io6 PHYSICAL SCIENCE 



that the extension of Van't Hoff's principles to 

 these cases required the assumption of the dis- 

 sociation of the molecules of salt in order that 

 the total number of particles in solution should 

 still be the number indicated by the observed 

 phenomena. According to this hypothesis, in a 

 dilute solution of common salt, the solute does 

 not exist as molecules of sodium chloride, but as 

 the dissociated parts, sodium and chlorine, which, 

 since the solution conducts a current of electricity, 

 must be associated with electric charges. Each 

 salt molecule thus gives two pressure-producing 

 particles in solution, and the double value of 

 the osmotic pressure is explained. In stronger 

 solutions this dissociation is not complete, and 

 the osmotic pressure is less than twice the normal 

 value ; but no exact correlation of pressure and 

 dissociation can be made, for the thermodynamic 

 theory as formulated above is only valid for very 

 dilute solutions. 



Like the thermodynamic theory of osmotic 

 pressure generally, this extension of it does not 

 involve any particular view as to the cause of the 

 pressure or the nature of solution. The dissocia- 

 tion hypothesis is concerned simply with the 

 difference between solutions of electrolytes and 

 non-electrolytes, and leaves entirely open the 

 more fundamental question, whether solution is 

 essentially chemical or physical in its nature. 



The dissociation theory of aqueous solutions 

 of electrolytes, originally indicated by osmotic 

 phenomena, is supported perhaps even more 

 clearly and strongly, by the study of the electrical 

 properties. During the years 1830 to 1840, 



