RADIOACTIVITY—ARTIFICIAL AND NATURAL 



313 



in Fig. 13 (which is really a pair of figures, as the caption says) and 

 Fig. 14. 



208 212 



216 



82 

 83 



Fig. 13 — Part of the thorium series of radioactive nuclei. To obtain the actinium 

 series, imagine each star and rosette transposed one unit to the left. 



210 



218 



\ 



1 

 84 >F '^C' ■-)fA 



Fig. 14 — Part of the radium series of radioactive nuclei. 



Taking Fig. 13 as it stands, we see five of the stars and one rosette 

 of Fig. 4 connected by arrows; each star marks a nucleus which 

 transforms itself by emission of a particle into the one next following 

 along the arrow-chain. The rosette stands for a stable nucleus, and 

 would be replaced by a circle were it not distinguished by being the 

 terminus of such a chain. These five radioactive isotopes and one 

 terminal nucleus-type belong to the "thorium series," and are known 

 as thorium A, thorium B, thorium C and so on, according to the 

 letters which adjoin their stars. This is an unlucky bit of terminology, 

 for it suggests that all are isotopes of the same element, which is 

 clearly far from the truth. 



Taking Fig. 13 again and imagining each star moved by one unit 

 to the left (so that e.g. the star A goes to 217), we now are thinking 

 of five more stars and another rosette of Fig. 4, duly connected by 

 arrows. These constitute (a part of) the "actinium series" and are 

 known as actinium A, actinium B and so forth. 



Taking Fig. 14 as it stands, we find ourselves confronted by eight 

 more of the stars and the last rosette of Fig. 4, connected by arrows. 

 These constitute a part of the "radium series" and are known as 



k 



