THE ELECTROLYTIC DISSOCIATION THEORY 385 



available energy on further dilution, with which alone we are 

 concerned in the transformations of dilute solutions, depends only 

 on the further separation of the particles . . . and so is a function 

 only of the number of dissolved molecules per unit volume and 

 of the temperature, and is, per molecule, entirely independent of 

 their constitution and that of the medium " — the assumption 

 being made that the particles are so far apart that their mutual 

 influence is negligible. The change of available energy is thus 

 brought into exact correlation with that which occurs in the 

 expansion of a gas. In the proof there is no assumption as to 

 the mechanism of the osmotic pressure ; it may be due to mole- 

 cular bombardments, to affinity between solvent and dissolved 

 substance, to surface tension, or to some other cause. It has 

 further been shown theoretically by Van 't Hoff 1 that the 

 osmotic pressure is independent of the nature of the membrane, 

 provided the latter is perfectly semi-permeable. 2 



Kahlenberg 3 has recently described a series of experiments 

 on osmotic pressure which, in his opinion, invalidate Van 't Hoff's 

 theory. He used a rubber membrane, and in his quantitative 

 experiments determined the osmotic pressures exerted by 

 lithium chloride and by sugar dissolved in pyridine. In both 

 cases the equilibrium pressures were much smaller than those 

 required by theory, and in equivalent concentration the sugar 

 exerted the higher pressure. In the course of the work it was 

 found necessary to stir the solutions inside the cells, and the 

 author considers that, on account of this omission, the results 

 of former observers are unreliable. It should be remembered, 

 however, that the pressures observed by Pfeffer and by Morse 4 

 were equilibrium pressures, and remained constant for long 

 periods. 



Whetham, 5 on the basis of the thermodynamical proof re- 

 ferred to above, contends that the experimental results obtained 

 by Kahlenberg have not the importance the latter claims for 

 them. 



Further, as the membrane was slightly permeable both for 



1 Loc. cit. p. 32. 



1 On the employment of ideal processes in thermodynamics, see Planck, 

 Annalen der Physik, 1903, 10, 436. 



s Journ. physical Chem. 1906, 10, 141. 



4 Loc. cit. 



5 Nature, 1906, 74, 54, 295 ; Kahlenberg, ibid. 222. 



