298 Mr. H. A. McTasreart on the 



&»' 



oppositely charged, and that the shock mechanically pulls 

 these charges apart. This separation of electrifications is 

 found to be produced by almost any kind o£ mechanical 

 disturbance acting at the surface if the forces in the dis- 

 turbance act quickly enough (Becker, Jalwb. d. Had. ix. 

 p. 52, 1912). The breaking away of a drop of water from 

 a dripping tap, the bubbling of a gas through the surface of 

 water, or the blowing of water into spray, are accompanied 

 by disturbances of this kind. Even the sudden separation 

 of two wetted towels which have been pressed together, will 

 charge the towels with one sign and the air with the opposite 

 sign. 



The changes that can be produced in the electrification 

 by mechanical means are modified by the nature of the gas 

 in contact with the liquid and by dissolved salts or other 

 impurities which it may contain. In experiments by 

 Professor J. J. Thomson (Phil. Mag. xxxvii. p. 341, 1894) 

 the action of the gas upon the electrification produced was 

 shown to be considerable. For example, distilled water 

 falling through air acquired a positive charge. When it fell 

 through hydrogen it became negatively charged, while, if 

 allowed to fall through its own vapour, it acquired no 

 charge. Small quantities of salts dissolved in the water 

 produced very remarkable effects on the electrification. The 

 most active in this respect were some of the organic salts 

 which are known to be readily adsorbed into the surface 

 (Freundlich, Kapillarch. p. 248). This fact has suggested 

 that the electrical double layer maybe a result of adsorption. 



It might be expected that the nature of this double layer, 

 if it actually exists, would be influenced not only by the 

 liquid and the salts dissolved in it, but to some extent also 

 by the gas. At liquid-solid surfaces the electrification is not 

 determined by the solution alone. The solid plays an im- 

 portant part — as experiments in cataphoresis and electric 

 endosmose clearly show — but the way in which the solid (or 

 gas) affects the charge in the double layer is not apparent. 

 Nor is it clear how the ions of the dissolved salts, or those of 

 the liquid itself, enter into the formation of the layer. To 

 add to the information on these points is the object of this 

 paper, which describes some experiments on liquid-gas 

 surfaces. 



In 1861 Quincke observed (Pogg. Ann. cxiii. p. 513, 1861) 

 that in water through which an electric current was passing, 

 small bubbles of air moved toward the anode, while in 

 turpentine they moved toward the cathode. This motion 



