296 Mr. C. S. Wright on Variations in the 



© 



type of radiation in the metal of the receivers; (3) radio- 

 active impurities present in the metals ; and (4) a possible 

 intrinsic radiation from the metals themselves. 



In view of the theoretical importance of ascertaining 

 whether metals generally possessed any specific activity, it 

 was thought highly desirable to endeavour to obtain metals 

 as free as possible from active impurities, and also to take 

 observations on the conductivity of air enclosed in vessels 

 made from them, in localities and under conditions, in which 

 the penetrating radiation from the earth, and the correspond- 

 ing radiation excited by it in the receiver, was very largely 

 cut off or at least reduced to a minimum. 



Several attempts have been made daring the last few years 

 to find some efficient screen for the earth's radiation. Cooke*, 

 while making some measurements on the conductivity of the 

 air enclosed in a brass cylinder, found a decrease of about 

 30 per cent, when the cylinder was completely surrounded 

 by large masses of lead. Shortly afterwards Elster and 

 Geitelf observed a fall of 28 per cent, in the conductivity 

 of air enclosed in an aluminium receiver, when the apparatus 

 was set up in a rock-salt mine. Later still Cooke J was able 

 to reduce the ionization 12 per Cent, by immersing his 

 cylinder in a reservoir of water to a depth of several feet. 

 The greatest decrease, however, was that observed by 

 McLennan and Burton §, who cut off 37 per cent, of the 

 conductivity of the air enclosed in a galvanized iron cylinder, 

 by surrounding it with a layer of water drawn from Lake 

 Ontario, 60 cms. thick. This last result, taken in conjunction 

 with an observation by Prof. McLennan that Lake Ontario 

 water contains no appreciable radioactive impurities, gave 

 rise to the idea that possibly a large body of water such 

 as the lake itself might furnish an efficient screen for the 

 earth's radiation. 



The object of the present investigation, therefore, was to 

 observe the conductivity of the air confined in metallic vessels 

 possessing little if any radioactive impurity, and from the 

 results to determine, if possible, what portion of the ioniza- 

 tion was due to an intrinsic activity in the metal. 



In carrying out the investigation, the conductivity of air 

 confined in vessels of lead, zinc, and aluminium was measured 



* H. L. Cooke, Phil. Mag. [6] vi. p. 403 (1903). 

 t Elster and Geitel, Phys. Zeit. Nov. (1) 1905, p. 733. 

 X H. L. Cooke, Phil. Mag. 1903, I c. 



§ McLennan and Burton, Phys. Rev. 3. 1903 : Burton, Phys. Rev. 3. 

 1904. 



