452 A NOTE ON ADSORPTION [ch. 



spicule-formation for the deposits to be laid down just in the 

 " interfacial " boundaries between cells or vacuoles, and how the 

 form of the spicular structures tends in many cases to be regulated 

 and determined by the arrangement of these boundaries. The 

 so-called collenchyma, in which an excess of cellulose is laid down 

 around the angles of contact of adjacent cells, in a kind of exag- 

 gerated "bourrelet," is another case in point*. 



No pure liquid ever forms a froth or foam. White of egg is no 

 exception to the rule; for the albumin is somehow changed, or 

 " denatured," and becomes a quasi-solid when we beat it up. But 

 in the frothing liquid there must always be some admixture present 

 to concentrate on, or be adsorbed by, the surfaces and interfaces 

 of the other ; and this dispersion must go on completely and uni- 

 formly, so as to leave the whole system homogeneous. The resulting 

 diminution of surface-tension facihtates the subdivision of the 

 bubbles and dispersion of the air ; and the adsorbed surface-layer 

 gives firmness and stability to the system. The sudden increase of 

 surface diminishes, for the moment, the concentration, or "thick- 

 ness" of the surface-layer; the tension rises accordingly, and the 

 cycle of operations begins anewf . 



In physical chemistry, a distinction is usually drawn between adsorption 

 and psevdo-adsorption, the former being a reversible, the latter an irreversible 

 or permanent phenomenon. That is to say, adsorption, strictly speaking, 

 implies the surface-concentration of a dissolved substance, under circumstances 

 which, if they be altered or reversed, will cause the concentration to diminish 

 or disappear. But pseudo-adsorption includes cases, doubtless originating in 

 adsorption proper, where subsequent changes leave the concentrated substance 

 incapable of re-entering the liquid system. It is obvious that many (though 

 not all) of our biological illustrations, for instance the formation of spicules 

 or of permanent cell-membranes, belong to the class of so-called pseudo- 

 adsorption phenomena. But the apparent contrast between the two is in 

 the main a secondary one, and however important to the chemist is of little 

 consequence to us. 



While this brief sketch of the theory of membrane-formation is 

 cursory and inadequate, it is enough to shew that the physical 

 theory of adsorption tends in part to overturn, in part to simphfy 



♦ Cf. G. Haberlandt, Zelle u. Elementarorgane, Biol. Centralbl. 1925, p. 263. 

 t Cf. F. G. Donnan, Some aspects of the physical chemistry of interfaces, Brit. 

 Ass. Address (Section B), 1923; Nature, Bee. 15, 22, 1923. 



