60 PRINCIPLES OF GENERAL PHYSIOLOGY 



the extent of surface, or the number of molecules situated there, is the controlling 

 factor, corresponding to the active mass. The surface of the same quantity of 

 matter may vary enormously, according to the degree of subdivision, as already 

 pointed out. The subdivision may indeed be carried out in imagination so far 

 that molecular dimensions are reached, in which case ordinary chemical action 

 is being dealt with. This possibility of the existence of every intermediate stage 

 is apparent, and it is, perhaps, this fact that has led to many of the loose state- 

 ments made by some writers on adsorption. Although theoretically, chemical 

 adsorption, as defined above, should be included under the general name, it is 

 usually understood that the more physical forms, due to changes in surface tension 

 or electrical charge, are meant when adsorption is spoken of as distinct from 

 chemical combination. 



In cases spoken of as " specific " adsorption, where a particular kind of surface 

 takes up preferentially a particular substance, it appears that the chemical con- 

 figuration of the surface must be taken into account. At the same time, when 

 we remember the manifold possibilities of differences in surface tension, electric 

 charge, etc., it seems unlikely that recourse need be had to chemical phenomena, 

 except in rare cases (see van Bemmelen, 1910, pp. 423-430 ; and Freundlich, 1909, 

 pp. 153-162 ; also Barger and W. W. Starling, 1915). 



Some examples may be given : Wohler and Pliiddemann (1908, p. 664) found that carbon 

 and red oxide of iron adsorb benzoic acid ten times as strongly as they do acetic acid. 

 Chromium oxide adsorbs both acids equally ; while platinum black adsorbs acetic acid 

 slightly more than benzoic acid, but neither to any great extent. These apparently specific 

 adsorptions can scarcely be of a chemical nature. Another case which may have a bearing 

 on the question of specific adsorption is given by Marc (1013, p. 692). Crystalline substances, 

 such as barium carbonate, only adsorb crystalloids when these are isomorphous, or crystallise 

 in a similar form to that of barium carbonate. They are supposed to be able to form a solid 

 solution on the surface of the adsorbent. Thus, potassium nitrate is adsorbed, since it, like 

 barium carbonate, belongs to the rhombic system. Sodium nitrate, of the hexagonal system, 

 is not notably adsorbed. Calcium carbonate, of the hexagonal system, on the other hand, 

 adsorbs sodium nitrate, but not potassium nitrate. Since calcium carbonate can be obtained 

 also in crystals of the rhombic system, it seems possible to test the hypothesis ; these 

 crystals should adsorb potassium nitrate but not sodium nitrate. It must be remembered 

 also that potassium nitrate can crystallise in the hexagonal system, isomorphous with sodium 

 nitrate. This deposition of a salt on an isomorphous crystal might be supposed to be merely 

 the ordinary growth of a crystal in a solution of an isomorphous salt, say, calc-spar increasing 

 in size by the addition of layers of sodium nitrate ; but, as it appears to follow a complex 

 parabolic law, surface concentration, according to the laws of adsorption, needs taking 

 account of. 



Freundlich (1909, p. 514) points out that gelatine only adsorbs sugar after having been 

 treated with formaldehyde. We have seen above that there is a considerable difference in 

 the structure of gelatine after the action of formaldehyde, as shown by Hardy (1899, i. p. 165). 

 But we must also remember that formaldehyde combines chemically with proteins, so that 

 the interpretation of this fact is not quite simple. 



Drury's work (1914) shows that the condensation of a solute on to a surface is markedly 

 influenced by the previous treat meiit of, or by the gas condensed on, that surface. 



The physical configuration of the surface may also play a part when both adsorbent 

 and body adsorbed have surfaces of a definite structure. A rough illustration may explain 

 what is meant. A flat surface and one covered with projecting points cannot get into close 

 contact, whereas two flat surfaces can do so. This idea is at present, however, purely 

 hypothetical. The problem of specific adsorption has not yet received adequate investigation. 



COMBINED EFFECTS 



The various forms of surface energy may be present at the same time on the 

 same surface, and it is of some interest to know how they affect one another. 

 The action of an electrical charge on mechanical surface tension is to diminish it, 

 as may be seen from the following consideration. Surface tension is due to the 

 mutual attraction of the elements of the surface ; when these elements receive an 

 electric charge, they repel one another, being of the same sign, an'l thus a force 

 is present in an opposite direction to that of surface tension. 



It appears that electrical adsorption exceeds in amount that due to diminution 

 of surface tension, so far as the cases at present known indicate. We see in the 

 experiments of Lewis with sodium oleate (1909, p. 494) that the amount adsorbed 

 by a water-oil interface was one hundred times greater than that calculated from 

 the Gibbs formula to be due to diminution of surface tension. 



