282 A NOTE ON ADSOEPTION [ch. 



of itself accumulating at the surface, but has also the property 

 of increasing the viscosity and mechanical rigidity of the material 

 in which it is dissolved or suspended, and so of constituting 

 a visible and tangible "membrane*." The "zoogloea" around a 

 group of bacteria is probably a phenomenon of the same order. 

 In the superficial deposition of inorganic materials we see the 

 same process abundantly exemplified. Not only do we have the 

 simple case of the building of a shell or "test" upon the outward 

 surface of a living cell, as for instance in a Foraminifer. but in a 

 subsequent chapter, when we come to deal with various spicules 

 and spicular skeletons such as those of the sponges and of the 

 Radiolaria, we shall see that it is highly characteristic of the 

 whole process of spicule-formation for the deposits to be laid 

 down just in the " interf acial " boundaries between cells or 

 vacuoles, and that the form of the spicular structures tends in 

 many cases to be regulated and determined by the arrangement 

 of these boundaries. 



In physical chemistry, an important distinction is drawn between adsorption 

 and pseudo-adsorption'f, the former being a reversible, the latter an irreversible 

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

 impUes 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 Uttle 

 consequence to us. 



* We may trace the first steps in the study of this phenomenon to Melsens, 

 who found that thin films of white of egg become firm and insoluble (Sur les modi- 

 fications apportees a ralbumine...par Taction purement mecanique, C. R. Acad. 

 Sci. XXXIII, p. 247, 1851); and Harting made similar observations about the same 

 time. Ramsden has investigated the same subject, and also the more general 

 phenomenon of the formation of albuminoid and fatty membranes by adsorption : 

 cf. KoaguHerung der Eiweisskorper auf mechanischer Wege, Arch. f. Anat. u. Phys. 

 {Phys. Ablh.) 1894, p. 517; Abscheidung fester Korper in Oberflachenschichten 

 Z.f. phys. Chem. XLVn, p. 341, 1902; Proc. R. S. lxxii, p. 156, 1904. For a general 

 review of the whole subject see H. Zangger, Ueber Membranen und Membranfunk- 

 tionen, in Asher-Spiro's Ergebnisse der Physiologic, vii, pp. 99-160, 1908. 



t Cf. Taylor, Chemittry of Colloids, p. 252, 



