550 



binding. If the differential binding heat is great, and if it decreases 

 on absorption of the substance, then follows from the equality of 

 the variations of free energy and of binding-heat that the binding 

 isotherm must have a course as Fkkundi,k:h must have found, i. e. 

 that it begins pretty well horizontally, and then turns its convex 

 side downwards. This appears to be the case in aqueous solutions 

 of sidphuric acid and phosphoric acid, and in (he swelling albumens 

 and polysaccharides. In all these cases Fheundmch's formula appears 

 to hold as approximating formula for small concentrations, even 

 particularly well in aqueous solutions of sulphuric acid and phos- 

 phoric acid, though we have certainly not to do here with real 

 surface adsorption, but with real mixing. 



Hence it is clear that the validity of Fbeundi.ich's formula does 

 not furnish the proof that roe have to do with surface adsorption. 

 Inversely the equality in the variation of free energy and heat- 

 effect is no proof either that there exists an ideal concentrated 

 solution. It does not seem improbable to me that this equality also 

 exists with pure surface adsorption, and possibly with many com- 

 plicated intermediary phenomena called sorption at present. I found 

 it confirmed in the absorption of water by ciipri ferro cyanide, in 

 which a strong change of colour from violet black to light brown 

 is found'). The next step is now in my opinion to test this relation 

 by a number of typical examples of genuine surface adsorption and 

 of sorption. For if it appears to be valid everywhere, this is an 

 important contribution to the knowledge of the sorption phenomena; 

 and if it holds in some cases and not in others, it may be studied 

 on what this depends. But apart from this it leads to a better 

 method of analysis of sorption and adsorption phenomena : the 

 simultaneous determination of the sorption isotherms and of the 

 sorption heats. This method gives a much deeper insight than the 

 prevalent one, which is restricted to the determination of the sorp- 

 tion isotherm for small concentrations. That Fheundlich's formula 

 is of such universal validity at these small concentrations, will 

 probably appear to mean that (in a system in which the variations 

 of free energy and of heat-effect are equal in approximation) the 

 differential sorption heat is very great at first, and diminishes 

 gradually during the absorption; the longer the (almost) asymptotic 

 horizontal initial part of the isotherm, the longer the differential 

 sor|)tion heat will preserve a great value. What is important in this 

 method of investigation of the sorption phenomena is further that 



') Verslag van de gewone vergadering der wis- en naluurk. Afd. Kon. Akad. v. 

 Wet. Dl. XXXI, Nos. 9-10, p. 542. 



