OSMOTIC PRESSURE 37 



that all electrolytes behave abnormally in aqueous solutions* 

 and that the presence of hydrates may affect the result. 



The method of determining the molecular weight from the 

 vapor tension of the solution is not yet applied so generally; 

 but its scope is even greater, because the range of solvents and 

 of temperatures is so largely increased. Raoult and Planck 1 

 have shown independently of each other that the formula 

 for the molecular weight of the dissolved substance is 



where M is the molecular weight of the solvent, p the per- 

 centage of substance in 100 grams of solvent, / and /' the 

 tensions of the pure solvent and the solution respectively. 



Raoult measures / and /' by the heights of the mercury in 

 eudiometers containing the liquids. A knowledge of the true 

 value of p depends upon the determination of /', as part of 

 the solvent leaves the solution as vapor under that tension; 

 therefore an error in determining /' affects the result three 

 times in the same direction. The method is less exact and 

 more inconvenient than that of determining the freezing- 

 point. 



A dynamic way, applied under Ostwald's direction by 

 Walker, 2 is very much easier: determine the amount of water 

 which different solutions will yield to the same amount of air. 

 The measurements are made on the balance, and the appa- 

 ratus consists of one set of bulbs to contain the solution, 

 another to hold an absorbent for water, and an aspirator. 

 The method has been generalized by Will and Bredig 3 for 

 other solvents. 



Lowering the tension means raising the point of ebullition 



1 Z. physik. Chem. 2, 353 seq., 405 seq. (1888). 



2 Z. physik. Chem. 2, 602 (1888). 

 8 Berichte, 21, 1084 (1889). 



