446 A NOTE ON ADSORPTION [ch. 



fluid, exposed to air, and containing droplets of oil : we know that 

 the specific surface-tension of oil in contact with air is much less 

 than that of water, and it follows that, if the watery surface of 

 our drop be replaced by an oily surface the specific surface-energy 

 of the system will be notably diminished. Now under these circum- 

 stances it is found that (quite apart from gravity, which might 

 cause it to float to the surface) the oil has a tendency to be drawn 

 to the surface; and again this phenomenon of molecular attraction 

 or adsorption represents work done, equivalent to the diminished 

 potential energy of the system*. In more general terms, if a Hquid 

 be a chemical mixture, some one constituent in which, if it entered 

 into or increased in amount in the surface layer, would have the 

 effect of diminishing its surface-tension, then that constituent will 

 have a tendency to accumulate or concentrate at the surface: the 

 surface-tension may be said, as it were, to exercise an attraction 

 on this constituent substance, drawing it into the surface-layer, 

 and this tendency will proceed until at a certain "surface-con- 

 centration" equilibrium is reached, its opponent being that osmotic 

 force which tends to keep the substance in uniform solution or 

 diffusion. In other words, in any "two-phase" system, a change 

 of concentration at the boundary-surface and a diminution of 

 surface-tension there accompany one another of necessity ; positive 

 adsorption means negative surface-tension, and vice versa. Further- 

 more, the lowering of surface-tension (as by saponin) will permit 

 (caeteris paribus) an extension of surface, manifesting itself in 

 "froth." Thus the production of a froth and the concentration 

 of appropriate substances therein are two sides of one and the same 

 phenomenon. 



In the complex mixtures which constitute the protoplasm of the 

 living cell, this phenomenon of adsorption has abundant play: for 

 many of its constituents, such as fats, soaps, proteins, lecithin, etc., 

 possess the required property of diminishing surface-tension. 



* The first instance of what we now call an adsorptive phenomenon was 

 observed in soap-bubbles. Leidenfrost was aware that the outer layer of the 

 bubble was covered by an "oily" layer {De aquae communis nonnullis qualitatihus 

 tractatus, Duisburg, 1756). A hundred years later Dupre shewed that in a soap- 

 solution the soap tends to concentrate at the surface, so that the surface-tension 

 of a very weak solution is very little different from that of a strong one {Theorie 

 mdcanique de la chaleur, 1869, p. 376; cf. Plateau, ii, p. 100). 



