300 Prof. E. Rutherford on Slow 



be half transformed in 40 years. The activity 180 years 

 after the active deposit is formed will be about the same as 

 that observed after an interval of 9 days. 



Origin of Polonium and Radio- Tell avium. 



Contemporaneously with the experiments on the rate of 

 decay of radium F deposited on a bismuth plate, observations 

 were made on the decay of activity of a specimen of the radio- 

 tellurium of Marckwald, which had been obtained from 

 Dr. Sthamer of Hamburg. The activity was found to decay 

 exponentially with the time, falling to half value in 143 days. 

 During these experiments, results of a similar character upon 

 the decay of activity of radio-tellurium were published by 

 Meyer and Schweidler*, and by Marckwald f. The former 

 experimenters found that the activity fell to half value in 

 135 days, while Marckwald obtained a corresponding value 

 of 139 days. Considering the difficulty of making accurate 

 experiments of the rate of decay over long periods of time, 

 the numbers obtained by Meyer and Schweidler, Marckwald, 

 and the writer are in remarkably good agreement. 



We may thus conclude that radio-tellurium loses its activity 

 according to an exponential law, falling to half value in about 

 140 days. 



In my previous paper I pointed out that there were strong- 

 reasons for believing that the active constituent in radio- 

 tellurium was identical with the product radium F. Both 

 active substances possess similar physical and chemical 

 properties. Both only give out a rays, and both are deposited 

 on bismuth from the active solution. In addition, I showed 

 that the a rays from radio-tellurium and radium F were 

 identical in their power of penetrating matter. Such a 

 result afforded very strong evidence of the identity of the 

 two products. 



We have seen that radium F loses half its activity in 143 

 days — a value agreeing closely with the rate of decay observed 

 for radio-tellurium. The agreement of physical and chemical 

 properties, coupled with the identity of the radioactive con- 

 stant X, conclusively shows that the active constituent in 

 radio-tellurium is identical with radium F. We may thus 

 conclude that the active constituent present in radio-tellurium 

 is in reality a transformation product of radium, and that 

 the radio-tellurium obtained from radioactive minerals is 



* Wien. Ber. Dec. 1, 1904. 



f Ber. d. D. Chem. Ges. No. 2, p. 591 (1905). 



