an evolution of 18.5 gram-calories. At some inter- 

 mediate temperature, about 17° C, there is no heat either evolved or 

 absorbed. In all cases the absorption of heat is greatest at the lowest 

 temperature of observation, and diminishes as the temperature is 

 raised, to become positive in some cases, as in the example just 

 given, at temperatures common in an ordinary room. The greater 

 the dilution, the lower the temperature at which the heat of dilution 

 changes its sign. 



A very simple thermodynamic argument proves that the rate of 

 change of the heat of dilution with the temperature, for a fixed con- 

 centration, or what we may call the temperature coefficient of the 

 heat of dilution, is equal to the negative value of the rate at which 

 the heat capacity of the solution changes with an increase of volume 

 of the solution. Using / to represent the heat of dilution, taken 

 positive if heat is evolved, 6, the absolute temperature, H, the heat 



