280 A NOTE ON ADSORPTION [ch. 



much as to say that the molecules of the dissolved or suspended 

 substance or substances will so distribute themselves throughout 

 the drop as to lead towards an equilibrium, for each small unit 

 of volume, between the superficial and internal energy ; or so, in 

 other words, as to lead towards a reduction to a minimum of the 

 potential energy of the system. This tendency to concentration 

 at the surface of any substance within the cell by which the surface 

 tension tends to be diminished, or vice versa, constitutes, then, 

 the phenomenon of Adsorption ; and the general statement by 

 which it is defined is known as the Willard-Gibbs, or Gibbs- 

 Thomson law*. 



Among the many important physical features or concomitants 

 of this phenomenon, let us take note at present that we need 

 not conceive of a strictly superficial distribution of the adsorbed 

 substance, that is to say of its direct association with the surface 

 layer of molecules such as we imagined in the case of the electrical 

 charge; but rather of a progressive tendency to concentrate, 

 more and more, as the surface is nearly approached. Indeed we 

 may conceive the colloid or gelatinous precipitate in which, in the 

 case of our protoplasmic cell, the dissolved substance tends often 

 to be thrown down, to constitute one boundary layer after another, 

 the general effect being intensified and multiplied by the repeated 

 addition of these new surfaces. 



Moreover, it is not less important to observe that the process 

 of adsorption, in the neighbourhood of the surface of a hetero- 

 geneous liquid mass, is a process which takes time ; the tendency 

 to surface concentration is a gradual and progressive one, and will 

 fluctuate with every minute change in the composition of our 

 substance and with every change in the area of its surface. In 

 other words, it involves (in every heterogeneous substance) a 

 continual instability of equilibrium : and a constant manifestation 



movement (Ueber periodische Ausbreitung von Fliissigkeitsoberflachen, etc., SB. 

 Berlin. Akad. 1888, pp. 791-806; of. Pfluger's Archiv, 1879, p. 136). 



* J. WiUard Gibbs, Equilibrium of Heterogeneous Substances, Tr. Conn. Acad. 

 Ill, pp. 380-400, 1876, also in Collected Papers, i, pp. 185-218, London, 1906; 

 J. J. Thomson, Applications of Dynamics to Physics and Chemistry, 1888 (Surface 

 tension of solutions), p. 190. See also (int. al.) the various papers by C. M. Lewis, 

 Phil. Mag. (6), xv, p. 499, 1908, xvii, p. 466, 1909, ZeUschr. f. physik. Chemie, 

 Lxx, p. 129, 1910; Milner, Phil. Mag. (6), xiii, p. 96, 1907, etc. 



