IONS CONDUCT ELECTRICITY 19 



varies with the concentration of the solution, and 

 that at the dissociation falls less rapidly with 

 increasing concentration than at a higher tempera- 

 ture. 



The electro-motive force (E.M.F.) required to separ- 

 ate the ions from their electrical charges varies, which 

 Hans Jahn considers is probably a cause of apparent 

 divergences from Ostwald's law.* As the ratio of 

 ionic concentration can be found by E.M.F. observa- 

 tions, so likewise, assuming Ostwald's law, can the 

 actual concentration be determined. The ionic 

 concentration of a dilute solution of potassium 

 chloride was calculated from E.M.F. observations and 

 compared with that of six other solutions of varied 

 strength, and the value of these was found to vary 

 from 0-001638 to 0-001642. Similar calculations 

 were made from solutions of sodium chloride and 

 hydrochloric acid. By assuming the value of the 

 most dilute solution, Hans Jahn calculated from it 

 the ionic concentration of the stronger solutions, 

 and he found these values agreed with those ob- 

 tained by the dilution law. 



The presence of ions in a solution influences the 

 osmotic pressure. All substances in solution behave 

 like gases, the dissolved molecules exerting pressure 

 on the sides of the containing vessel in their effort 

 to diffuse through space. But the osmotic pressure 

 depends upon whether the substance in solution is 

 an electrolyte or a non-electrolyte. The osmotic 

 pressure of non-electrolytes in solution is propro- 

 tionate to the number of contained molecules ; but 

 in the case of electrolytes the pressure is in propro- 



* Loc. cit. 



22 



