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IV. The Relative Activity of Radium and the Uranium 

 with which it is in Radioactive Equilibrium*. By 

 J. H. L. Johnstone and B. B. Bolt wood f. 



ALTHOUGH the matter has been under consideration 

 and discussion for a number of years, the genetic 

 relationship between the earlier members of the uranium 

 family of radio-elements is still a matter of considerable 

 uncertainty. It is generally conceded that both ionium and 

 actinium are products of the radioactive disintegration of 

 uranium, but the exact point of origin of actinium and its 

 immediate parentage have remained somewhat obscure and 

 uncertain. 



The work of BoltwoodJ on the relative a-ray activity of 

 uranium minerals and the uranium which they contained 

 demonstrated a constancy of relationship between the radio- 

 active constituents of the older minerals and clearly indicated 

 a close genetic relation between uranium and actinium. His 

 determination of the activities of the more stable a-ray 

 products relative to the activity of the associated uranium 

 showed a simple and direct relation to exist between the 

 products ionium, radium, and polonium, but showed an 

 abnormally low value for the ratio in the case of the actinium 

 series, which could only be explained on the assumption that 

 actinium originates as a branch product and belongs to what 

 may be termed a collateral branch of the ionium-radium- 

 polonium family. 



The values obtained by Boltwood in the course of his 

 experiments showed that the activity of the uranium was 

 about 2*2 times that of the radium with which it was in 

 equilibrium, although at that time the range of the a-particle 

 from uranium was supposed to be about 2'7 cm., which is less 

 than that of the a-particle from radium. Since the ionizing 

 power of an a-particle is nearly proportional to the range, 

 and since, on the basis of the disintegration theory, an equal 

 number of a-particles are emitted per second by each of two 



* The experimental results given in this paper and the general theo- 

 retical conclusions are taken from a dissertation on the " Relative 

 Activity of Uranium and Radium" presented on April 27, 1916, by 

 J. H. L. Johnstone in fulfilment of the requirements for the degree of 

 Doctor of Philosophy in Yale University. The chief reason for the delay 

 in publication was the entry of Dr. Johnstone into active military service 

 with the Canadian forces in May 1916. The work was carried out in 

 the Sloane Physical Laboratory of Yale University. 



t Communicated by the Authors. 



J Am. Journ. Sci. xxv. p. 269 (1908). 



