the Natural Ionization in a Closed Vessel. 



217 



vessel. From some experiments with an ionization vessel 

 with a movable side, Campbell * calculated the ranges of the 

 particles given off by the walls, and has found that they are 

 in general greater than those of the a particles from known 

 radioactive elements ; but McLennan f has lately measured 

 the natural ionization in vessels of different material, and 

 attributes it to external radiation differences coming in on 

 account of the various metals not giving out equal amounts 

 of secondary radiation. 



Apparatus. 



It was decided to measure directly the ionization in an 



electroscope containing gas at the desired pressure. The 



pressure-chamber, which was kindly lent by Dr. Hutton and 



Prof. Petavel, has been described by them in Phil. Trans. 



Fisr. 1. 



A. 207. p. 421. It consists briefly of a cylindrical iron vessel, 

 a section of which is shown diagrammatically in fig. 1. The 



* Campbell, Phil. Mag. Sept. 1906. 

 t McLennan, Phil. Mag. Dec. 1907. 



