298 Prof. J. H. Poynting on Osmotic Pressure. 



(7 and la) , we see that the lowering due to a given osmotic 

 pressure in the solution is greater than that due to an equal 

 pressure on the pure solvent in the ratio 



111 



P P P 



or 



In the case of ice and water the ratio is 1 : P092 — 1 



= 1 : -092 

 = 11 : 1 



It now remains to see if we can give any reasonable 

 account of the decrease in mobility in a liquid when a salt is 

 present in solution. If the molecules of salt were simply 

 mixed with those of the solvent, or if they combined to form 

 stable non-evaporating compounds with the solvent, which 

 compounds were simply mixed, then the mixture should have 

 the same vapour-tension as the pure solvent. For we might 

 regard the salt or compound molecules at the surface as equally 

 reducing the effective evaporating and the effective condensing 

 area, somewhat as a perforated plate or gauze laid on the sur- 

 face would do. But the salt probably combines with the 

 solvent to form unstable molecules which continually inter- 

 change constituents, so that when near the surface they may 

 serve equally with those of the pure solvent to entangle the 

 molecules of vapour coming downwards, these descending 

 vapour molecules taking the place of molecules attached to 

 the salt. Probably, however, they are less energetic than the 

 pure solvent molecules and do not contribute so much to 

 evaporation. We shall make the supposition that they do not 

 contribute at all. 



Let then each of the salt molecules combine on the average 

 with a of the solvent molecules, and in such a way that it 

 prevents those a molecules from evaporating while the com- 

 pound molecules formed will entangle returning molecules, 

 each of the a being replaceable by a vapour molecule. Then 

 we may regard the solution as solvent, having a number of 

 molecules simply mixed up and inactive as regards evapora- 

 tion but active in effecting condensation. 



If N is the number of gramme molecules of solvent per 

 litre, and n the number of gramme molecules of salt added, 

 the number of solvent molecules left is N — an. Were the n 

 compound molecules quite inactive both as regards evaporation 

 and condensation the mobilities outwards and inwards would 



