gadolinite, and yttrotitanite. This metal, the oxide of which exists only in quantities of a few grammes, and which no person, perhaps, other than the author of the discovery has had in his hands, possesses considerable scientific importance, because its atomic weight of 44, as determined by Nikon, is precisely that indicated by Mendeleef for ekabor, an element the existence of which was predicated by the periodic law.
In 1794, Gadolin had separated from the gadolinite of Ytterby an earth which he called the earth of Ytter, and which was afterward known under the forms of erbia, terbia, and yttria proper. These earths were found in a considerable number of rare minerals, but the oxides extracted from these minerals exhibited different natures and aspects, presenting themselves rather like mixtures in which the separation of the different constituents was attended by considerable difficulties, for the different elements gave no very distinct reaction. It was necessary to recur to spectrum analysis and to the determination of atomic weights, and to try to isolate them by repeated fractionings, under the action of sulphate of potassium or of ammonia, or else by the partial decomposition of the nitrates by heat. The bulk of these analyses, the results of which are not, however, entirely clear as yet on some points, have been performed within the last quarter of a century, and, besides securing more precise knowledge of scandium and yttrium, have revealed the existence of numerous other rare elements, the reduction of which does not seem impossible; among which we cite erbium, holmium, thulium, dysprosium, terbium, gadolinium, samarium, decipium, and ytterbium.
Cerium, lanthanum, and didymium have been the object recently of very attentive researches having a practical end in view—the constitution of mantles for incandescent gaslights. Didymium has been long suspected of not being a simple substance; but Carl Auer von Welsbach, the inventor of this method of illumination, is entitled to the credit of having succeeded, in 1855, in separating didymium into its two elements of prsesodidymium and neodidymium. The utilization of monazite afterward permitted the preparation of the salts of these remarkable metals in larger quantities, and the practical use of them.
The existence of metacerium, announced by M. Brauner, does not yet appear to be fully established, nor that of russium, which M. Crushchow has found associated with thorium in some zircons and in monazite, and the atomic weight of which is calculated at 220. The jargonium of Sorby, the austrium of Linneman, the norvegium of Dahll, the actinium of Phipson, the idumium of Websky, the masrium of Richmond and Off, and