580 The Evolution of Matter 
We can now exhibit in tabular form the amazing pedigree of 
radio-active change shown by this one family of elements. An im- 
mediate descent is indicated by ~, while one which may either be 
immediate or involve an intermediate step is shown by ; 
No place is found in this pedigree for thorium and its derivatives. 
They seem to form a separate and independent radio-active family. 
Atomic Time of half Radio- 
weight decay activity 
Uranium 238°5 a 
Uren X 2 22 days By 
Actinium 2 q no rays 
Actinium-X q 10-2 days a (B, ¥) 
Actinium Emanation q 3°9 seconds a 
Actinium-A q 35°7 minutes no rays 
Actinium-B 2 2°15 minutes a, B, y 
Radiat 225 about 2600 years | @ 
shad Emanation q 38 days a 
Radium-A 2 3 minutes a 
Radium-B 2 21 minutes no rays 
Radium-C 2 28 minutes a, B, y 
Radium-D q about 40 years no rays 
Radium-E q 6 days B (y) 
Radium-F q 143 days a 
Lead 207 q no rays 
As soon as the transmutation theory of radio-activity was accepted, 
it became natural to speculate about the intimate structure of the 
radio-active atoms, and the mode in which they broke up with the 
liberation of some of their store of internal energy. How could we 
imagine an atomic structure which would persist unchanged for long 
