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188 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 
The process of the production of Th X is continuous, and no 
alteration has been observed in the amount produced in the given 
time after repeated separations. After 23 precipitations extending 
over 9 days, the amount produced in a given interval was about 
the same as at the beginning of the process. 
These results are all in agreement with the view that the 
Th X is being continuously produced from the thorium compound | 
at a constant rate. The amount of active matter produced from 
1 gram of thorium is probably extremely minute, but the elec- 
trical effects due to its activity are so large that the process of 
production can be followed after extremely short intervals. With 
a sensitive electrometer the amount of Th X produced per minute 
in 10 grams of thorium-nitrate gives a rapid movement to the 
electrometer needle. For larger intervals it 1s necessary to add 
additional capacity to the system to bring the effects within range 
of the instrument. 
124. Rate of decay of activity. It has been shown that 
the activity of Ur X and Th X decays according to an exponential 
law with the time. This, we shall see later, is the general law of 
decay of activity im any type of active matter, obtained by itself, 
and freed from any secondary active products which it may, itself, 
produce. In any case, when this law is not fulfilled, it can be 
shown that the activity is due to the superposition of two or 
more effects, each of which decays in an exponential law with 
the time. The physical interpretation of this law still remains 
to be discussed. 
It has been shown that im uranium and thorium compounds 
there is a continuous production of active matter which keeps the 
compound in radio-active equilibrium. The changes by which 
the active matter is produced must be chemical in nature, since 
the products of the action are different in chemical properties 
from the matter m which the changes take place. The activity 
of the products has afforded the means of followmg the changes 
occurring in them. It now remains to consider the connection 
between the activity at any time, and the amount of chemical 
change taking place at that time. 
In the first place, it is found experimentally that the saturation 
