CAESIUM, a metal of the alkali (q.v.) group (symbol Cs, atomic number 55, atomic weight 132.81). Its name is derived from the Lat. caesius, sky-blue, from two bright blue lines of its spectrum. It was the first metal to be discovered by the aid of the spectroscope (R. Bunsen, Pogg. Annal., cxiii., 337; Berlin Acad. Ber., 186o), although caesium salts had undoubtedly been handled before, but had been mistaken for potassium salts. The most im portant mineral containing caesium is pollucite, a silicate of alum inium and caesium. This mineral contains from 31 of and occurs in commercial quantities in Maine, U.S.A., and the Isle of Elba. In 1846 C. F. Plattner analysed the mineral and obtained as total constituents 93.75%, reckoning the as He was mystified by the result which was cleared up in 1864 by F. Pisani when he found the alkali present to be caesium. Pollucite is attacked quite readily by hydrochloric or nitric acid and after the removal of the silica the caesium may be precipitated by forming the insoluble double chloride with antimony chloride or crystalliz ing out the fairly insoluble caesium aluminium sulphate, an alum (q.v.). Caesium is found in the mineral springs of Franken hausen, Montecatini, in Val di Nievole, Tuscany, and Wheal Clifford, near Redruth, Cornwall, and, associated with rubidium, at Durkheim ; it is also found in lepidolite, leucite, petalite, triphyl line and in the carnallite from Stassfurt. The separation of caesium from the minerals which contain it is an exceedingly diffi cult and laborious process. According to R. Bunsen, the best source of rubidium and caesium salts is the residue left after ex traction of lithium salts from lepidolite. This residue consists of sodium, potassium and lithium chlorides, with small quantities of caesium and rubidium chlorides. The caesium and rubidium are separated from this by repeated fractional crystallization of their double platinum chlorides, which are much less soluble in water than those of the other alkali metals. The platinichlorides are re duced by hydrogen, and the caesium and rubidium chlorides ex tracted by water. W. Feit and K. Kubierschky (1892) separated rubidium and caesium from the other alkali metals by converting them into double chlorides with stannic chloride; whilst J. Red tenbacher (1865) separated them from potassium by conversion into alums, which are very slightly soluble in a solution of potash alum. In order to separate caesium from rubidium, use is made of the different solubilities of their various salts. The bitartrates and have been employed, as have also the alums. The double chloride of caesium and antimony 3CsC1.2SbCl3 has been used, the corresponding compound not being formed by rubidium. The metal has been obtained by electrolysis of a mix ture of caesium and barium cyanides, and by heating the hydrox ide with magnesium or aluminium. L. Hackspill finds, however, that metallic caesium can be prepared more readily by distilling the chloride with metallic calcium. It is a silvery white metal which burns on being heated in air. It melts at 26° to C and has a specific gravity of 1.88 (15° C).
Caesium hydroxide, CsOH, obtained by the decomposition of the sulphate with baryta water, is a greyish-white deliquescent solid which melts at a red heat and absorbs carbon dioxide rapidly. It readily dissolves in water, with evolution of much heat. Caesium chloride, CsCl, obtained by dissolving the hydroxide or carbonate in hydrochloric acid, forms small cubes which melt at a red heat and volatilize readily; it deliquesces in moist air. The bromide, CsBr, and iodide, CsI, resemble the corresponding potas sium salts. Many trihaloid salts of caesium are also known such as etc. (H. L. Wells and S. L. Penfield, 1892). Caesium sulphate, may be prepared by dissolving the hydroxide or carbonate in sulphuric acid. It crystal lizes in short hard prisms, readily soluble in water but insoluble in alcohol. It forms caesium alum, Caesium nitrate, is obtained by dissolving the carbonate in nitric acid, and crystallizes in glittering prisms, which melt readily, and when heated evolve oxygen and leave a residue of caesium nitrite. The carbonate, is soluble in water and in alcohol.
Caesium compounds can be readily recognized by the two bright blue lines (of wave length 4555 and 4593) in their flame spectrum, although these are not present in the spark spectrum. The other lines include three in the green, two in the yellow, and two in the orange. Caesium has an important technical application in the manufacture of radio tubes or "valves." Its nitrate or chloride is mixed with cerium or magnesium and compressed into small tablets which are introduced into the tubes. In evacuating the tubes, these tablets are flashed, helping to secure a high vacuum.