I9I2.] MAGIE— THERMAL RELATIONS OF SOLUTIONS. 239 



nection with the heat of dikition for the same concentration we can 

 determine the constants of the formula. With these we can then 

 calculate the osmotic pressure at ioo° C. and compare it with the 

 value found for this quantity from observations of the elevation 

 of the boiling point. I was able to do this with the observations of 

 Kahlenberg on the freezing and boiling points of sodium chloride 

 solutions combined with those of Thomsen on the heat capacities 

 and of myself on the heats of dilution. The agreement of the ob- 

 served boiling points with those predicted from the formula was 

 excellent. Incidentally this agreement confirms the validity of the 

 assumption from which the formula was derived, that a is indepen- 

 dent of the temperature. 



Another test of a less searching character can be made by using 

 the osmotic pressures given by the formula at different temperatures 

 to calculate the ratio of the vapor pressure of the solution to that of 

 the pure solvent. According to von Babo's law this ratio should be 

 independent of the temperature. Calculations for sodium chloride 

 solutions show that while it is not strictly the same at all tempera- 

 tures between o° and ioo° C. yet the dififerences between ratios at 

 different temperatures are excessively small, and lie within the 

 errors of the observations by which von Babo's law has been tested. 



The terms a and e of the formula for the heat of dilution are 

 manifestly not quantities which are fundamentally kinetic in their 

 nature. They express rates of change of energy with change of 

 volume. Their appearance in the formula for the osmotic pressure 

 indicates that the osmotic pressure is not to be explained as a kinetic 

 phenomenon, as the pressure of a gas is, but as the result of forces 

 acting between the solute — its molecules and ions — and the solvent. 

 These thermal relations, therefore, afford strong evidence, and 

 evidence with as little admixture of hypothesis as is possible in the 

 nature of the case, first of the validity of the dissociation hypothesis 

 by which the laws of electrolytic conduction are explained, and sec- 

 ondly, of the dependence of the osmotic pressure on the forces 

 which are exerted between the parts of the solution and the pure 

 solvent. 



Palmer Physical Laboratory, 

 Princeton University. 



