x] RADIO-ACTIVE PROCESSES 295 
190. Radio-activity of thorium. The radio-active pro- 
cesses occurring in thorium are far more complicated than those 
In uranium. It has been shown that a radio-active product 
Th X is continuously produced from the thorium. This Th X 
breaks up, giving rise to the radio-active emanation. This radio- 
active emanation, in turn, produces from itself the active matter, 
emanation X, which is responsible for the phenomenon of excited 
radio-activity. It has also been shown (section 176) that there is 
very strong evidence that the emanation X of thorium goes through 
two further changes, before the radio-active processes are at an 
end. 
The peculiarities of the initial portions of the decay and recovery 
curves of Th X and thorium respectively (Curves A and B, Fig. 34, 
p- 180), will now be considered. It was shown that when the 
Th X was removed from the thorium by precipitation with ammonia, 
the radiation increased about 15 per cent. during the first day, passed 
through a maximum, and then fell off according to an exponential 
law, decreasing to half value in four days. At the same time the 
activity of the separated hydroxide decreased for the first day, 
passed through a minimum, and then slowly increased again, rising 
to its original value after the lapse of about one month. 
When a thorium compound is in a state of radio-active equi- 
librium, the series of changes in which Th X, the emanation, and 
emanation X are produced go on simultaneously. Since a state of 
equilibrium has been reached for each of these products, the 
amount of each product changing in unit time is equal to the 
amount of that product supplied from the preceding change im 
unit time. Now the matter Th X is soluble in ammonia, while 
the matter emanation X is not. The Th X is thus removed from 
the thorium by precipitation with ammonia, but the emanation X 
is left behind with the thorium. Since the emanation X 1s pro- 
duced from the emanation, which in turn arises from Th X, on the 
removal of the exciting cause Th X, the radiation due to this 
emanation X will decay, since the rate of production of fresh 
emanation X no longer balances its own rate of change. Disregard- 
ing the mitial irregularity in the decay curve of emanation X 
(section 170), the activity of the emanation X will have decayed 
to half value in about 11 hours and to one quarter value at the 
