'2'-'>4 ]V(ttiii(i l'<nr(r iiiid Ifs !>< hriuiiKitlon 



tendency being to increase rather than decrease; the interfacial tension, 

 on the other hand, decreases progressively. This diflference is evidently 

 to be explained by surface concentration phenomena, which are not 

 nearly so marked at the interface of the oil and soap solution as at 

 that of the soap solution and air. We have carried out exactly similar 

 deterniiiiations on solutions of castor oil, sulphonated castor oil, linseed 

 oil, resin and commercial soft soaps, and in every case we have found 

 that, whilst the surface tension, at concentrations higher than 0-1 % 

 of fatty acid, has shown but little variation with the concentration, 

 tlie interfacial tension has decreased progressively up to about 2 % 

 of fatty acid, when the interfacial tension usually approaches zero. 



It is for this reason that in the actual practical method of com- 

 parison of the wetting power of dips and sprays, we do not recommend 

 the determination of the three tensions 7',. T., and 1\.,. 



It is obvious from the results given in Table II and illustrated 

 graphically in Fig. 2, that of these three tensions, it is the interfacial 

 tension {'1\.^ which exerts the predominating influence, so far as wetting 

 is concerned. It is for this reason, coupled with the fact that the inter- 

 facial tension is the least affected of any by surface concentration 

 effects, that we consider that the interfacial tension forms a better 

 criterion of the wetting power than the expression T,— (Tj H- Tj,), 

 which, since it contains Tj, is itself vitiated by such concentration 

 effects. 



The method which we have employed for the determination of 

 surface tension of the soap solutions (see Tabic II) is Serle's Torsion 

 Balance Method (1913) which proved very convenient for the purpose. 

 It is rapid and fairly accurate, and, where a large number of com- 

 parative determinations have to be made, can be confidently recom- 

 mended. As, however, a determination of the surface tension is not 

 necessary in the method of comparison of wetting power which we 

 advocate, it is beyond the scope of this paper, and the reader is referred 

 to the briginal communication. 



Our method of estimating wetting power merely resolves itself 

 into a determination of the interfacial tension of a standard thick 

 paraffin oil with the liquid under investigation. There are several 

 methods by which this interfacial tension may be measured, but the 

 one most usually adopted now is Donnan's (1809) drop-pipette method; 

 and, after some proliniinary trials with most of the other methods, it 

 quickly became evident that Donnan's method was the only one suitable 

 for our purpose. 



