VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 185 
may be. If J, is the initial activity of the separated product, the 
activity J; after an interval t is given by 
Li 
ao, 
0 
Thus, the two assumptions of uniform production of active 
matter and the decay of its activity in an exponential law from 
the moment of its formation satisfactorily explain the relation 
between the curves of decay and recovery of activity. 
122. Experimental evidence. It now remains to consider 
further experimental evidence in support of these hypotheses. The 
primary conception is that the radio-active bodies are able to 
produce from themselves matter of chemical properties different 
from those of the substance that produces it, and that this process 
goes on at a constant rate. This new matter initially possesses 
the property of activity, and loses it according to a definite law. 
The fact that a proportion of the activity of radium and thorium 
can be concentrated in small amounts of active matter like Th X 
or Ur X does not, of itself, prove directly that a material con- 
stituent responsible for the activity has been chemically separated. 
For example, in the case of the separation of Th X from thorium, 
it might be supposed that the non-thorium part of the solution is 
rendered temporarily active by its association with thorium, and 
that this property is retained through the processes of precipita- 
tion, evaporation, and ignition, and finally manifests itself in the 
residue remaining. According to this view it is to be expected 
that any precipitate capable of removing the thorium completely 
from its solution should yield active residues similar to those ob- 
tained from ammonia. No such case has however been observed. 
For example, when thorium nitrate is precipitated by sodium or 
ammonium carbonate, the residue from the filtrate after evapora- 
tion and ignition is free from activity and the thorium carbonate 
obtained has the normal amount of activity. In fact, ammonia is 
the only reagent yet found capable of completely separating Th X 
from thorium. <A partial separation of the Th X can be made by 
shaking thorium oxide with water owing to the greater solubility 
of Th X in water. 
Thorium and uranium behave quite differently with regard to 
