AV. F. Cooper and W. H. Nuttall 229 



the imiiiersioii of the sohd ; the difference iu the two weights represented 

 the amount of Hquid removed on the surface of the solid. Two experi- 

 ments with distilled water gave 0-0282 gm. and 0-0356 gm. respectively, 

 as the weight of adhering liquid. Since water does not wet a collodion 

 surface, theoretically no water should be removed by the solid. In 

 actual practice, a variable quantity of water is removed in the form of 

 adherent droplets, and this fact alone would seriously limit the possi- 

 bilities of the test. With the 1 % saponin solution, which possesses 

 the property of surface concentration to a marked degree, the weight 

 of adhering liquid was 0-07 04 gm. With the green oil emulsion, a liquid 

 which had a low surface tension and interfacial tension, and 'ran' well, 

 the corresponding weight was 0-0520 gm. Considering the very great 

 difference in wetting power of the three liquids used, the difference 

 in weight of the adhering lic^uid is very small, exactly as theoretical 

 considerations had led us to surmise; such differences in weight are 

 altogether too small to serve as criteria of wetting power. 



Lastly, there is the question ol foaming power. It has been rightly 

 claimed that a solution containing a small quantity of oil in a finely 

 emulsified state, possesses a high wetting power, and it will be seen later 

 why this is so. Does the power of forming a permanent foam afford 

 any indication of the fineness of the emulsion and therefore of the 

 wetting power of the liquid? If, for a moment, we consider those 

 properties upon which foaming and emulsification depend, we shall 

 see that they are but indirectly connected. 



Donnan (1899, pji. 43-9) has shown that a soap solution acts as an 

 emulsifier by virtue of its low interfacial tension with respect to the 

 oil. The result of this low interfacial tension is twofold : firstly, the 

 soap becomes concentrated at the surface of the globules of oil, and 

 owing to the surface viscosity produced thereby, the film of licfuid 

 intervening between two oil globules resists thinning and the con- 

 sec[uent coalescence of the globules. Secondly, the force tending to 

 break the intervening film is that of the interfacial tension, and this 

 being low, is not strong enough to withdraw the aqueous film separating 

 the globules. Hillyer (1903, pp. 516-21), working independently, has 

 arrived at similar conclusions as to the cause of the permanence of 

 emulsions. Stable emulsions of oil in soap solution are therefore 

 dependent upon a low interfacial tension between the oil and the aqueous 

 liquid, and the surface tension of the latter plays no part in the process. 



The property of foaming and the factors which lead to the formation 

 of a permanent foam, have formed the subject of a considerable amount 



