Radioactivity Excited in Air at the Foot of Waterfalls. 419 



unsaturated solution, then along the boiling-point curve back 

 to the point T //y , &c. It is well worth while looking into, 

 whether it is not possible, by means of such cycles, to dis- 

 close some new thermodynamic connexions between the 

 different phenomena included in the above diagrams. 



Davy-Faraday Laboratory of the Royal Institution, 

 June 1902. 



XLT. Induced Radioactivity Excited in Air at the Foot of 

 Waterfalls. By J. C. McLennan, Associate Professor of 

 Physics. University of Toronto *. 



1 . — Introduction. 



ri^HE fine drops of spray into which water is broken on 

 X- passing over waterfalls was found by Lenard f to 

 communicate, on striking the wet rocks at the foot of the 

 fall, a negative charge of electricity to the surrounding air 

 and a positive charge to the water. 



From a number of laboratory experiments with jets im- 

 pinging on metal plates he obtained the same results with 

 pure water, but found that the presence of any impurity 

 greatly lessened the effect. With certain solutions — notably 

 sodium chloride in water — the action weakened with an in- 

 crease in the strength of the solution, and was finally reversed r 

 a positive charge being communicated to the air and a nega- 

 tive charge to the liquid. 



The splashing of rain he found imparted a negative electri- 

 fication to the surrounding air, while the breaking of waves 

 on the sea-shore electrified it positively. 



The experiments of J. J. Thomson} and of Elster and 

 Geitel § have confirmed these observations, and, while showing 

 that the sign and the amount of the electrification im- 

 parted varied both with the liquid and with the gas in which 

 the splashing occurred, yet the splashing of pure water in air 

 always gave a negative electrification. 



A little over a year ago Elster and Geitel || found that, if 

 a negatively electrified wire were exposed for some hours in 

 the open air or in a very large room, it became temporarily 

 radioactive. Since then a number of observations have been 

 made upon this effect, and the consensus of opinion appears 

 to be that it is due to the presence in the atmosphere of some 



* Communicated by Professor J. J. Thomson. 



f Lenard, Wied. Ann. xlyi. p. 581 (1892). 



% J. J. Thomson, 'Discharge of Electricity through Gases,' p. \~ . 



§ Elster and Geitel, Wied. Ann. xlvii. p. 496 (189i>). 



|j Elster and Geitel, Phys. Zcit. No. 40. p. 590 (1901). 



