i84 PHYSICAL SCIENCE 



its activity. The radio-activity of the pure thorium 

 seems to be a consequence of its change into 

 thorium-X and to accompany that change. The 

 activity of the thorium-J^, in a similar way, 

 accompanies, and is a consequence of, its continual 

 change into other bodies, in this case, the thorium 

 emanation. The constant activity of a thorium 

 compound, as ordinarily found, is thus due to a 



URAM/UM 



60 so wo tZO 



Time' irv days 



Fig. 33. 



balance in the rate of production of the active 

 thorium-^ and the rate of its loss of radio-activity. 

 What view are we to take of the changes In the 

 thorium or uranium which result in the formation 

 of the X products, and what further changes must 

 we suppose to go on when the X products give 

 rise to emanations or other bodies ? Are these 

 changes of the nature of ordinary chemical action, 

 in which atomic or molecular combinations, or 

 rearrangements of the atoms in a molecule, are 



