liJjO Wetting Puiver and its DctiniiiiKilioii 



of investigation by various workers, amongst whom may be mentioned : 

 Plateau (187:5, pp. 1-11!)), Quincke (1888, pp. 580 et seq.), Kayleigh 

 (1890 a, pp. 85 et seq.), Donnan (1899, p. 49), Shorter (1912, pp. 629-32; 

 1914, pp. 718-20). The generally accepted view of the nature of a 

 foam is that it is an emulsion of air in a ii(juid. Support is lent to this 

 view by tlie fact that the re((uisite conditions for the production of a 

 jiernMnenl foam are exactly analogous to those necessary for the forma- 

 tion of a permanent emulsion of oil in an aqueous liquid. These we have 

 seen to consist in (1) a Iiigh surface viscosity of the intervening film, 

 and (2) an interfacial tension so low as to be incapable of breaking 

 this film. In the case of an oil emulsion, the intervening matrix prevents 

 the coalescence of the oil globules, and similarly, in the case of a foam, 

 it is the intervening matrix which prevents the coalescence of the 

 minute bubbles of air. With emulsions of oil, therefore, it is the inter- 

 facial tension of the aqueous liquid to the oil, which is the determining 

 factor; with foams on the other hand, it is the interfacial tension of 

 the aqueous liquid with air, i.e. the surface tension of the aqueous 

 liquid, which is of importance. The property of giving a lasting foam 

 therefore indicates that the hquid possesses (1) the property of surface 

 concentration, such as saponin solution, solutions of which show quite 

 exceptional frothing properties ; and (2) a low surface tension, such as 

 soap solutions. Pure liquids of low surface tension, as benzene, ether, 

 do not give permanent foams. This indicates that surface concen- 

 tration plays an essential part in the production of foams. It is now 

 evident that, since a fine oil emulsion is dependent upon a low interfacial 

 tension between the oil and uijueous liquid, and since foaming is depen- 

 dent upon a low surface tension of the liquid, the power to give a lasting 

 foam gives no indication of the emulsifying properties of the liquid 

 towards oil, and therefore of the state of the oil emulsion contained 

 in the liquid. In other words, foaming power is in no way indicative 

 of a high wetting power. As a matter of fact, one emidsion wMch we 

 examined, was found to have an extremely low interfacial tension, 

 but the foaming power of the liijuid was practically nil. 



From the criticism which has been made of the methods hitherto 

 suggested for the comparison of tlic wetting powei- (if varicnis solutions, 

 it will be evident that the determination of wetting ])ower is fraught 

 with no small ditliculty. And so far as liquids, which contain saponin, 

 gelatine and other substances exhibiting surface concentration to a 

 marked degree, are concerned, we do not know of any method by 

 which these difficulties may be overcome. In the case of fluids made 



