224 Wettimj I'tniur (iiid its Detcnniuation 



a low interfacial tension (Tjo). Furtlicr, it is evident from the con- 

 sideration of Fig. 1, that the effect of reducing the interfacial tension 

 Tja will be to cause the liquid to spread over the surface of the solid, 

 even if the surface tension T, is fairly high ; or in other words, it is the 

 interfacial tension Tjg, rather than the surface tension of the li(juid Tj, 

 which is the determining factor in wetting power. 



The process of wetting, however, is not so simple as would appear 

 from a consideration of the above equation. There are at least two 

 other factors which must be taken into account, which, under certain 

 conditions, may cause the above relationship to fail entirely. 



The first of these disturbing factors is concerned with the solvent 

 properties of the wetting li(juid. It almost always happens in actual 

 practice that the surface to be wetted is already coated with a layer 

 of some greasy or waxy secretion. Since wetting depends partly upon 

 the interfacial tension of the liquid to the solid, actual contact between 

 the two is essential. It is necessary therefore that the wetting liquid 

 should have a certain solvent action on the grease or wax, with which 

 the solid may be coated. This solvent power need not be very pro- 

 nounced, provided that the interfacial tension of the liquid is small, 

 for, provided the surface of the solid is reached in any one place, the 

 greasy or waxy layer will be displaced by the wetting liquid, because 

 its interfacial tension to the solid will be lower than that of the grease 

 or wax to the solid. If, on the other hand, the grease or wax is com- 

 pletely insoluble in the liquid, there can be no actual contact between 

 the solid and the liquid, and therefore no wetting. Thus, for example, 

 it is probable that the reason why mercury fails to wet the surface of 

 so many solids is not entirely due to its high surface tension, but to 

 the fact that it is incapable of dissolving the surface layer of moist air 

 which adheres to the surfaces of most solids (Freundlich, 1909, p. ITO). 



The second factor, liable to vitiate conclusions drawn from the 

 equation conditioning wetting, is concerned with the phenomenon 

 known as surface coneentralion (capillary adsorption). If. with in- 

 creasing concentration, an aqueous solution of a substance decreases 

 the surface tension, it will be found that the solute tends to aggregate 

 in the surface layers of the solution (Milner, 1907, p. 96). Saponin, 

 proteins and various other substances possess this property to a marked 

 degree. Thus, for example, Lewis (1908, p. .513) has calculated that 

 with a 0-25 % solution of sodium glycocholate, there is an extremely 

 thin surface layer, the degree of concentration of which is about IGO 

 times that of the bulk. The result of this surface concentration is the 



