RADIOACTIVITY—ARTIFICIAL AND NATURAL 



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nuclei — ^neutrons and protons, presumably, and whatever others there 

 may be — cannot unite into stable systems.^ All of these nuclei 

 beyond 209 which are found in Nature are seeking for stability by the 

 emission of particles, but never finding it until they have emitted 



80 81 62 83 84 85 86 87 88 89 90 91 92 



ATOMIC NUMBER 



Fig. 15 — The three series of radioactive substances. 



sufficiently many to convert themselves (or rather, the residues of 

 themselves) into one or another of the three isotopes of element 82 — ■ 

 lead — which are marked by rosettes in Fig. 4. For an obvious reason 

 these three are called thorium lead, actinium lead and radium lead. 

 "Ordinary" lead as it comes from most mines is a mixture of many 

 isotopes, but the lead which is found in close association with thorium 

 or with uranium proves its origin from vanished atoms of these metals 



' I recall from an earlier footnote that some of these very heavy nuclei (notably 

 thorium 232 and uranium 238) are so very long-lasting that "unstable," while strictly 

 correct, seems much too strong a word for them. 



