The probability that one element might be changed into another was increased by the demonstration of the connexions between uranium and radium. The metal uranium has been classed with the elements since it was isolated in 1840. In 1896, Becquerel found that compounds of uranium, and also the metal itself, are radio-active. In the light of what is now known about radio-activity, it is necessary to suppose that some of the minute particles of uranium emit particles lighter than themselves, and change into some substance, or substances, different from uranium; in other words, it is necessary to suppose that some particles of uranium are spontaneously disintegrating. This supposition is confirmed by the fact, experimentally proved, that uranium emits [alpha]-rays, that is, atoms of helium, and produces a substance known as uranium X. Uranium X is itself radio-active; it emits [beta]-rays, that is, it gives off electrons. Inasmuch as all minerals which contain compounds of uranium contain compounds of radium also, it is probable that radium is one of the disintegration-products of uranium. The rate of decay of radium may be roughly expressed by saying that, if a quantity of radium were kept for ten thousand years, only about one per cent. of the original quantity would then remain unchanged. Even if it were assumed that at a remote time the earth's crust contained considerable quantities of radium compounds, it is certain that they would have completely disappeared long ago, had not compounds of radium been reproduced from other materials. 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.