SURFACES OF DISCONTINUITY 543 



where x is the mass of gas or solute adsorbed per unit mass of 

 adsorbing material, c the concentration of the solution in the 

 bulk or the partial pressure of the gas in a gaseous system, n 

 an exponent which in general is less than unity. The exponent 

 n and the constant k are in general functions of temperature. 

 For substances feebly adsorbable n approaches unity. Ap- 

 parently this type of equation appears to have been first applied 

 to adsorption of gases by Saussure as early as 1814, and in 1859 

 Boedecker extended it to solutions. It has since been em- 

 ployed by a large number of workers. The most complete 

 examination of its applicability in relatively recent times has 

 been made by Freundlich, whose name is now very generally 

 associated with the relation itself. In his Colloid and Capil- 

 lary Chemistry (English translation of the third German edition, 

 p. 93 (1926)), he draws attention to the fact that some of the 

 experimental results at liquid-liquid interfaces fit it fairly well ; 

 for in them there appears a striking feature, corresponding to what 

 is known to be true at solid boundaries, viz., a surprisingly large 

 relative amount adsorbed at low concentrations, followed by a 

 growth as the concentration rises which is not in proportion to 

 the concentration but increases much less rapidly, ending up at 

 high concentrations with a saturation which hardly changes. 

 Actually the exact formula is only roughly valid numerically at 

 high concentrations, but when the conditions are sufficiently 

 removed from saturation it holds quite well. Although only 

 one of many relations suggested, it is still regarded as one of 

 the most convenient and reasonably exact modes of represent- 

 ing existing data, especially for systems consisting of finely 

 divided solids as adsorbing agents. For a discussion of the limi- 

 tations of its applicability the reader is referred to Chapter V 

 of An Introductio7i to Surface Chemistry by E. K. Rideal (1926). 



19. Approximate Form of Gihhs' Equation and Thomson's 



Adsorption Equation 



Actually Gibbs' equation is the earhest theoretically derived 

 relation; but in 1888, about ten years after its publication, 

 J.J. Thomson obtained by an entirely different method a relation 

 which resembles that of Gibbs. There is a rather prevalent 



