[149] _Met. Chem. Eng._ 1911, ~9~, 543. By electrolytic reduction of the mixed chlorides of the cerium elements, a mixture known as ‘Misch metal’ is obtained; this has powerful reducing properties, and, like aluminium, reduces the oxides of iron, chromium, etc., with great development of heat.[150] The yttrium metals have not yet been obtained in the pure state, the electrolytic method giving unsatisfactory results on account of the high melting-points of the metals, and the volatile nature of their chlorides. [150] A full account of the properties and preparation of the cerium metals and their alloys will be found in the monograph of Kellermann, ‘_Die Ceritmetalle und ihre pyrophoren Legierungen_, Wilhelm Knapp, Halle, 1912. The cerium metals are white or slightly yellowish in colour, and are moderately stable in dry air. In moist air they tarnish slowly, lanthanum, as the most positive, being most readily oxidised. The melting-points and specific gravities are as follows: Element Melting-point Specific Gravity Cerium 623° 7·0242 Lanthanum 810° 6·1545 Praseodymium 940° 6·4754 Neodymium 840° 6·9563 Samarium 1300°-1400° 7·7-7·8 The metals decompose water slowly in the cold, but rapidly at the boiling-point, with evolution of hydrogen. They have a great affinity for oxygen, the heats of formation of the oxides being of the order of those of alumina and magnesia: Heat of Formation per Equivalent Weight of Oxide[151] ¹⁄₃La₂O₃ 74·1 K ¹⁄₃Nd₂O₃ 72·5 „ ¹⁄₃Pr₂O₃ 68·7 „ ¹⁄₄CeO₂ 56·1 „ ¹⁄₃Al₂O₃ 64·3 „ ¹⁄₂MgO 71·9 „ [151] Muthmann and Weiss, _loc. cit._; K = 1 kilogram-calorie, or 1000 cal. In consequence of the high values of the heats of combustion, the metals have powerful reducing properties. The cerium metals form alloys with magnesium, zinc, aluminium, and iron, and combine with boron and silicon. The alloys of cerium, and the metal itself, are remarkable for their property of emitting brilliant sparks when scratched (see Chapter XXI). Cerium also forms an amalgam with mercury. The metals burn brilliantly when heated in oxygen, and dissolve readily in dilute mineral acids. When heated to a temperature of 200°-300° in a current of hydrogen, they absorb the gas very readily, forming the _hydrides_. These compounds are also obtained by heating the oxides with magnesium in a current of hydrogen. They were first prepared by Winkler,[152] who deduced from his analyses the general formula RH₂; the more recent work of Muthmann and Beck,[153] however, points to the formula RH₃. [152] _Ber._ 1890, ~23~, 2642; 1891, ~24~, 873. [153] _Annalen_, 1904, ~331~, 58.