Again, the most likely hypothesis is that compounds of radium are being produced from compounds of uranium.
Uranium is a substance which, after being rightly classed with the elements for more than half a century, because it had not been separated into unlike parts, must now be classed with the radium-like substances which disintegrate spontaneously, although it differs from other radio-active substances in that its rate of change is almost infinitively slower than that of any of them, except thorium.[12] Thorium, a very rare metal, is the second of the seventy-five or eighty elements known when radio-activity was discovered, which has been found to undergo spontaneous disintegration with the emission of rays.

The rate of change of thorium is considerably slower than that of uranium.[13] None of the other substances placed in the class of elements is radio-active.
[12] The life-period of uranium is probably about eight thousand million years.
[13] The life-period of thorium is possibly about forty thousand million years.
On p.

192 I said, that when the radio-active substances had been labelled _elements_, the facts of radio-activity led some chemists to the conclusion that the other bodies which had for long been called by this class-name, or at any rate some of these bodies, are perhaps not true elements, but are merely more stable collocations of particles than the substances called compounds.

It seems to me that this reasoning rests on an unscientific use of the term _element_; it rests on giving to that class-name the meaning, _substances asserted to be undecomposable_.

A line of demarcation is drawn between _elements_, meaning thereby forms of matter said to be undecomposable but probably capable of separation into unlike parts, and _true elements_, meaning thereby groups of identical undecomposable particles.