i8o SCIENCE PROGRESS. 



in some cases smaller and in others greater than it should 

 be according to the above rule. 



There is little difficulty in explaining those exceptions 

 to the osmotic pressure rule which occur when the osmotic 

 pressure found is smaller than the theoretical. Such cases 

 usually arise when associating compounds are dissolved in 

 monomolecular liquids, and it is now generally admitted 

 that in these cases the dissolved compound is still in an 

 associated condition, so that the number of molecules pre- 

 sent in the solution is less than that calculated on the 

 assumption that the dissolved compound is monomolecular. 

 But if a smaller number than the calculated number of 

 molecules is present, the observed osmotic pressure will 

 also be smaller than that calculated, and this is what is 

 actually found to be the case. 



A greater osmotic pressure than the theoretical is met 

 with in the case of aqueous solutions of metallic salts, and 

 also of solutions of the same salts in formic acid and some 

 other associating liquids. In these cases the explanation 

 first advanced by Arrhenius is commonly accepted, and the 

 dissolved compound is believed to be more or less in a state 

 of electrolytic dissociation. Assuming that the molecules 

 of the dissolved salt are dissociated into their ions, and 

 that each ion has the same effect in establishing osmotic 

 pressure that a separate molecule has, the observed os- 

 motic pressure must of course be greater than that calcu- 

 lated on the assumption that the dissolved compound 

 has gone into solution without any change in molecular 

 composition. 



But the abnormal osmotic pressures of electrolytes can 

 be explained without recourse to the theory of electrolytic 

 dissociation, and by reference solely to the facts regarding 

 liquids which have been just considered. When a solution 

 is formed by dissolving a given compound in a given sol- 

 vent, four possible cases are presented :— 



(i) The solvent and the dissolved compound may be 

 both monomolecular. 



(2) The solvent and the dissolved compound may be 

 both associating. 



