OSMOTIC PRESSURE 41 



From these data the molecular weight M may be 

 calculated on the basis of Avogadro's law according to the 

 relation (p. 19) 



PV 



G : 24-45 ~ T - = M: 2, 



where G is the weight in grams (= i), V the volume in 

 litres (01006), so that 



*= 0.813 x. 



The numbers thus calculated are given in the last 

 column, and are in good accord with the number, 342, 

 obtained from the formula of sugar, C l2 H 29 O n . Notwith- 

 standing this and several other researches l , direct measure- 

 ment of osmotic pressure has not yet led to a practicable 

 method for determining molecular weights. 



2. Indirect Methods for Molecular Weight Determination. 



Whilst direct measurement of the osmotic pressure has 

 so far met with difficulties for want of a satisfactorily 

 resistent semipermeable membrane, so that the experiment 

 is difficult or even impossible, attention has been paid to 

 other properties of solutions, which bear calculable relations 

 to the osmotic pressure. 



With regard to these methods, we may say generally, 

 with Nernst, that every mode of separation between solvent 

 and dissolved body involves a method for determination of 

 molecular weight. This is so because every such separation 

 renders possible a cyclic process in which the solvent, after 

 having been separated in the way in question, may be again 

 brought together with the separated or dissolved substance 

 osmotically, i. e. by means of a semipermeable membrane. 

 If this cyclic process be carried out reversibly e. g. if the 

 osmotic action be supposed to take place in a cylinder with 

 piston, such that the wall of the cylinder enclosing the 



1 Ladenburg, BerL Ber. 22. 1225 ; Adie, .7. C. S. 1891, p. 344 ; Ponsot, 

 Compt. Rend. 125. 867 ; Naccari, Rend. Ace. Lin-cei, 1897, i. 32. 



