THE TRANSFORMATION OF ELEMENTS 237 



to be reached. Uranium and radium give a good example of 

 such a fixed ratio. 



Again, the ultimate disintegration product — of course a non- 

 radioactive element — will accumulate in ever-increasing amount 

 as the parent substance, and the mineral- containing it, grow 

 older. Consequently the amount of this product in any mineral 

 must depend on the age of the mineral. Lead is supposed 

 to be the ultimate disintegration product of the uranium-radium 

 series. 



The evidence put forward to support these statements will 

 now be considered. 



The method employed to estimate the amount of radium in 

 a mineral has been gradually evolved from those used by Bolt- 

 wood and Strutt. A known weight of the mineral is decomposed 

 with acids, and the solution boiled to expel accumulated emana- 

 tion completely. It is then kept in a closed vessel, and the 

 emanation allowed to accumulate for a known period. The 

 solution is again boiled, the emanation removed, and measured 

 quantitatively. From the amount of emanation formed in a 

 given time the amount of radium can be calculated. If thorium 

 or actinium be present in the mineral, then its emanation will 

 also be present. By observing the initial rate of decay of the 

 mixed emanations, the relative amounts of the thorium and 

 radium emanations can be determined. Actinium emanation 

 decays so rapidly that it can scarcely be detected in such a 

 mixture unless present in relatively large amount. The amounts 

 of thorium and uranium present in minerals are determined by 

 the usual methods of chemical analysis, though Strutt has found 

 it convenient and accurate to estimate the thorium from the 

 amount of its emanation. 



The following figures are taken from a much larger table 

 published by Boltwood in 1905 ; the emanation equivalent is 

 given in arbitrary units : 



