id and dissolving the residue in ice-cold water; sulphuretted hydrogen
is passed through the solution, which is then filtered, acidified with
hydrochloric acid, and precipitated as oxalate by oxalic acid; the
oxalate is then converted into oxide by ignition. From the crude oxide
so obtained (which contains lanthanum and didymium oxides) the cerium
may be separated by conversion into its double sulphate on the addition
of potassium sulphate, the sulphates of the cerium group being insoluble
in a saturated solution of potassium sulphate. The sulphate is
subsequently boiled with water, when a basic sulphate is precipitated.
For the preparation of pure cerium compounds see Auer v. Welsbach,
_Monatshefte_, 1884, v. 508.

The metal was first obtained, in an impure state, by C.G. Mosander, by
fusing its chloride with sodium. W.F. Hillebrand and T. Norton have
prepared it by the electrolysis of the melted chloride (_Pogg. Ann._,
1875, 156, p. 466); and C. Winkler (_Berichte_, 1891, xxiv. 884)
obtained it by heating the dioxide with magnesium powder. The metal has
somewhat the appearance of iron, and has a specific gravity of 6.628,
which, after melting, is increased to 6.728. Its specific heat is
0.04479 (W.F. Hillebrand). It is permanent in dry air, but tarnishes in
moist air; it can be hammered and rolled; it melts at 623 deg. C. It
burns readily on heating, with a brilliant flame; and it also combines
with chlorine, bromine, iodine, sulphur, phosphorus and cyanogen. In the
case of the two former elements the combination is accompanied by
combustion of the metal. With water it is slowly converted into the
dioxide. Cold concentrated nitric and sulphuric acids are without action
on the metal, but it reacts rapidly with dilute nitric and hydrochloric
acids. The dioxide is used in incandescent gas mantles (see LIGHTING).

Three oxides of cerium are known. The sesquioxide, Ce2O3, is obtained
by heating the carbonate in a current of hydrogen. It is a
bluish-green powder, which on exposure rapidly combines with the
oxygen of the air. By the addition of caustic soda to cerous salts, a
white precipitate of cerous hydroxide is formed. Cerium dioxide, CeO2,
is produced when cerium carbonate, nitrate, sulphate or oxalate is
heated in air. It is a white or pale yellow compound, which becomes
reddish on heating. Its specific gravity is 6.739, and its specific
heat 0.0877. It is not reduced to the metallic condition on hea