CONTEMPORARY ADVANCES IN PHYSICS 75 



Tn 86Th Thoron, thorium emana- 

 tion 54.55 a. 



ThA 84ThI Thorium A 0.1455 a 



ThB 82ThI Thorium B 10.6^ 



ThC 83ThI Thorium C 60.6w /3 to ThC, a to ThC" 



The 84ThII Thorium C IQ-n^ {est.) a 



ThC" 81 Th Thorium C" SWm 



ThD 82ThII Thorium D, thorium lead apparently stable 



Actinium Series: 



Pa 91Ac protactinium 1.6-10% a 



Ac 89Ac actinium 20a 8 



RdAc 90Ac radioactinium 18.9i a 



AcX 88Ac actinium X n.ld a 



An 86Ac actinon, actinium emana- 

 tion 3.925 a 



AcA 84AcI actinium A 2.005 a 



AcB 82AcI actinium B 36. Iw /3 



AcC 83Ac actinium C 2.16m a to AcC", /3 to AcC 



AcC 84AcII actinium C 00^5 a 



AcC" 81Ac actinium C" 4.71m 



AcD 82AcII actinium D, actinium lead apparently stable 



UY, K, Rb not assigned to series. They emit beta-rays, and their half-periods are 

 given respectively as 24.6/j (St. Meyer, I. c. footnote 1), 1.5- lO+^^a and 10"o (Holmes 

 and Lawson, /. c. footnote 13). 



To measure a disintegration-constant seems an easy task, since one 

 has only to choose the most convenient effect of the rays of the sub- 

 stance in question, and measure it at sufficiently many times to 

 establish a sufficiently long arc of its decay-curve. Yet there is, I 

 suppose, no other problem of which the general solution involves as 

 many of the typical difficulties of research in this field ; partly because 

 some of the half-periods to be measured are so exceedingly short and 

 some so tremendously long, largely because no radioactive substance 

 ever exists by itself. Some can be separated completely from their 

 ancestors, but none can ever be totally isolated from its posterity, 

 especially since its rate of producing its posterity is the very thing 

 which is being measured. Its own gradually-declining rays are mixed 

 with the gradually-augmenting rays of its descendants, and while the 

 specific effects of the former can indeed in some cases be distinguished 

 from those of the latter, this is often difficult and sometimes im- 

 practicable. Frequently the observer is required to deduce the half- 

 periods of individual substances from observations upon a continually- 

 changing mixture; and most of the mathematical formulae used in the 

 study of radioactivity are developed out of equations (1) and (2) for 

 interpreting such observations, or inversely for predicting the evolution 



