ELECTRIFICATION OF WATER AND OSMOTIC PRESSURE. 



By JACQUES LOEB. 

 {From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, July 21, 1919.) 

 I. INTRODUCTION. 



When a watery solution is separated from pure water by a strictly 

 semipermeable membrane we call osmotic pressure the additional 

 pressure upon the solution which is required to cause the migration 

 of as many molecules of water from the side of the solution to the 

 side of the pure solvent as migrate simultaneously from pure water to 

 the solution. Van't Hoff's theory demands that this pressure should 

 increase with the number of molecules in the solvent and that it should 

 be equal to the gas pressure of the solute in the volume of the solution. 

 The actual measurements of osmotic pressure of solutions of cane 

 sugar made by Morse^ and his collaborators as well as by the Earl 

 of Berkeley^ and his fellow workers show a rather close approximation 

 to van't Hoff's theory. 



When the solution is separated by a strictly semipermeable mem- 

 brane, the difference in the rate of diffusion of water in opposite 

 directions must be the greatest at the beginning of the experiment, 

 and the difference must diminish steadily during the experiment as 

 a consequence of the increase of hydrostatic pressure on the side of 

 the solution. The value of the osmotic pressure of a solution as 

 defined above must therefore vary with the difference in the rate 

 of the diffusion of water molecules in the two opposite directions at 

 the beginning of the experiment. When we double the concentration 

 of the solute we also double the initial difference in the rate of diffusion 

 of water molecules in opposite directions, and the additional pressure 



1 Morse, H. N., The osmotic pressure oia-qaeoussolntions, Carnegie Institution 

 of Washington, Publication 198, 1914. 



2 Earl of Berkeley, and Hartley, E. G. J., Proc. Roy. Soc. London, Series A, 

 1916, xcii, 477. 



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