Compounds of the yttrium group have at present no technical importance. They were formerly used to some extent for the manufacture of filaments for Nernst lamps, but with the introduction of metal filament lamps in electric lighting, the demand for Nernst lamps and consequently for the yttria oxides, has to a very great extent died away. Zirconium and its compounds, on the other hand, promise to become of some technical importance. The metal received considerable attention in the earlier stages of experimental work on metallic filaments for electric lighting, but it has been shown that its melting-point is not sufficiently high to allow of extended use in this direction. The carbide has been proposed for the same purpose, but is even less suitable; this compound, however, on account of its great hardness, is likely to find employment as an abrasive, and in glass-cutting. The oxide, which occurs in nature in an impure form as the mineral Baddeleyite (_q.v._), is employed in the manufacture of ‘Siloxide’ glass and of enamels, as a pigment and polishing agent, and in various forms of lamps, _e.g._ the Nernst and Bleriot lamps, the Drummond light, etc. Far more important, however, is its use for fire-resistant crucibles, furnace linings and supports, etc., for which its refractory nature renders it particularly suitable. On account of its high specific gravity and non-poisonous character, it has been proposed for use in the Röntgen ray examination of the human body. Quite recently, metallic zirconium has been employed in metallurgy; addition of small quantities, in the form of suitable alloys, is said to secure sound castings, with increased strength and resistance to acids. THE CERIUM GROUP ~Pyrophoric Alloys.~--It has long been known that the metals of the cerium group possess the property, when scratched or struck, of throwing off glowing particles; this power of emitting sparks is not lost when the metals are alloyed, so long as the percentage of foreign metal is not allowed to become too high. In a patent[555] protecting the use of various ‘pyrophoric alloys,’ as these spark-giving alloys are called, Auer states that the pure metals do not show this property, which only appears when foreign metals are present; he accordingly patents alloys of the cerium metals with iron, specifying particularly the alloy with 30 per cent. of the latter element. Auer’s statement has been contradicted,[556] and it seems to be generally accepted that misch-metal[557] of ordinary technical purity has the property of sparking when scratched. This alloy of the cerium metals, however, is far too soft to be useful for the purpose, and the addition of some foreign element is required to obtain the strength, hardness, and brittleness necessary in the various forms of ‘lighters.’ Besides the addition of iron, the use of tin, lead, zinc, cadmium, silicon, etc., has been patented.[558] [555] _E._ 16853, 1903; _D. R. P._ 154807.