WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 101 



volume-normal system was based on the presumption — already exten- 

 sively abandoned — that the phenomenon of pressure in the osmotic 

 cell is due simply to the bombardment of the membrane by the solute 

 molecules. If we had employed the volume-normal system for solutions, 

 our colleagues could have convicted us, by circumstantial evidence, 

 of being under the dominion of a discarded conception of the cause 

 (and therefore of the proper magnitude) of osmotic pressure. Inas- 

 much as the obviously immediate cause of the pressure observed in 

 the cell is a dilution of the solution within by solvent acquired from 

 without, the weight-normal appeared to involve less of hypothesis and 

 to be more rational than the volume-normal system. 



As to the part which is played by the membrane in bringing about 

 this dilution of the imprisoned solution, upon which the existence of the 

 pressure in the cell depends, the original idea of Graham — that the 

 passage of the aqueous solvent through it is due to some sort of hydration 

 of the colloidal material of the membrane on the side bathed by the 

 more dilute solution, and a dehydration on the side covered by the 

 more concentrated solution— has always appealed to us as more simple 

 than and quite as satisfactory as any of the numerous other explana- 

 tions which have been offered. It is certainly in accordance with the 

 observed fact that the amount of water which a colloid can acquire 

 and retain depends on the concentration of the solution to which it is 

 exposed. If Graham's view concerning the modus operandi of the 

 transmission of water is correct, the real problem to be studied in this 

 connection would seem to be the dependence of the hydration of the 

 colloidal membrane upon the concentration of the solutions and upon 

 pressure. 



The course of reasoning which led to the adoption of the volume of 

 the solvent as the standard for the computation of the gas pressure of 

 the solute is quite elementary and appears to involve very little of 

 hypothesis. 



The essential difference between a substance in gas form and in 

 solution, which strikes one at once and first of all, is the fact that the 

 molecules of a gas are moving through space otherwise unoccupied, 

 while in a solution the molecules of the solute are moving through 

 space occupied by the solvent. The analogy of the "free space," in the 

 case of a gas, to the free or pure solvent in a solution is, in this particular, 

 obvious and unmistakable. Moreover, it was to be presumed that the 

 space occupied by the solute molecules would bear, in general, somewhat 

 the same relation to osmotic pressure that the aggregate volume of 

 the gas molecules bears to the pressure of a gas — in other words, that 

 a correction would have to be employed for osmotic pressure which is 

 equivalent to the correction symbolized by the term b in the equation of 

 van der Waals for gases. It was also clear that, if the pressure of a gas 

 could always be computed on the basis of, or referred to, the volume 



