ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 233 



causing this accumulation is deliberately left open, i.e., it is im- 

 material for the present whether it is caused by physical or by the 

 so-called chemical forces. At the time when Rutherford's atomic 

 model is constantly furnishing greater support for the common basis 

 of the physical and chemical properties of matter, it appears quite 

 futile to attempt to differentiate between mechanical forces of ad- 

 hesion and chemical attractions. We shall therefore use the term 

 adsorption not in any sense of distinction from chemical union, 

 and we shall take it to include cases which theoretical chemists would 

 recognize as chemical reactions, as well as such cases which, to an 

 investigator of the older school, would not convey the least sug- 

 gestion of chemical affinity, such as, for example, the adsorption by 

 charcoal of an alcohol dissolved in water. The important feature of 

 our definition is that this binding process occurs at the boundary 

 surface of two phases, and not in the interiors of these phases. 



This definition of adsorption leads to a peculiar consequence. 

 According to Gibbs' principle, surface-active substances dissolved in 

 w^ater must concentrate in the surface layer of the water, even though 

 no special "adsorbent" is present, and even when the water is bound 

 only by a gaseous space. In this case, according to our definition, 

 the gaseous space would be the "adsorbent." While it is true that 

 this designation is a more or less borrowed one, it cannot, nevertheless, 

 be neglected. Thus let us take for an example two phases, A and B, 

 and dissolved in phase A is substance C, which is either entu'ely 

 insoluble in B, or has reached in B its equilibrium of distribution. 

 Then the composition of the boundary layer will be determined by 

 the effects of the following molecular interrelationships: 1. those of 

 the molecules A - A, 2. A - B; 3. A - C; 4. B -B;5. B -C;and 

 6. C — C, But when phase B (a gas) is practically without mass in 

 comparison with A, then the forces A — B, B — B, and B — C need 

 not be considered. In spite of this, and because of the interference 

 of the remaining forces, there occurs a concentration of substance C 

 in the surface layer of phase A. One could offer the definition: — in 

 this case the adsorbent is lacking; hence an adsorbent is a substance 

 which changes such a state of distribution. But since the amount of 

 C accumulated in the free surface layer cannot be experimentally 

 determined, practically it is preferable to choose for our point of 

 departure that state in which the dissolved substance C is uniformly 

 distributed throughout in phase A, and to designate every accmnu- 



