328 SCIENCE PROGRESS 



The great majority of substances, including the common 

 dyestuffs, cause a decrease in the surface tension of a solvent in 

 which they are dissolved. Willard Gibbs showed from thermo- 

 dynamical considerations that in such cases the solute will tend 

 to accumulate in the surface layer of the solvent. In a system 

 such as the colloidal solution of a dye in the presence of a solid 

 colloid, such as a textile fibre, the amount of surface involved is 

 really enormous, and in consequence we should expect to find 

 changes of concentration in one phase (the liquid) at its boundary 

 surface with the other phase (the solid). It is in fact such 

 changes of concentration on a surface, especially a solid colloidal 

 surface in contact with the solution, due to a tendency of the 

 surface energy to attain a minimum that are designated collec- 

 tively as " adsorption phenomena." The absorption of gases and 

 of iodine by animal charcoal and the clarifying action of certain 

 colloid-gels, such as alumina, on organic solutions belong to this 

 type of phenomena. Investigations by Freundlich and others 

 have shown that in all such cases an equilibrium is attained more 

 or less quickly and that the relative concentrations in the two 

 phases can be expressed by a simple exponential formula. If a 

 is the original total amount of solute (e.g., dye) before adsorption, 

 ajv is the original concentration of the solution. If x is the 

 amount of dye adsorbed when m grms. of solid (e.g., textile 

 fibre) are employed, x/m is the amount adsorbed per grm. of 

 solid, a — x is therefore the amount left unadsorbed, and hence 



a — x 



■ = c 



V 



is the final concentration of the solution. 

 Then 





where /3 and p are constants depending on the nature of the 

 solutions and the adsorbent. It is obvious that this equation 

 is merely an extension of Henry's law with one side raised to 



a power - ; that it is in fact a general statement applicable 



to various kinds of equilibria, and of which the partition law 

 mentioned above is a particular case. In all adsorption 

 phenomena the value of p is found to lie somewhere between 

 2 and io. 



