448 A NOTE ON ADSORPTION [ch. 



pores; both alike are "two-phase" systems, and in both ahke the 

 phenomenon of adsorption has free play. The occlusion of gases, 

 including water-vapour, by charcoal is a familiar phenomenon of 

 adsorption, and is due to the minuteness of the pores only in so far 

 as surface-area is increased and multipUed thereby. For surface- 

 energy is surface-strain or surface-tension x surface-area, and is 

 vastly increased by minute subdivision. And surface-energy is 

 such that, whenever a substance is introduced into a two-phase 

 system — which merely means two things in touch (or surface- 

 contact) with one another — it is apt to concentrate itself on the 

 surface where the two phases meet. Absorption implies uniform 

 distribution, as when a gas is absorbed by a liquid; adsorption 

 imphes a heterogeneous field, and a concentration localised on the 

 surfaces therein. 



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 sub- 

 stance, that is to say of its direct association with the surface-layer 

 of molecules such as we imagined in the case of an electrical charge ; 

 but rather of a progressive tendency to concentrate more and more, 

 the nearer the surface is 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 repetition 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 con- 

 tinual instabihty: and a constant manifestation of motion, some- 

 times in the mere invisible transfer of molecules, but often in the 

 production of visible currents, or manifest alterations in the form 

 or outline of the system. 



Cellular activity is of necessity associated with cellular structure, 



