Radioactivity of Lead and other Metals. 713 



may have been due to traces of some foreign active 

 substance. 



If one could surround the cylinder with some substanee 

 ■which would act as a screen, and so cut off entirely the 

 earth's penetrating rays, and consequently also the induced 

 secondary radiation, any ionization within the cylinder would 

 then be due to active impurities present in the metals. The 

 difficulty, however, is in finding a suitable screen. In some 

 experiments made in this direction by the writer* in colla- 

 boration with E. F. Burton some years ago, screens of water 

 were used, and with them it was found possible to make a 

 reduction as high as 37 per cent, in the ionization within a 

 closed cylinder. About the same time H. L. Cooke f, in 

 .studying the conductivity of air enclosed in a brass vessel, 

 w r as able to reduce the ionization 30 per cent, by sur- 

 rounding the brass with a screen of lead. Later still Elster 

 and Geitel t observed a fall of 28 per cent, in the con- 

 ductivity of the air enclosed in an aluminium cylinder on 

 removing the apparatus from the surface of the earth to a 

 closed space in a mine surrounded by a wall of rock salt. 

 But in none of these experiments is there clear evidence that 

 the penetrating radiation was entirely cut off. On the other 

 hand, in several of the experiments which have been made 

 with this object in view, it has been found that active im- 

 purities were present in the substances used as screens, and 

 the screens themselves were observed to contribute a pene- 

 trating radiation which masked any falling off in the intensity 

 of the external radiation arising from absorption. 



Although many of the surface waters of the earth which 

 have been examined, among other substances, by different 

 experimenters, have been shown to contain minute traces of 

 radium, it is possible that such waters as those of the great 

 lakes of Canada might be fairly free from such an impurity, 

 and if so might serve to screen off radiations from an ioniza- 

 tion chamber immersed in them. Some experiments made a 

 few years ago by the writer failed to show the existence of 

 any measurable amount of the emanation from radium in the 

 water of Lake Ontario; and from this result it would appear 

 that the w T ater of this lake would seem to afford the substance 

 requisite to carry out an experiment such as that just indi- 

 cated. The experimental difficulties, however, are con- 

 siderable, and it is doubtful if they could be overcome in a 



* McLennan and Burton, Phys. Rev. no. 3 (1903); Burton, Plivs. Rev. 

 no. 3 (1904). 



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

 % Elster and Geitel, Phys. Zeit. Nov. 1, 1905, p. 733. 



Phil Mag. S. 6. Vol. 14. No. 84. Dec. 1907. 3 F 



