THERMAL RELATIONS OF SOLUTIONS. 



By WILLIAM FRANCIS MAGIE. 



(Read April 20, 191 2.) 



The thermal relations of solutions afford evidence of a peculiar 

 and valuable kind about the nature of solutions. The electrical con- 

 ductivity of solutions has been explained by the hypothesis that the 

 molecules of the solute are partly dissociated into ions in the solu- 

 tion. When this hypothesis is further tested on the assumption that 

 the osmotic pressure is proportional to the number of free molecules 

 and ions in the solution, the experimental results of Dieterici, of 

 Kahlenberg, of Jones and others shov^ disaccord with the predictions 

 of the hypothesis. I venture to believe that a study of the heat 

 capacities and the heats of dilution of solutions will confirm the 

 view that the reason for this disaccord lies in the assumed relation of 

 the osmotic pressure to the dissociation, so that while the dissocia- 

 tion hypothesis is confirmed, the relation of the osmotic pressure to 

 the dissociation is shown to be different from that which was orig- 

 inally assumed. 



Five years ago I presented to this Society a paper in which I 

 discussed the heat capacities of solutions. I will summarize here the 

 principal results described in that paper, in order to render the 

 present discussion more complete. 



The heat capacity of a gram-molecular solution of an electrolyte 

 diminishes with increasing dilution. The change in the heat capacity 

 is directly proportional to the change in the dissociation, determined 

 from the electrical conductivity. In most cases, at ordinary concen- 

 trations, the heat capacity of the water is diminished, and we are led 

 to infer an interaction between the water and the molecules and ions 

 of the solute of such a sort as to diminish the freedom of the water. 

 A study of the constants of the formula by which the heat capacity 

 is expressed leads to the conclusion that the heat capacity of the 



235 



