m 



Dr. F. a. Donnan 



on a 



are subjected to a statistically almost uniform molecular 

 bombardment, and hence possess only very small quasi- 

 molecular motions. These complexes, moreover, are such 

 that changes of temperature, or the addition o£ comparatively 

 small quantities of other substances, frequently cause the 

 sudden precipitation in mass of the substance C. 



In what follows an attempt is made to show that all these 

 phenomena can be explained by the application of a well- 

 known hypothesis, namely by means of Laplace's theory of 

 intermolecular attractive forces. In doing this it is necessary, 

 and in the present case essential, to carefully distinguish 

 between the kinetic molecular equilibrium and the statical* 

 (mechanical) equilibrium at the interface of solid and liquid. 



Let ns consider first the state of affairs between a crystal- 

 line solid and a liquid medium which dissolves it. 



Here we have, at any given temperature, a definite con- 

 centration of the dissolved solid. This equilibrium is a 

 molecular-statistical one, and consists in the balancing of two 

 statistically equal and oppositely directed molecular fluxes. 

 At the same time, owing to the fact that a definite interface 

 exists between the solid and the liquid, it is clear that any 

 small volume-element of the solid lying near the interface is 

 in statical equilibrium. The resultant of all the inter- 

 molecular forces is, in fact, an inwardly-directed force urging 

 this volume-element towards the inner part of the solid. 



Thus a crystalline solid immersed in its saturated solution is 



(a) in statical equilibrium under a compressive stress ; 



(b) in kinetic-molecular equilibrium under equal and 



opposite molecular fluxes. 



Hence, during the process of solution of a crystalline solid, 

 although there is a net outward flux of molecules, we must 

 suppose that the resultant mechanical force acting on any 

 small volume-element of the solid in the immediate neighbour- 

 hood of the solid-liquid interface remains inwardly directed, 

 so that the l( molar " integrity of the mass is preserved, 

 although '' molecular" disintegration is taking place. 



The theory which is here proposed for the pseudo-solution 

 of colloidal matter regards this phenomenon as a process of 

 molar or mechanical disintegration due to the non-existence 

 of statical equilibrium in the thin surface-layers of the solid 

 when the latter is in contact with certain liquid media. 



Consider the solid, I, bounded by the planes AB, CD, and 

 surrounded by the liquid medium (it is sufficient to con- 

 sider the problem in two dimensions). Let de be any small 

 volume-element of I situated just at the bounding interface 

 * In this connexion see Larmor, ' ^Ether and Matter ' (1900), passim. 



