RADIO-ACTIVITY 177 



Soon after appreciable quantities of radium 

 were available for investigation, Giesel drew atten- 

 tion to the fact that a radium compound gradually 

 increases in activity after formation, and only 

 reaches a constant state after a month's interval. 

 Similar phenomena were observed by Curie and 

 Dewar for the heat effect. These results are 

 readily explained if we consider the properties of 

 the emanation as elucidated by the experimental 

 evidence that has now accumulated. 



When a salt of radium is dissolved in water, 

 and the solution boiled, the emanation previously 

 stored in the salt is evolved and removed. The 

 residual activity of the salt is then found to be 

 much diminished. This activity must include 

 that due to the radium itself, and also that of 

 the active deposit, which has been developed by 

 the emanation, but is not removed with it. The 

 effect of the active deposit decays rapidly ; after 

 a few hours it will nearly have vanished, and we 

 then get the true activity of the pure radium salt 

 alone, uncomplicated by that of the emanation, 

 or by that of the active deposit which is produced 

 by the emanation. 



This residual, non-separable activity is found 

 to consist entirely of a rays, and, measured 

 electrically, is about 25 per cent, of the normal 

 activity of a radium compound after a month's 

 existence ; a normal activity which comprises 

 the combined effects of radium, of the radium 

 emanation, and of the active deposit. 



Rutherford and Soddy studied these relations 

 in detail. They dissolved a radium compound, 

 removed the emanation, and waited till the activity 

 of the deposit had subsided. The solution was 



N 



