THE ASSOCIATION THEORY OF SOLUTIONS. 



By WILLIAM FRANCIS MAGIE. 



(Read April 19, 1907-) 



When it was discovered, by van't Hoff, that the osmotic pres- 

 sure of sokitions obeys the laws of gases, the conchision was drawn, 

 in many quarters, that the osmotic pressure is to be explained as 

 a pressure arising from the impacts of the dissolved molecules or 

 ions against the bounding surface of the solution, and that the part 

 played by the solvent is simply that of a medium in which the 

 solute is suspended, or in which it assumes a state practically equiv- 

 alent to that of a gas. This conclusion was not received without 

 opposition, especially by the advocates of the so-called hydrate 

 theory, according to which the act of solution involves a more or 

 less intimate or structural union of each molecule of the solute with 

 one or more molecules of water. The trend of opinion at present 

 is toward some form of the hydrate theory, but it does not seem 

 to have been noticed, except by Tammann,^ that certain properties 

 of solutions are demonstrative against the theory of free solute and 

 inactive solvent, and in favor of the theory that there exists such 

 an interaction between the solute and the solvent that certain prop- 

 erties of the solvent are modified thereby. As the evidence which 

 we have does not go so far as to prove the existence of true 

 hydrates in all solutions, and indeed is rather unfavorable to the 

 hydrate theory, as strictly construed, I prefer to call the theory 

 based upon it the association theory of solutions. 



The evidence which proves an efficient interaction between the 

 solute and solvent, and a modification of the properties of the sol- 

 vent, was discovered by Julius Thomsen.- On investigating the 

 heat capacities of many aqueous solutions of electrolytes, Thomsen 

 found that the apparent heat capacity of the solute (that is, the 

 difference between the heat capacity of the solution and that of the 



^ Zeifs. fill' pliys. Cliem., Vol. 18, p. 625, 1895. 



^ " Thermod. Untersuchimgen," Vol. i ; IVied. Ann., 142, p. 237, 1871. 



138 



