VI] ON SURFACE-CONCENTRATION 279 



or "adsorption" represents the work done, equivalent to the 

 diminished potential energy of the system*. In more general 

 terms, if a liquid (or one or other of two adjacent liquids) 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 the complex mixtures which constitute the protoplasm of 

 the living cell, this phenomenon of "adsorption" has abundant 

 play : for many of these constituents, such as oils, soaps, albumens, 

 etc. possess the required property of diminishing surface tension. 



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

 surface tension of the Uquid 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 or oils, not only in visible drops, but also in the 

 finest suspension or "colloidal solution." 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) 

 would have in contact with the surrounding water 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 f. And this is all as 



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

 observed in soap-bubbles. Leidenfrost, in 1756, was aware that the outer layer 

 of the bubble was covered by an "oily" layer. 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 httle different from that 

 of a strong one (Theorie me'canique de la chaleur, 1869, p. 376; cf. Plateau, ii, 

 p. 100). 



t This identical phenomenon was the basis of Quincke's theory of amoeboid 



