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 thoria in the burner mantles were absolutely pure or contained traces of some of the other elements that were so difficult to separate.

This sudden change of venue from pure to applied science came about through a Viennese chemist, Dr. Carl Auer, later and in consequence known as Baron Auer von Welsbach.  He was trying to sort out the rare earths by means of the spectroscopic method, which consists ordinarily in dipping a platinum wire into a solution of the unknown substance and holding it in a colorless gas flame.  As it burns off, each element gives a characteristic color to the flame, which is seen as a series of lines when looked at through the spectroscope.  But the flash of the flame from the platinum wire was too brief to be studied, so Dr. Auer hit upon the plan of soaking a thread in the liquid and putting this in the gas jet.  The cotton of course burned off at once, but the earths held together and when heated gave off a brilliant white light, very much like the calcium or limelight which is produced by heating a stick of quicklime in the oxy-hydrogen flame.  But these rare earths do not require any such intense heat as that, for they will glow in an ordinary gas jet.