heme might be devised in which the elements could be filed away in the
order of their atomic weights so that one could see just how a certain
element, known or unknown, would behave from merely observing its
position in the series. Mendeleef, a Russian chemist, devised the most
ingenious of such systems called the "periodic law" and gave proof that
there was something in his theory by predicting the properties of three
metallic elements, then unknown but for which his arrangement showed
three empty pigeon-holes. Sixteen years later all three of these
predicted elements had been discovered, one by a Frenchman, one by a
German and one by a Scandinavian, and named from patriotic impulse,
gallium, germanium and scandium. This was a triumph of scientific
prescience as striking as the mathematical proof of the existence of the
planet Neptune by Leverrier before it had been found by the telescope.

But although Mendeleef's law told "the truth," it gradually became
evident that it did not tell "the whole truth and nothing but the
truth," as the lawyers put it. As usually happens in the history of
science the hypothesis was found not to explain things so simply and
completely as was at first assumed. The anomalies in the arrangement did
not disappear on closer study, but stuck out more conspicuously. Though
Mendeleef had pointed out three missing links, he had failed to make
provision for a whole group of elements since discovered, the inert
gases of the helium-argon group. As we now know, the scheme was built
upon the false assumptions that the elements are immutable and that
their atomic weights are invariable.

The elements that the chemists had most difficulty in sorting out and
identifying were the heavy metals found in the "rare earths." There were
about twenty of them so mixed up together and so much alike as to baffle
all ordinary means of separating them. For a hundred years chemists
worked over them and quarreled over them before they discovered that
they had a commercial value. It was a problem as remote from
practicality as any that could be conceived. The man in the street did
not see why chemists should care whether there were two didymiums any
more than why theologians should care whether there were two Isaiahs.
But all of a sudden, in 1885, the chemical puzzle became a business
proposition. The rare earths became household utensils and it made a big
difference with our monthly gas bills whether the ceria and the