334 RADIO-ACTIVE PROCESSES [CH. 
thus forced to the conclusion that radium is being continuously 
produced in the earth, unless the very improbable assumption is 
made, that radium was in some way suddenly formed at a date 
recent in comparison with the age of the earth. It has been 
suggested that radium may be a disintegration product of one 
of the radio-elements found in pitchblende. Both uranium and 
thorium fulfil the conditions required in a possible source of 
production of radium. Both are present in pitchblende, have 
atomic weights greater than that of radium, and have rates of 
change which are slow compared with that of radium. In some 
respects, uranium filfils the conditions required better than thorium; 
for it has not been observed that minerals rich in thorium contain 
much radium, while on the other hand, the pitchblendes contain- 
ing the most radium contain a large proportion of uranium. 
If radium is not produced from uranium, it is certaimly a 
remarkable coincidence that the greatest activity of pitchblende 
yet observed is about five or six times that of uranium. Since 
radium has a life short compared with that of uranium, the 
amount of radium produced should reach a maximum value after 
a few thousand years, when the rate of production of fresh radium 
—which is also a measure of the rate of change of urantum— 
balances the rate of change of that product. In this respect the 
process would be exactly analogous to the production of the 
emanation by radium, with the difference that the radium changes 
much more slowly than the emanation. But since radium itself 
in its disintegration gives rise to at least four changes with the | 
corresponding production of « rays, the activity due to the radium 
(measured by the a rays), when in a state of radio-active equili- 
brium with uranium, should be about four times that of the 
uranium that produces it; for it has been shown that only one 
change has so far been observed in uranium in which a rays are 
expelled. Taking into account the presence of polonium and 
actinium in pitchblende, the activity in the best pitchblende is 
about the same as would be expected if the radium were a dis- 
integration product of uranium. If this hypothesis is correct, the 
amount of radium in any pitchblende should be proportional to 
the amount of uranium present, provided the radium is not 
removed from the mineral by percolating water. On the other 
