516 



SCIENCE. 



[N. S. Vol. IV. No. 93. 



and with improved apparatus have made 

 many measurements of very satisfactory 

 character. These results show, that equi- 

 molecular solutions of non- electrolytes show 

 equal osmotic pressures. The osmotic pres- 

 sure is directly proportional to concentration 

 (expressed in gramme equivalents) . The 

 osmotic pressure is proportional to the 

 absolute temperature. 



The similarity of these laws to those of 

 gaseous pressure, is at once apparent. Van't 

 Hoflf further, upon the bases of absolute 

 measurements and the applications of the 

 ordinary equations for properties of gases, 

 enunciates the laws : That the molecules of 

 the dissolved substance exert pressures in 

 osmotic action, equal to the pressure which 

 would be exerted by the same number of 

 molecules in the gaseous state upon the 

 sides of a containing vessel of the volume 

 of the solution, the temperature remaining 

 the same. 



Osmotic pressure seems then to be merely 

 a molecular kinetic effect. On this as- 

 sumption thermodynamic considerations led 

 to the same result as experiment. 



The first two laws of osmotic pressure 

 could be satisfactorily explained under the 

 ordinary supposition of molecular attrac- 

 tion; the last two, however, are difficult to 

 reconcile with any explanation other than 

 the one that the pressure is due to molecu- 

 lar impact. 



The osmotic pressure of electrolytes is 

 considerably greater than that of non- elec- 

 trolytes ; in dilute solutions they, however, 

 follow the same laws of variation. In quite 

 a number of binary compounds it is just 

 twice as great numerically as in the non- 

 electrolytes. The behavior is, as though the 

 number of molecules contained in the elec- 

 trolyte solution per gramme equivalent, was 

 greater than in non- electrolytes. If we 

 imagine that solution has dissociated the 

 molecule into two groups, the anomaly is 

 easily explained. There is evidence here. 



independent of electrolytic behavior, that 

 electrolytes are dissociated in solution. 



Blagden, as early as 1788, recognized that 

 salts in solution lowered the freezing point of 

 water ; his experiments were made mainly 

 on sea water. He found that in very dilute 

 solutions, the lowering of the freezing point 

 was very nearly proportionate to the amount 

 of substance dissolved. Eiedorf, Coppett 

 and Eaoult carefully studied the subject 

 and found that the molecular depression of 

 the freezing point was equal for salts of 

 similar composition. For non-electrolytes, 

 equimolecular solutions of different salts 

 gave very nearly the same values, show- 

 ing, that the effect was purely a molecular 

 one independent of the nature of the sub- 

 stance. 



The problem may be treated from the 

 dynamical point of view upon this suppo- 

 sition : Knowing the osmotic pressure and 

 imagining the change in state in the nature 

 of a reversible cycle, the necessary energy 

 changes are calculable. The depression of 

 the freezing point calculated in this way 

 for a number of substances gave values 

 closely agreeing with experiment. The 

 evidence that this effect is purely a molec- 

 ular kinetic process is very strong. 



Electrolytes cause a depression of the 

 freezing point, experimentally determined,, 

 far greater than non-electrolytes ; we have 

 here another evidence of dissociation by 

 solution. 



The investigation of the effect of sub- 

 stances in solution upon vapor pressure 

 and boiling point made by Berthelot, Beck- 

 mann, Eaoult, Ciamician, Ostwald and 

 others lead to precisely similar laws. 



We may summarize these laws thus :, 

 Equimolecular solutions of different sub- 

 stances made with equal masses of the same 

 solvent, show equal osmotic pressure, equal 

 relative diminution of vapor pressure, equal 

 elevation of boiling point and equal depres- 

 sion of the freezing point. 



