VI] OF SURFACE CONCENTRATION 447 



Moreover, the more a substance has the power of lowering the 

 surface-tension of the Hquid in which it happens to be dissolved, 

 the more will it tend to displace another and less effective substance 

 from the surface-layer. Thus we know that protoplasm always 

 contains fats, not only in visible drops, but also in the finest sus- 

 pension or "colloidal solution"; and if under any impulse, such for 

 instance as might arise from the Brownian movement, a droplet of 

 oil be brought close to the surface, it is at once drawn into that 

 surface and tends to spread itself in a thin layer over the whole 

 surface of the cell. But a soapy surface (for instance) in contact 

 •with the surrounding water would have a surface-tension even less 

 than that of the film of oil: and consequently, if soap be present 

 in the water it will in turn be adsorbed, and will tend to displace 

 the oil from the surface pellicle*. And all this is as 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, in other words, so as to 

 reduce towards a minimum the potential energy of the system. 

 This tendency to concentration at a 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 lawf, and was arrived at not by experi- 

 mental but by theoretical and hydrodynamical methods. 



An assemblage of drops or droplets offers a great extension of 

 surface, but so also does an assemblage of equally minute cells or 



* This identical phenomenon was the basis of Quincke's theory of amoeboid 

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

 Berlin. Akad. 1888, pp. 791-806; cf. Pfiuger's Archiv, 1879, p. 136). We must bear 

 in mind that to describe an amoeboid cell as "naked" does not imply that its outer 

 layer is identical with its internal substance. 



t J. Willard 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.) various papers by C. M. Lewis, 

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

 Lxx, p. 129, 1910; Milner, Phil. Mag. (6), xm, p. 96, 1907; A. B. Macallum, The 

 role of surface-tension in determining the distribution of salts in living matter, 

 Trans, loth Int. Congress on Hygiene, etc., Washington, 1912; etc. 



