22 MORRIS LOEB 



The vapor tension of a liquid is not constant within the 

 range of ordinary experimentation, since it is a complex func- 

 tion of the temperature and of the nature of the substance. 

 But Raoult has shown that its depression shows a relation 

 to percentage of foreign molecules which is independent of 

 temperature, provided he expresses the depression, not in 

 absolute measure, but as a fraction of the tension of the pure 

 solvent at the same temperature. Then the molecular de- 

 pression, i.e., the product of the molecular weight of the sub- 

 stance dissolved into the relative depression of a one per 

 cent solution, becomes a definite constant for any substances 

 which that particular liquid may dissolve. It is noticeable 

 that this constant's numerical value always approaches very 

 nearly -j^ of the molecular weight of the solvent. The con- 

 stant is therefore likewise independent of the nature of the 

 solvent; one may generalize for all solutions, that the ten- 

 sion of the pure solvent is to the actual depression as is the 

 number of molecules of solvent to the number of molecules of 

 substance dissolved. 



Laws as simple as these point to conditions, in solutions, 

 very like those existing in the gaseous state. How great this 

 analogy is has been shown in the well-matured papers of J. H. 

 van 't Hoff 1 on osmotic pressure. Osmotic pressure is the 

 name given by van 't Hoff to the force with which a liquid 

 will enter into a cell containing the solution of some sub- 

 stance in that liquid through walls which are pervious to the 

 solvent alone. Pfeffer 2 has shown that when such a cell is 

 put into a vessel containing the pure solvent, the latter will 

 enter the cell, increasing the bulk of the solution within, until 

 this tendency is counterbalanced by the difference of level 

 in the two vessels or some other pressure. The pressure 



' Z. physik. Chem. 1, 481 (1887). 



* Monograph, Osmotische Untersuchungen. Leipzig, 1877. 



