460 Mr Lushy, Some Experiments on 



could be thoroughly dried, then it should acquire a strong positive 

 charge. The present paper gives an account of some experiments 

 made to try and detect such an action in air. 



(2) Experimental conditions. The problem was first attacked 

 directly. A stream of undried, unfiltered air was drawn through 

 brass tubing by means of a water pump at an approximate rate of 

 50 cubic centimetres per second. Near the end of the apparatus 

 where the air entered was a roll of filter paper, 1 decimetre long, 

 on which uranium oxide had been fixed by means of acetyl 

 collodion : this served as the ionising agent. The ionised air then 

 passed through a copper U-tube, which could be surrounded by 

 liquid air, and then passed into a Faraday cylinder which was 

 connected to a Wilson electroscope of ordinary type — a sensitive- 

 ness of 60 divisions per volt being generally employed. In order 

 to avoid diffusion effects as far as possible, the brass piping Avas 

 chosen with a fairly large diameter (4 centimetres); the U-tube 

 was necessarily smaller, being only 1'5 centimetres in diameter, 

 and the total length immersed in liquid air was about 15 centi- 

 metres. Liquid air was chosen as the drying agent in order to 

 make the action as perfect as possible ; this method has the 

 further advantage that it does not cause any mechanical filtering 

 effect such as the use of calcium chloride or sulphuric pumice 

 might produce. 



The experiment consisted in merely placing a cylinder of 

 liquid air round the U-tube and testing the electroscope for 

 charge. In no case could any charge be detected, even though 

 the electroscope's sensitiveness was pushed as high as 80 divisions 

 per volt ; special care was of course taken to render all insulations 

 perfect. 



To further test the matter, the Faraday cylinder was replaced 

 by a condenser of the type used by Zeleny and other workers with 

 blast methods, so that the positive and negative charges could be 

 measured separately and compared, by raising the outer wall of 

 the condenser to an appropriate positive or negative potential 

 whilst the inner electrode was connected to the electroscope. The 

 diameter of the outer brass tube of the condenser was 4 centi- 

 metres, that of the inner electrode 7 millimetres, and the total 

 length was 15 centimetres; a potential was applied sufficient to 

 drive on the inner electrode all ions whose mobilities exceeded 

 1 centimetre per second. In order to avoid effects due to the 

 charging up of the ebonite insulation, the air was made to enter 

 and leave the condenser through earthed tubes. 



On measuring the positive and negative charges in the dried 

 air blast, a surprising phenomenon manifested itself; the two 

 charges were exactly equal, but were two or three times the 

 corresponding quantities in the undried air. Results were variable 



