THE NEW THEORY OF SOLUTIONS. 19 



hypothesis demands, precisely similar exceptions have 

 been recognised in the case of solutions. In some 

 solutions association of simple dissolved molecules is 

 supposed to take place, while in a whole class of 

 solutions, namely, those of salts, strong acids and bases, 

 all of which are electrolytes, the dissolved molecules are 

 supposed to undergo dissociative changes resembling in 

 many respects those occurring in the case of gases. 



To begin with, we will deal with non-electrolytic or 

 indifferent solutions, and further with those in which no 

 association of molecules is supposed to take place. 



OSMOTIC PRESSURE. 



Absolute Measurements. — Measurements of osmotic 

 pressure have only been made for aqueous solutions, and 

 only three observers, namely Pfeffer, Adie, and Tarn- 

 man, have obtained absolute measurements. Adie used 

 Pfeffer's method, Tamman employed a new method, 

 which consists in separating two solutions by a semi- 

 permeable wall, and altering the pressure on one of them, 

 till neither loses or gains water. The external pressure 

 thus applied is evidently equal to the difference of the 

 osmotic pressures of the solutions, and by adding a 

 soluble substance to one of the solutions, its osmotic 

 pressure may be obtained in terms of the adjustable 

 external pressure. The method is, however, very trouble- 

 some in practice. 



As regards the substances examined, Pfeffer, who in- 

 vestigated the subject from a botanical point of view, 

 used solutions of cane sugar v dextrin, gum-arabic, and the 

 nitrate, sulphate, and tartrate of potassium. Adie used 

 salt solutions only, and Tamman studied cane sugar and 

 isobutyl alcohol when dissolved in copper sulphate 

 solution. 



Relative Measurements. — A large number of observa- 

 tions on osmotic pressure have been made by relative 

 methods. De Vries in 1884 described three botanical 

 methods of obtaining isotonic solutions. Of these, the 



