previsions; but his triumph was destined to be still more complete, for to gallium were afterward added scandium (ekaboron), discovered by M. L. F. Nilson in 1875, and germanium (ekasilicon), discovered by me in 1886.
The discovery of germanium, predicted under the name of eka-silicon by Mendeleef, bears a resemblance to the discovery of the planet Neptune, the existence of which had been shown by the calculations of Adams and Leverrier. That discovery was not due to a concurrence of favorable circumstances or to a happy accident, but was the result of researches inspired by theoretic previsions, and the concordance between the predicted and the real properties was so great that Mendeleef regarded the discovery of germanium as an important verification of the periodic law. On only one point—that touching its formations in Nature—did germanium completely fail expectations. The search for it would be more likely made as an oxide in the rare minerals of the north, along with titanium and zirconium, than as a sulphide accompanying similar compounds of arsenic and antimony in gangues of silver-bearing minerals. This fact, with the comparative rarity of its mineral, argyrodite, has contributed no little to delaying the elucidation of its real character. For myself, I was at first inclined to regard it as eka-antimony, while Mendeleef, after my first incomplete communications, thought it was ekacadmium. At the same time, M. von Richter expressed the conviction that germanium was nothing else than the long-expected eka-silicon, a conclusion that was justified by the correspondence of atomic weights.
The success of the bold speculations of Mendeleef permits the affirmation that the elaboration of the periodic system constitutes a great forward step for science. In the course of only fifteen years all the predictions of the Russian chemist have been confirmed. New elements have come to fill the vacant spaces in his table, and there is every reason to hope that a like fulfillment awaits the rest of the natural system.
Yet the two elements last discovered, argon and helium, do not seem to present any relation with the periodic system. The physical properties of argon are very distinct; its characteristic spectrum distinguishes it with great certainty from all other substances; but chemically the gas manifests an extraordinary indifference, and it has not so far been possible to make it enter into the usual compounds with other elements. This peculiarity, and the impossibility of introducing a simple body of the molecular weight of argon (39.88) into the periodic system, have given occasion to all sorts of hypotheses concerning the gas; and the question of its relations has not yet been answered.
