Caesium, a metal discovered in 1860-'61 by Bunsen and Kirchhoff by means of spectrum analysis. It so closely resembles potassium in its properties that it had escaped the notice of chemists who pursued their investigations according to the ordinary methods of analysis. Bunsen, in his capacity of sanitary inspector of the mineral springs of Baden, received the residues from the evaporation of Durkheim water, and in the course of a critical examination into its properties detected by two blue lines on the spectrum that something different from potash was manifestly present. He precipitated the new body with chloride of platinum, and by repeated washings separated it from its surroundings. In consequence of the color of the lines on the spectrum he gave the name of caesium (Lat. ccesius, sky-blue) to the new element. The first publication on the subject was in the Annalen of Liebig and Wohler for July, 1861. In 1864 Pisani found caesium to the extent of 34 per cent, in a rare mineral from the island of Elba, called pollux.

The metal has since been detected in carnallite and tri-phylline; in numerous salt brines; in the lepi-dolite of Hebron, Me., which Johnson found to contain 0.3 per cent.; in petalite, various sea shells, basaltic rocks, and occasionally in the ashes of plants, though it appears not to be readily assimilated by vegetables, and in this respect to resemble sodium. Caesium is the most electro-positive of all the metals, and oxidizes so rapidly that Bunsen has not been able to give a full description of its properties. Compounds of caesium, analogous to those obtained from potassium and sodium, have been prepared by chemists. They yield alums, soaps, and organic bodies, none of which have at present any application in the arts. The atomic weight of caesium is 133, and its symbol is Cs.