But what is there the least of on Earth? There is very little gold, platinum and platinum metals. That is why they are valued so highly. But it is a curious paradox that gold was the first of the metals to become known to man. Platinum was discovered before oxygen, silicon or aluminium had ever been heard of.

   The noble metals possess a unique feature. They do not occur in nature as compounds but in the native state. No effort is required to smelt them from their ores, that is why they were found on Earth, precisely found, so very long ago.

   However, these metals do not "take the cake" for rarity. This lamentable prize goes rather to the secondary radioactive elements. We could rightly call them ghost elements. The geochemists tell us that the amount of polonium on Earth totals only 9600 tons; the amount of radon is still smaller, 260 tons; there is 26 thousand tons of actinium.

   Radium and protactinium are veritable giants among the ghosts: they total about 100 million tons, but compared to gold and platinum this is a very small quantity.

   As to astatine and francium, they can hardly be classed even as ghosts, because they are still less material. The terrestrial reserves of astatine and francium are measured, ridiculous though it sounds, in milligrams. The name of the rarest element on Earth is astatine (69 milligrams in all of the Earth's crust). No further comments are necessary.

   The first transuranium elements, neptunium and plutonium, have also been found to exist on Earth. They are born in nature as a result of very rare nuclear reactions between uranium and free neutrons. These ghosts can "boast" of hundreds and thousands of tons. But as to promethium and technetium, which are also due to uranium (the latter is capable of spontaneous fission, with its nuclei breaking up into two approximately equal fragments), there is nothing that can be said of them.

   Scientists have found hardly perceptible traces of technetium, and are still looking for promethium in uranium minerals. The balance has yet to be invented on which the Earth's "reserves" of promethium and technetium could be weighed.

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