THE PROBLEMS OF SOLUTION 105 



insight into the intimate structure of atoms and 

 molecules. In such matters we are driven back 

 to molecular theory, which offers an alternative 

 method of correlating other phenomena also, 

 equally definite, and supported by an ever in- 

 creasing number of experimental concordances. 



Thermodynamic theory, as well as practical 

 experiment, thus indicates that the osmotic 

 pressure of a solution depends only on the 

 number of dissolved particles, and not on their 

 nature or on the nature of the solvent. The 

 phenomena of gases show that the number of 

 molecules in two systems may be compared by 

 a knowledge of the total masses and of the 

 chemical molecular weights. Thus, two solutions, 

 one of sugar, let us suppose, and one of alcohol, 

 which are prepared so as to contain the same 

 number of molecules in the same volume, both 

 in theory and practice, possess equal osmotic 

 pressures. But, if equimolecular solutions of 

 sugar and salt be examined, the osmotic pressure 

 of the salt is found to be greater, and, if the 

 solutions be dilute, nearly twice as great as that 

 of the sugar. These abnormally great osmotic 

 pressures were discovered at an early date in 

 the history of the subject ; and further investiga- 

 tion showed that, at all events when the solvent 

 was water, they occurred in the cases of those 

 solutions which were conductors of electricity. 



When Van't Hoff formulated the physical 

 theory of the osmotic pressure, he treated these 

 abnormal values as exceptions to the usual law. 

 It was reserved for the physicists Arrhenius of 

 Stockholm and Planck of Berlin to point out 



