70 BELL SYSTEM TECHNICAL JOURNAL 



AcC, transmute themselves by ejecting electrons. As the disintegra- 

 tion of a sample of any of these substances proceeds, the relative pro- 

 portions of the atoms disintegrating in the two ways remain unchang- 

 ing. This makes it seem inadvisable to describe ThC (for instance) as 

 a mixture of two distinct substances; rather it appears that the atoms 

 may be all alike, but the destiny of each particular atom is a matter of 

 "chance," with the chances favoring one type of disintegration over 

 the other by nearly two to one. This is not the only circumstance in 

 radioactivity which suggests the operations of "chance." 



The substances labelled C and C", which result from the dual 

 disintegration of any of the three substances labelled C, differ in atomic 

 weight and in atomic number, and in radioactivity as well; for the C 

 substances which were born out of beta-ray transformations emit 

 alpha-rays, while the C" substances which resulted from alpha-ray 

 transmutations send forth beta-rays. Consequently their immediate 

 descendants, the two grandchildren of each C-substance, are isotopes 

 with one another — and isotopes which should be alike not only in 

 atomic number but in atomic weight as well. Is there any respect in 

 which they differ? We cannot tell. Both of the grandchildren of 

 ThC are apparently non-radioactive and stable; probably they are one 

 and the same isotope of lead. Both grandchildren of AcC likewise seem 

 to be stable. The predominant grandchild of RaC is the radioactive 

 substance RaD ; but in this case the number of atoms of RaC electing 

 the less popular path of disintegration is so exceedingly small that we 

 can neither discern any distinctive radiation to be ascribed to a sub- 

 stance isotopic with RaD but distinct from it, nor yet conclude from 

 our failure that no such substance exists. Concerning the fourth of 

 the known branchings, which occurs at UXi, the state of affairs is the 

 same as with RaC ; we can neither detect more than one kind of grand- 

 child, nor be sure that there is only one. In this case, by the way, both 

 modes of transmutation of the parent element involve the emission of 

 beta-rays. 



Although among the four substances which are known to disintegrate 

 in two alternative ways there is thus none for which both of the two 

 lines of posterity can be traced through more than two generations, 

 it is believed by many that there must be a fifth such substance in the 

 uranium series, from which the actinium series goes off as a branch 

 while the main proportion of the atoms continue evolving down the 

 radium sequence. The reason for this idea is that in the ores of 

 uranium the members of the actinium sequence are as a rule to be 

 found about three per cent as abundantly as the members of the 

 radium sequence. This fact could be deduced by assuming that 



