22(5 Wcttliifi Poircr and ifs DctcrinliKitidii 



defined ; its value is dependent upon the age of the surface, and gradually 

 passes into that of the stnlic tension. The explanation of this peculiar 

 behaviour of soap and other solutions is to be found in the property of 

 surface concentration. Since the presence of soap reduces the surface 

 tension, soap will become concentrated in the surface layer. As the 

 surface tension is a function of the surface layer only, the presence of 

 this concentrated surface ])o]lic]o of soap solution will be evidenced in 

 a low surface tension. The peculiar behaviour of soap referred to 

 above, would therefore find a ready explanation in tiio production of 

 a surface pellicle of soap, tlie concentration of which is but little affected 

 by that of the bulk of the solution. The formation of this surface 

 pellicle is not instantaneous, but requires a certain amount of time, 

 and it is upon this fact, that the existence of static and dynamic tensions, 

 with the gradual passage of the one into the other, is due. 



We are now in a better position to consider the various methods, 

 which have been suggested from time to time for the measurement of 

 wetting power, and to see how far these are suited to the purpose. 



Firstly, can the surface tension (static) of a lic^uid, containing soap, 

 be accepted as a rehable guide to its wetting power? We think not, 

 for two good reasons. As we have seen, the conditions for a li(|uid to 

 wet a solid surface are expressed in the equation : 



T^>T^+ Tio. 

 Since wetting is thus dependent upon three distinct tensions, it is 

 obvi(msly unwise to draw any conclusion as to its value, based on the 

 surface tension of the wetting Hquid Tj only, even if the other two 

 tensions are indeterminate. Vermorcl and Dantony (191"2, p. 1300 

 et seq.) have proved experimentally that solutions of difl'ercnt surface 

 tensions may have the same wetting power. Thus, a solution of sodium 

 oleate at 2-5 parts to 10,000 parts does not wet 'feuiiles dc Gamay,' but 

 wets sulphur very well. The wetting power thus manifests itself very 

 differently according to the nature or state of the substance to be 

 wetted. The frequently recurring statement, that liquids with a low 

 surface tension wet more readily than those M'ith a higher one is not 

 strictly true, because it may happen that a liquid with a high surface 

 tension and low interfacial tension will wet better than one with a low 

 surface tension and a high interfacial tension. In other words, the 

 interfacial tension has a greater determining value than the surface 

 tension, and for this reason, if it is desirable to determine one tension 

 only, the interfacial affords a much more reliable guide to the wetting 

 power than does the surface tension. 



