462 



SCIENCE 



[N. S. Vol. XL. No. 1031 



osmotic pressure is a case such as we are 

 considering, and equilibrium throughout 

 the membrane is only obtained when the 

 necessary difference in pressure exists be- 

 tween the two sides of the membrane. 



This condition would eventually be 

 reached no matter how thick the membrane 

 was. It is sometimes helpful to think of 

 the membrane as being very thick. It pre- 

 cludes any temptation to view molecules as 

 shooting across from one liquid to the other 

 through some kind of peepholes in the 

 membrane. 



The advantage in a thin membrane in 

 practise is simply that the necessary mois- 

 ture is rapidly applied to the active sur- 

 face, thus enabling the pressure on the side 

 of the solution to rise quickly, but it has no 

 effect on the ultimate equilibrium. 



As far as that goes, the semi-permeable 

 membrane or saturated medium might be 

 infinitely thick, or, in other words, there 

 need be no receptacle or place for holding 

 the pure solvent outside the membrane at 

 all. In fact, the function of the receptacle 

 containing the pure solvent is only to keep 

 the medium moist, and is no more or no less 

 important than the vessel of water supplied 

 to the gauze of the wet-bulb thermometer. 

 It is merely to keep up the supply of water 

 to the medium. 



The real field where the phenomenon of 

 osmosis takes place is the surface of separa- 

 tion between the saturated semi-perme- 

 able medium and the solution. Imagine 

 a large mass of colloidal substance satu- 

 rated with water and having a cavity con- 

 taining a solution. The pressure will now 

 tend to rise in the cavity until it reaches 

 the osmotic pressure — ^that is, until there is 

 established an equilibrium of surface trans- 

 fer of molecules from the solution into the 

 medium and back from the medium into the 

 solution. 



No doubt, the phenomenon as thus de- 



scribed occurs often in nature. It is just 

 possible that the high-pressure liquid cav- 

 ities, which mineralogists find in certain 

 rock crystals, have been formed in some 

 such manner in the midst of a mass of semi- 

 permeable medium ; the pure solvent in this 

 case being carbon dioxide and the medium 

 colloidal silica, which has since changed 

 into quartz crystal. 



In considering equilibrium between a 

 saturated semi-permeable medium and a 

 solution there seems to me to be a point 

 which should be carefully considered before 

 being neglected in any complete theory. 

 That is, the adsorption layer over the sur- 

 face of the semi-permeable medium. We 

 have seen that solutions are profoundly 

 modified in the surface layers adjoining 

 certain solids, through concentration or 

 otherwise of the salts in the surface layer, 

 so that the actual equilibrium of surface 

 transfer of water molecules is not between 

 the unmodified solution and the semi-per- 

 meable medium, but between the altered 

 solution in the absorption layer and the 

 saturated medium. Actual determinations 

 of the adsorption by colloids are much 

 wanted, so as to be able to be quite sure of 

 what this correction amounts to or even if 

 it exists. It may turn out to be zero. If 

 there is adsorption, however, it may pos- 

 sibly help to account for part of the unex- 

 pectedly high values of the osmotic pres- 

 sure observed at high concentrations of the 

 solution, the equilibrium being, as we have 

 seen, between the saturated medium and a 

 solution of greater concentration than the 

 bulk of the liquid, namely, that of the ad- 

 sorption layer. In addition, when above 

 the critical adsorption point, there may be 

 a deposit in the solid state. This may pro- 

 duce a kind of polarized equilibrium of 

 surface transfer in which the molecules 

 which discharge from the saturated medium 

 remain unaltered in amount, but those 



