LEAD THE FINAL PRODUCT 105 



The figures in the upper line are the atomic 

 numbers, the figures after each element the atomic 

 weights, both in terms of that of hydrogen as unity. 

 Radium, when discovered, fell naturally into the 

 vacant place No. 88, and polonium and actinium 

 are now known to occupy Nos. 84 and 89. The 

 three radioactive emanations of Rutherford, products 

 of radium, actinium and thorium respectively, are 

 chemically analogous to Ramsay's inert family of 

 atmospheric gases, and occupy the place No. 86. 

 No. 91 is known to be occupied by a product of 

 uranium, having a period of average life of only if 

 minutes, called Brevium. The numbers 85 and 87 

 in the above figure now alone remain vacant. 



Thus radioactivity has peopled all but two of 

 these vacant places, but it has done more. It has 

 crowded into ten of the above places, between Nos. 

 8 1 and 92, no less than 39 distinct elements, and all of 

 the elements occupying any one place isotopes as 

 they are called are invariably identical', their whole 

 chemical character. Ionium is isotopic with thorium, 

 mesothorium I. with radium, and so on. To the 

 chemist and the spectroscopist they would be taken 

 as one. Not so, however, to the newer methods 

 of radioactivity. 



When the whole sequences of changes of uranium 

 and thorium are set forth in the Periodic Table 

 according to the a- and /3-change rules mentioned, 

 it is found that all the final products occupy the 

 place, No. 82, occupied by lead. The atomic 

 weight of the end product of uranium should be 

 206 and that for thorium 208, whereas the atomic 

 weight of common lead is 207-2. This suggests 

 that common lead is a mixture of isotopes rather 

 than a single homogeneous element. The view 

 rapidly received complete vindication. For the 

 atomic weight of lead derived from minerals rich 



