CERIUM, serl-Om, a metallic element the oxide of which was discovered and recognized as a new substance in 1803 by Klaproth, and, independently, by Berzelius and Hisinger. It was named for the minor planet Ceres, which was also discovered at ab(2tit the same time. The principal source of cerium is the mineral monazite (q.v.), which is a silicate of cerium and certain other allied elements. Cerium forms a basic nitrate that is insoluble in water, and this fact affords a ready means of separat ing the metal from the other elements with which it is almost invariably associated. Metallic cerium may be obtained by electrolysis of the anhydrous chloride, or by melting the anhydrous chloride with • metallic sodium. Thus prepared, cerium is a steel-gray metal, ductile and malleable, and melting at a tem perature probably not far from the melting point of silver. Its chemical symbol is Ce, and its atomic weight is 139.1 if H =1. It has a specific gravity of from 6.6 to 6.75, and a specific heat of about 1).0448. It does not change in dry air, but in moist air it oxidizes superficially. It decomposes in cold water slowly, and in hot water quickly. Two oxides of the
metal certainly exist, one having the formula Ce203 and the other the formula CeO,. Three other oxides have also been described, with the formula; CeO,, Ce,O, and Ce.40.; of these the first probably exists, but the other two still need confirmation. Oxide of cerium is also used in the manufacture of the better grades of incandescent gas-mantles. A fabric of cot ton is woven of the desired form and this is impregnated by repeated dipping in a solution of the nitrates of cerium and thorium. On ignition the cotton burns away, and the nitrates are converted into oxides, which give the in tense luminosity desired. A mixture of 99 per cent of thorium oxide to 1 of cerium oxide gives the best results.
In medicine the insoluble salts of cerium are used, cerium oxalate alone being officinal. It resembles the insoluble bismuth compounds in its action, being a sedative to mucous mem branes, and it is much used as an antemetic, particularly in the nausea of pregnancy. The soluble salts of cerium are poison, their action being similar to the soluble salts of bismuth.