CERIUM, the commonest metallic element belonging to the rare earth group, discovered in 5803 by Klaproth and independ ently by Berzelius and Hisinger. Cerium (symbol Ce, atomic number 58, atomic weight 140.2) occurs most abundantly in the minerals monazite, cerite and allanite, accompanied by the other members of the group. It is formed by electrolysis of fused an hydrous chloride. It is of an iron-grey colour, about as soft as lead, and is attacked slowly by cold water and more rapidly by hot water. It melts at 623°C. It burns brilliantly when heated in the form of wire, and is a powerful reducing agent. It forms alloys with other metals, the best known being that with iron which is used because of its well-known property of giving off showers of brilliant sparks when struck or filed. Of all the rare earths it is most easily purified. This is due to the fact that it is the only one that forms a higher series of compounds (CeX,). These com pounds readily suffer hydrolysis with the precipitation of basic substances practically free from other earths. The purification is readily brought about by carefully boiling the mixed nitrates with potassium bromate and then gradually adding coarsely ground marble to keep down the acidity. The precipitated basic nitrate is washed with a 1 cc solution of nitric acid. The product may be purified if necessary by repeating the process.
The common oxide, produced when the salts of cerium with volatile acids are heated, is the dioxide This oxide, which is a pale-yellowish powder, is insoluble in nitric and hydrochloric acids, but soluble in sulphuric acid. The salts corresponding to this oxide are of the type Ce (S0,) 2 and possess a yellow or orange-red colour. The salts derived from trivalent cerium [type are much more stable, colourless and show no absorp tion spectrum. Cerium compounds have a few uses. The oxalate is used to prevent various forms of sickness such as seasickness, etc. Cerium nitrate is used in the manufacture of «"elsbach mantles, in the ceramic and textile industries, etc. (See RARE EARTHS.) (C. J.)