18 Prof. Rutherford and Miss Brooks : Comparison of 



greater than that o£ the 

 thorium and radium, which 



of the excited radiations is much 



ordinary nondeviable rays of 



give rise to the emanations. The radiations are apparently 



fairly homogeneous in character. 



The penetrating power of the radium-excited radiation is 

 independent of the substance in which it is produced. 



In 



Decay of Excited Radioactivity. 

 a previous paper it has been shown that the excited 



radiation due to thorium compounds decays regularly with 

 the time, falling to half its value in about eleven hours. The 

 decay of excited radioactivity due to radium is irregular and 

 depends upon the sample of radium employed. 



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Fig. 7 shows the decay curves for the two samples of 

 radium labelled "einfach " and "concentrated " respectively. 

 The decay curve for the former is more regular than that for 

 the latter. Fig. 8 is for radium (c) when the plate had been 

 excited by exposure for a short interval to its emanation. In 

 obtaining the curve of fig. 7 the plates were exposed for 

 several hours in the presence of the emanations. 



Curie * has given a curve of decay for excited radi©activity 



* Paris Report, 1900. 



