Electrification at Liquid Gas-Surfaces. 367 



angle o£ scattering. The penetrating power of this scattered 

 radiation may become modified in its final passage through 

 matter, but the penetrating power depends essentially on 

 the angle of scattering, and not on the material of the 

 radiator. 



I wish to thank Prof. Sir Ernest Kutherford for the large 

 quantities of emanation which have made it possible to carry 

 out experiments using thin radiators. 



XLV. Electrification at Liquid Gas-Surfaces. By H. A. 

 McTaggart, M.A., Toronto ; Research Student of Gonville 

 and Cams College, Cambridge *. 



IN" a previous paper (Phil. Mag. Feb. 1914, p. 297), the 

 velocity of small bubbles of air, hydrogen, and oxygen 

 in distilled water subject to a fall of potential was examined 

 and found to be about 4. 10~ 4 cm./sec./volt./cm. This corre- 

 sponds — on the assumption of an electric double layer — to a 

 densitv of electrification over the surface of the bubble of 

 4.10 -5 coulomb approximately. In the same paper it was 

 shown that the surface charge may be varied by the presence 

 in solution of minute quantities of some inorganic salts and 

 acids. The salts which are most effective in altering the 

 surface charge are these containing ions of large valency. 

 In some way not clearly understood, these ions enter the 

 surface layer and make their presence evident in the charge 

 they impart to it. There is a kind of selective adsorption in 

 liquid-gas surfaces — an adsorption which also plays a part in 

 the cataphoresis of solid and liquid particles. 



The degree of adsorption of a dissolved substance is 

 connected — qualitatively — with the change in the surface 

 tension by the well-known formula : — 



c da 

 ' l ~~lifdc~' 

 where n = excess of salt in unit area of surface, 

 a = surface tension, 

 c = concentration of salt in the liquid. 



This equation expresses the fact that substances which 

 lower the surface tension are present in the surface layer in 

 excess, while substances which raise the surface tension are 

 present in smaller concentration in the surface than in the 

 .body of the liquid. To the first class belong a number of 



* Communicated by Sir J. J. Thomson, O.M., F.R.S. 



