62 A. IV. Stewart on 



lighter element. Originally three of these series were assumed 

 to exist, one starting from uranium, another from the element 

 actinium, and a third from thorium ; but recent woi'k l>y Pro- 

 fessor Soddy has proved that there are only two, since actinium 

 itself is a descendant of radium. 



The following Table IV shows the relations of the radio- 

 elements, and gives some idea of the numljer of these unstable 

 materials now known : — 



Table IV. 



THE KADIOACTIVE SERIES OF ELEMENTS. 



I. — The Thorium Series. 



Element 



Atomic 

 Weight 



Average 

 Life 



Rays 

 Emitted 



Thorium 



232 



2-6 -f 10^° years 



a 



Mesothorium-1 



228 



7-9 years 



i« 



Mesothorium-2 



228 



8-9 hours 



fS 



Radiothorium 



228 



2-91 years 



a 



Thoriuni-X 



224 



5-25 days 



a 



Thorium emanation 220 



78 seconds 



a 



Thorium-A 



216 



0-2 seconds 



a 



Thovium-B 



212 



15-4 hours 



(^ 



Thorium-C 



212 



87 minutes 



a and f3 



Thorium C^ 



212 



10"^^ seconds 



a 



Thorium-D 



208 



4-5 minutes 



(i 



End-product 



208 



Permanent 



No rays 



Note. — Thorium U and Thorium C^ are 

 'J'horium-C which decomposes 



botli disintegration products 

 in two different ways. 



II.-The 



Uranium-R 



ADiuM Series. 





Element 



Atomic 

 Weight 



Average 

 Life 



Rays 

 Emitted 



Uranium-1 



238 



8 X 10'^ years 



a 



Uranium-Xi 



234 



35-5 days 



/3 



Uranium Xo 



234 



1*65 minutes 



i8 



Uranium-2 



234 



3 X 10'' years 



a 



