Electrification associated ivitli Dust-Clouds. 481 



This equation can of course only be true so long as the ions 

 behave as perfect gases. The increase in osmotic pressure 

 of an ion for a change in potential of 0*1 volt is about 80 

 fold. So that if the solution were decinormal potassium, 

 chloride it would scarcely be legitimate to use the formula 

 for a greater value of V than 0*1 volt. Unfortunately, the 

 change in surface tension for an alteration in the applied 

 electromotive force of 0*1 volt when the surface tension is at 

 its maximum is so small that it cannot be accurately measured ,, 

 I estimate from S. W. J. Smith's* electrocapillary curves 

 and Quincke's value for the surface tension of mercury in 

 contact with h} T drochloric acid (assuming that the surface 

 tension of mercury in contact with hydrochloric acid is the 

 same as that of mercury in contact with a solution of potas- 

 sium chloride), that the fall in surface tension when Y is- 

 0*L volt lies between 3'5 and 4 C.G.s. units when the mercury 



is in contact with — - KC1. The fall in surface tension 



calculated from the above formula for the same change in 

 the applied electromotive force is 4*8 C.G.s. units. It is 

 therefore very probable that the observed change in surface 

 tension can, as Helmholtz postulated, be largely accounted 

 for by the electric charges in the double layer. 



It is obvious that what is most needed to elucidate the 

 Txiechanism of the capillary electrometer is a very thorough 

 examination of the electrocapillary curve in the region of 

 maximum capillarity, preferably with solutions which give no 

 electromotive force in contact with mercury, such as those 

 used by Palmaerf to investigate the electromotive force of 

 the calomel electrode. 



Jesus College, Oxford. 



LII. On the Electrification associated with Dust- Clouds. By 

 W. A. Douglas Rudge, Professor of Physics, University 

 College, Bloemfontein %. 



[Plates VI. & VII.] 



THE author has shown § that a considerable amount of 

 electrification is imparted to the atmosphere by clouds 

 of dust raised by the wind, and it seemed of interest to 

 investigate the electrification developed by raising clouds^ 



* Zeit,f PJujs. Ck.xxxil p. 467 (1900). 

 t Zeit. Elehtrochemie, ix. p. 754 (1903). 

 X Communicated by the Author. 



§_ Phil. Mag. June 1912 ;. Proc. Roy. Soc. South Africa, 1912 ; South- 

 African Journ. Sci. 1912. 



Phil. Mag. S. 6. Vol. 25. No. 148. April 1913. 2 L 



