CHAPTER X. 
RADIO-ACTIVE PROCESSES. 
189. Radio-activity of uranium. It has already been 
shown in section 118 that a radio-active constituent Ur X can 
be separated from uranium by several different processes. The 
activity of the separated Ur X decays with the time, falling to half 
value in about 22 days. At the same time the uranium, from 
which the Ur X has been separated, gradually regains its lost 
activity. The law of decay of Ur X and the recovery of the lost 
activity of the uranium are expressed by the equations 
fe —leme sane h =1-e-*, 
0 0 
where X is the radio-active constant of Ur X. The constant radio- 
activity of uranium thus represents a state of equilibrium, where 
the rate of production of new active matter is balanced by the rate 
of change of the Ur X already produced. 
The radio-active processes occurring in uranium present several 
points of difference from the processes occurring in thorium and 
radium. In the first place, uranium does not give off an emanation, 
and in consequence does not produce any excited activity on bodies, 
So far only one active product Ur X has been observed in uranium. 
This active product Ur X differs from Th X and the emanations, 
inasmuch as the radiation from it consists almost entirely of @ rays. 
This peculiarity of the radiations from Ur X initially led to some 
confusion in the interpretation of observations on Ur X and the 
uranium from which it had been separated. When examined by 
the photographic method, the uranium freed from Ur X showed no 
photographic action, while the Ur X possessed it to an intense 
