578 The Evolution of Matter 
With this idea in mind we see at once the significance of the con- 
stitution of uranium minerals. Formed in the remote antiquity of 
past geological ages, these minerals must become store-houses of all 
the products of uranium except those which may have escaped as 
gases or possibly liquids. Even gases may be expected to some 
extent to be retained by occlusion. Among the contents of uranium 
minerals, then, we may look for the descendants of the parent 
uranium. If the descendants are permanent or more long-lived than 
uranium, they will accumulate continually. If they are short-lived, 
they will accumulate at a steady rate till enough is formed for the 
quantity disintegrating to be equal to the quantity developed. A 
state of mobile equilibrium will then be reached, and the amount of 
the product will remain constant. This constant amount of substance 
will depend only on the amount of uranium which is its source, and, 
for different minerals, if all the product is retained, the quantity of 
the product will be proportional to the quantity of uranium. In a 
series of analyses of uranium minerals, therefore, we ought to be able 
to pick out its more short-lived descendants by seeking for instances 
of such proportionality. 
Now radium itself is a constituent of uranium minerals, and 
two series of experiments by R. J. Strutt and B. B. Boltwood have 
shown that the content of radium, as measured by the radio-activity 
of the emanation, is directly proportional to the content of 
uranium’. In Boltwood’s investigation, some twenty minerals, with 
amounts of uranium varying from that in a specimen of uraninite 
with 74°65 per cent., to that in a monazite with 0°30 per cent., gave a 
ratio of uranium to radium, constant within about one part in ten. 
The conclusion is irresistible that radium is a descendant of 
uranium, though whether uranium is its parent or a more remote 
ancestor requires further investigation by the radio-active genea- 
logist. On the hypothesis of direct parentage, it is easy to calculate 
that the amount of radium produced in a month by a kilogramme of a 
uranium salt would be enough to be detected easily by the radio- 
activity of its emanation. The investigation has been attempted by 
several observers, and the results, especially those of a careful ex- 
periment of Boltwood, show that from purified uranium salts the 
growth of radium, if appreciable at all, is much less than would be 
found if the radium was the first product of change of the uranium. 
It is necessary, therefore, to look for one or more intermediate 
substances. 
While working in 1899 with the uranium residues used by M. and 
Mme Curie for the preparation of radium, Debierne discovered and 
1 Strutt, Proc. Roy. Soc. A, Feb, 1905; Boltwood, Phil. Mag. April, 1905. 
