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XXIII. On the Diffusion of Liquids, 

 By Thomas Graham, F.R.S., F.C.S.* 



ANY saline or other soluble substance, once liquefied and 

 in a state of solution, is evidently spread or diffused 

 uniformly through the mass of the solvent by a spontaneous 

 process. 



It has often been asked whether this process is of the nature 

 of the diffusion of gases, but no satisfactory answer to the 

 question appears to be obtained, owing, I believe, to the sub- 

 ject having been studied chiefly in the operations of endos- 

 mose, where the action of diffusion is complicated and obscured 

 by the imbibing power of the membrane, which is peculiar 

 for each soluble substance, but no way connected with the 

 diffusibility of the substance in water. Hence also it was not 

 the diffusion of the salt, but rather the diffusion of the solution, 

 which was generally regarded. A diffusibility like that of 

 gases, if it exists in liquids, should afford means for the sepa- 

 ration and decomposition even of unequally diffusible sub- 

 stances, and being of a purely physical character, the neces- 

 sary consequence and index of density, should present a scale 

 of densities for substances in the state of solution, analogous 

 to vapour densities, which would be new to molecular theory. 



M. Gay-Lussac proceeds upon the assumed analogy of 

 liquid to gaseous diffusion in the remarkable explanation 

 which he suggests of the cold produced on diluting certain 

 saline solutions, namely, that the molecules of the salt expand 

 into the water like a compressed gas admitted into additional 

 space. 



The phaenomena of solubility are at the same time con- 

 sidered by that acute philosopher as radically different from 

 those of chemical affinity, and as the result of an attraction 

 which is of a physical or mechanical kind. The characters 

 indeed of these two attractions are strongly contrasted. Che- 

 mical combination is uniformly attended with the evolution of 

 heat, while solution is marked with equal constancy by the 

 production of cold. The substances which combine chemically 

 are the dissimilar, while the soluble substance and its solvent 

 are the like or analogous in composition and properties. 



In the consideration of solubility, attention is generally en* 

 grossed entirely by the quantity of salt dissolved. But it is 

 necessary to apprehend clearly another character of solution, 

 namely, the degree of force with which the salt is held in so- 

 lution, or the intensity of the solvent attraction, quite irre- 



* From the Philosophical Transactions for 1850, part i. ; having been 

 received by the Royal Society November 16, and read December 20, 1849. 



