﻿394 Electrification at Boundary between Liquid and Gas. 



action of the air-water surface is very marked, a positive 

 charge being acquired by the surface with very minute 

 concentrations of the salt. The positive ions available for 

 selection are Th + and H + , but neither of these separately 

 can be responsible for the unusual activity of the salt. The 

 mere presence of H + ions, as, for example, in the form of an 

 acid, does not produce so great an influence on the surface 

 charge. Nor can free Th + ions have much effect, for they 

 disappear in the dialysis and yet leave the pure colloidal 

 solution practically as active as before. The real agent 

 must be the particles of colloidal thorium hydroxide which 

 gather about them groups of H + ions and carry them into 

 the surface in larger numbers than would be possible for the 

 H + ions alone. 



The nature of this selective action must be connected with 

 the shape of the surface, or, to put it in another way, a 

 particle must reach a certain size before it can be regarded 

 as having a surface-layer about it with a tension and an 

 electric charge. We have at present in order of size — ions, 

 ionic micelles (Prof. McBain, " Soap Solutions," Nature, 

 March 10, 1921), ultra-microscopic colloidal particles, micro- 

 scopic and macroscopic particles including gas-bubbles. At 

 what stage a surface-layer is formed it is difficult to say, but 

 it seems reasonable to suppose that the curvature of such a 

 surface would have an effect on the charge adsorbed. The 

 change of sign with decreasing size of air-sphere shown in 

 these experiments seems to bear out this idea. 



The information obtained regarding the effect of thorium 

 nitrate on the electrification of air-water surface layers may 

 be summarized as follows : — 



1. Thorium nitrate in aqueous solution and in concentra- 

 tions as small as 8xl0~ 6 normal gives a positive electric 

 charge to the surface of a sphere of air immersed in it. (In 

 distilled water the charge is always negative.) 



2. For concentrations in the neighbourhood of 6 x 10~ b 

 normal a sphere initially negative becomes gradually positive 

 as the sphere diminishes in size. 



3. Colloidal thorium hydroxide in small concentrations of 

 the same order also gives a positive electric charge to a 

 sphere of air immersed in it. 



4. Colloidal thorium hydroxide also exhibits the reversal 

 of the sign of the charge with a decrease in the size of the 

 bubble. 



5. It is suggested that this reversal of sign is experimental 

 evidence of a relation between the curvature of the surface 

 and its adsorptive power. 



