Electrification at Liquid-Gas Surfaces. 377 



corresponding in this respect to its effectiveness in reducing 

 the surface tension of water. This reduction of surface tension 

 is caused by a positive adsorption of alcohol into a surface 

 layer of small thickness. In this layer there exists — on the 

 electric double layer theory — a double layer of ions, the 

 outer surface negative, the inner positive. The number of 

 ions, i. e.y the charge, will be influenced by the relative 

 proportions of alcohol and water in the layer. Consider a 

 bubble of air in a mixture of alcohol and water. It is 

 enclosed in a layer of small thickness which contains 

 relatively more alcohol than the liquid around it. Suppose 

 this surface to diminish in area by the gradual absorption of 

 the air into the liquid. If, as the diameter of the sphere gets 

 smaller and the curvature of the surface greater, the pro- 

 portion of water in the surface layer increases, one would 

 expect the electric charge to approach more nearly to the 

 value it has in pure water — i. e., one would expect a bubble 

 to move faster under a given potential difference as it 

 becomes smaller. This is exactly what happens in the case 

 of: the alcohols and other substances which exhibit a positive 

 adsorption. It seems probable that this equalization of the 

 concentration in the surface layer with that in the solution 

 goes on as the bubble becomes smaller, until at the moment 

 of disappearance the surface layer has the same concentration 

 as the rest of the liquid. In other words, this effect indicates 

 a dependence of the degree of adsorption upon the curvature 

 of the surface — an effect analogous to the change of vapour 

 pressure in pure liquids with the curvature of the surface. 



An instance of an effect the converse of that shown by 

 these organic substances was given in the last paper. In a 

 solution of thorium nitrate a bubble of air decreased in 

 velocity as it became smaller, and even reversed its direction 

 of motion before disappearing. This behaviour is explained 

 as before if the salt is negatively adsorbed — i. e., if the 

 surface layer at first contains less salt than the liquid. 



It is rather unexpected to find that alcohol appears to show 

 no electrical surface charge in these experiments, while in 

 waterfall experiments or in bubbling experiments it does 

 give a small electrification. Prof. Thomson (Joe. cit.) in 

 waterfall experiments found alcohol to give an effect, but 

 much smaller than water. Bloch also (Ann. de Chimie, 

 pp.22 & 23, 1911) found it to be "active" in bubbling 

 experiments. The difference in behaviour is probably due 

 to the difference in the conditions of experiment. In cata- 

 phoresis experiments there is no violent mechanical rupture 

 of the surface as in the other types of experiment. In 



