412 



SCIENCE PROGRESS. 



FIG. I. 



Let two solutions A and B (fig. i) be introduced into a 



hollow ring-shaped vessel and let 

 them be separated by a semi-perme- 

 able membrane MM. If the solutions 

 are isotonic, at the outset there will 

 be no change in their concentrations 

 due to passage of solvent through 

 MM. If, however, the vapour-pres- 

 sure of A be supposed greater than 

 that of B, solvent will pass from A to 

 B by evaporation, and to restore the 

 osmotic equilibrium solvent will have to pass from B to A 

 through MM. A continuous isothermal circulation of solvent 

 would thus proceed automatically at the expense of the heat 

 of surrounding objects and a result would thus be obtained 

 which is opposed to the second law of thermodynamics. In 

 this way it may be shown that the vapour-pressures of 

 isotonic solutions must be equal. 



Now Raoult's empirical results have proved that solutions 

 having the same vapour-pressure have the same molecular 

 concentration. These results furnish, therefore, another 

 means of showing that Avogadro's hypothesis is applicable 



to solutions, that, in fact, solu- 

 tions of the same molecular 

 concentration have the same 

 osmotic pressure. 



Indeed, the following simple 

 discussion, basedupon considera- 

 tions due mainly to Arrhenius, 

 serves to prove that Raoult's re- 

 sults taken collectively constitute 

 an indirect demonstration of the 

 general validity of the equation 

 PV = 'oSig T as applied to 

 dilute solutions. 



Let a quantity of solvent S 

 be placed in an evacuated enclosure ABCD (fig. 2) kept at 

 constant temperature T, and let an upright vessel, the base 

 of which is a semi-permeable partition MM, dip into the sol- 



Fic.2. 



