CH. XVIII.] 



RADIO-ACTIVITY 



179 



leakage ; the remaining seventy per cent, seemed due 

 to internal radio-activity. Materials such as brick 

 seem specially radio-active, and any metals which 

 have been exposed to the outside atmosphere are 

 more radio-active than virgin metal. 



But, in addition to this induced or excited activity, 

 Strutt and others tried a good many materials and 

 found different characteristic effects with each. 



D. 



E. 



FIG. 24. Working part of the above electroscope, magnified four times. 

 The rod AB with the gold leaf C is cemented to a thin rod of melted 

 quartz D by means of a drop of mastic or guttapercha at the bottom of a 

 minute metallic capsule E attached to the rod AB; thereby lessening 

 any tendency of the charge to creep, and keeping the capacity exceedingly 

 small. 



The question of whether radiation is emitted 

 spontaneously by metals of every kind, and not only 

 by those few substances which are conspicuously 

 radio-active, has been still further examined by Mr. 

 Norman E. Campbell of Trinity College, Cambridge 

 (Phil. Mag., Feb. 7, 1906). He adduces very strong 

 experimental proof that such radiating power actually 

 exists, since they all ionize the air in their neigh- 

 bourhood. Moreover, the experiments indicate a 





