[556] _Vide_ Böhm, _Chem. Zeitg._ 1910, ~34~, 361. [557] The crude mixture of cerium, lanthanum, neodymium, praseodymium, samarium, etc., with small quantities of iron and other metals, obtained by reduction of the earth-compounds formed as by-products in the thorium industry, is technically known as ‘misch-metal.’ [558] _F._ 439058, March, 1912. Various forms of these lighting devices are manufactured;[559] in all of these the sparks produced by scratching the pyrophoric alloy with hardened steel, by means of some simple mechanical device, is caused to ignite a fragment of tinder, or a wick supplied with a suitable liquid, _e.g._ methyl alcohol, benzene, or petrol. In the numberless forms of cigarette-lighters at present before a somewhat indifferent public, the friction is obtained by means of a toothed wheel, actuated by a spring which is released when the device is opened. Many forms of gas lighter are also on the market, but the demand for them is very small. Many attempts have been made to adapt the device to the ignition of the Davy miners’ lamp, but none have been successful, since it is impossible to prevent the sparks flying through the gauze. Much work has also been spent in efforts to utilise the pyrophoric alloys for the automatic ignition of incandescent gas-lamps, but these have been equally unsuccessful, so that it may be said that important technical applications of this interesting property have still to be made. [559] _Vide_ Böhm, _Chem. Zeitg._ 1910, ~34~, 377; also Kellermann, _Die Ceritmetalle und ihre pyrophoren Legierungen_, Wilhelm Knapp, Halle, 1912, pp. 94 _et seq._ Auer prepared his alloys by addition of iron, or other heavy metal, to the fused mixture of cerium metals obtained in the electrolytic apparatus employed for the production of the latter. They can, however, be prepared by fusing together the required quantities of foreign metal and misch-metal, the latter being obtained by processes other than those of electrolysis usually employed. The rare metals were obtained by the earlier chemists in a very impure state by reduction of the halogen or double halogen compounds with sodium or potassium. More recently[560] much purer products have been obtained--especially in the case of zirconium--by the action of metallic calcium, in the form of powder, on the oxides. Another method,[561] which has been employed in the preparation of metallic filaments for lamps, consists in heating the oxides with powdered magnesium in an atmosphere of hydrogen or nitrogen; by this means, hydrides or nitrides are obtained, which on heating decompose into the gas and the metal. [560] _Vide_ Kuzel and Wedekind, _E._ 23215, 1909. [561] _Electrodon Gesellschaft_, _D. R. P._ 154691, September, 1904.