No, this is not the case. It is simply that they keep reappearing, because they are fed by an inexhaustible source, the terrestrial reserves of uranium and thorium. As long as these radioactive "patriarchs" keep travelling down their long and complex path of transformations, ultimately to form stable lead, they must continue changing into the intermediate elements. Thus, among the chemical elements we can distinguish two large groups, namely, the primary and the secondary elements.

   The primary elements are all the nonradioactive elements and uranium and thorium whose half-life periods are greater than the Earth's age. They witnessed the formation of the solar system. All the rest are secondary elements.

   Still, there will come a time when the Periodic System will find itself lacking several elements. These will be uranium and thorium, the eternal source of secondary elements which are, however, eternal only relatively.

   At some future time they will also disappear from the face of the Earth, in a matter of a few hundred billion years. And together with them will vanish the products of their radioactive transformations.

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