The ease with which misch-metal and its alloys with iron and other elements throw off glowing particles when struck is due to the low ignition temperature of cerium, and the energy with which it combines with oxygen. When such alloys are scratched, small fragments are struck off, which are raised to the ignition temperature by the heat of friction. It is generally accepted, however, that this explanation is by no means a complete one, and the existence of a pyrophoric suboxide was suggested.[562] The theory was advanced that the pyrophoric properties of the alloys were due primarily to the formation on the surface of a film of this sub-oxide, and the partial oxidation of cerium alloys, protected by patent (_loc. cit._), was said to cause a marked increase in the ease with which sparks could be obtained. In this connection, an experiment of Hirsch, who has thoroughly investigated the properties of metallic cerium,[563] is of interest. He found that when the element is warmed in a sealed glass bottle, a black powder forms on the surface, which, when the bottle is opened, ignites spontaneously. It is probable that this black sub-oxide plays an important part in the production of sparks from the ordinary pyrophoric alloys. [562] _Vide_ _F._ 407117, December, 1909. [563] _Met. Chem. Eng._ 1911, ~9~, 543. ~Other Alloys.~--On account of the great affinity of the cerium metals for oxygen, misch-metal has been suggested as a reducing agent,[564] as have also the alloys of cerium and magnesium;[565] the formation of the latter is endothermic, so that they act much more vigorously than either metal separately. The alloys of cerium with tin and aluminium have been thoroughly investigated from the stand-point of metallography, by Vogel.[566] It has recently been claimed that the addition of very small quantities of cerium to aluminium has a very marked effect, the rare earth metal acting as a purifying agent,[567] and greatly improving the properties of the aluminium. The cerium may be introduced as fluoride, either to the electrolytic bath in which the aluminium is being prepared, or to the latter metal, after preparation, in the fused state. The most favourable effect is said to be produced by 0·2 per cent. of cerium. [564] _Vide_, _e.g._ Escales, _D. R. P._ 145820, October, 1903. [565] Hirsch, _loc. cit._ [566] _Zeitsch. anorg. Chem._ 1911, ~72~, 319; 1912, ~75~, 41. [567] Borchers and Barth, _D. R. P._ 246484, May, 1912.