554 Dr. T. M. Lowry on Osmotic Pressure from 



Poynting's Theory. 



Much interest was aroused by the publication in 1896 of a 

 paper by Professor Poynting (Phil. Mag. [5] xlii. pp. 289-301) 

 in which an explanation was given of the osmotic pressure of 

 liquids which afforded an alternative to van't HofPs gas- 

 theory, and had the advantage that the active part in osmosis 

 was assigned to the solvent rather than to the solute. In 

 many respects the view he advocated is very similar to that 

 described below, but the two theories differ in one essential 

 point, and consequently lead to widely different explanations 

 of the origin of osmotic pressure. In particular, Poynting 

 was led to assume that osmotic pressure was due to the 

 formation of labile hydrates; and it became necessary, as 

 Whetham pointed out (Nature, Oct. 15, 1896), to assume 

 that all substances which gave a normal osmotic pressure 

 were monohydrated, whilst the double osmotic pressure of 

 binary salts might be ascribed to the formation of a dihydrate 

 or of two monohydrated ions*. The invariable formation of 

 loose monolrydrates was so improbable, and is so far in con- 

 tradiction to recent work on the hydrates present in solution, 

 that Poynting's theory has failed to secure general acceptance 

 as an explanation of the osmotic phenomena. 



Osmotic Pressure as a Kinetic Phenomenon. 



The theory of osmotic pressure now described formed the 

 subject of a paper read before the Chemical Society of the 

 Central Technical College as long ago as May 1896; but it 

 was only recently that it was recognized as being sufficiently 

 novel to warrant further publicit}'. 



The starting-point of the theory is a consideration of the 

 conditions prevailing at the surface of separation of the 

 solution and the semi-permeable membrane, which may be 

 either a layer of copper ferrocyanide or merely the boundary 

 between liquid and vapour or liquid and ice. The simplest 

 of these cases is undoubtedly that which involves the equi- 

 librium between liquid and vapour. In this case the kinetic 

 theory postulates a continual process of evaporation, whereby 

 rapidly moving particles are constantly escaping from the 

 surface of the liquid into the vapour space. This is balanced 

 by the condensation of practically all the molecules of the 

 vapour that impinge on the liquid surface. When the 

 vapour reaches a certain concentration the rate of conden- 

 sation becomes equal to the rate of evaporation, and a con- 

 dition of equilibrium is attained, not because evaporation has 



* Compare I. Traube, Amu Phys. Chem. 1897, ii. 62. pp. 490-506. 



