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LII. Spontaneous Ionization of Air In ( 'losed Vessels and 

 its Causes. By Alexander Wood, B.Sc. Glasgoiv, B.A. 

 Cantab., late 1851 Exhibition Scholar of the University of 

 Glasgow; Emmanuel College, Cambridge* . 



Preliminary Discussion. 



TI^HE conductivity o£ air and other gases is now generally 

 JL attributed to the presence o£ free ions, and as the 

 free ions existing at any time are continually recombining, 

 it follows that some agency must be at work splitting up the 

 molecules and so maintaining the supply constant. Ioniza- 

 tion of* this kind, in which no artificial ionizing agent is 

 employed, was at first thought to be an intrinsic property of 

 the gas, and was known as " spontaneous " ionization. Later, 

 however, the hypothesis that it was due to the action of a 

 penetrating radiation constantly passing through the atmo- 

 sphere in all directions was advanced, and has since received 

 so much experimental support that it has come to be 

 universally adopted. McLennan f surrounded the vessel in 

 which he was measuring the ionization with water to a depth 

 of about a foot, and found that this reduced the leak by about 

 85 per cent. At the same time Rutherford and Cooke J 

 surrounded their vessel with a screen of leod and observed a 

 similar reduction of the leak. This pointed strongly to the 

 existence of the hypothetical penetrating radiation. On the 

 other hand, C. T. R. Wilson's " tunnel experiment w § rather 

 tended to throw doubt on this conclusion. It was at this 

 point, and before the publication of the more recent research 

 by Cooke ||, that the present series of experiments was under- 

 taken at the suggestion of Professor J. J. Thomson, whose 

 kindly interest and helpful advice it is a real pleasure to be 

 in a position to acknowledge. The earlier experiments were 

 made with a view to confirming the results of McLennan 

 and Rutherford & Cooke. The conclusion reached in the 

 course of the investigation was that the natural ionization of 

 the gas in a vessel had at least a threefold origin — part of it 

 being due to a penetrating radiation from without, part to a 

 secondary radiation from the walls of the vessel excited by 

 the former, and part to an intrinsic radiation from the walls 

 completely independent of the external radiation. The 



* Communicated by Prof. J. J. Thomson. 



t Phys. Rev. xvi. p. 184 (1903), and Phil. Mag. v. p. 419 (1903). 



X Amer. Phys. Soc. Dec 1902. 



§ Proc. Roy. Soc. lxviii. p. 151. 



li Phil. Mag-. Oct. 1903. 



