1911 Encyclopædia Britannica/Caesium
CAESIUM (symbol Cs, atomic weight 132.9), one of the alkali metals. Its name is derived from the Lat. caesius, sky-blue, from two bright blue lines of its spectrum. It is of historical importance, since it was the first metal to be discovered by the aid of the spectroscope (R. Bunsen, Berlin Acad. Ber., 1860), although caesium salts had undoubtedly been examined before, but had been mistaken for potassium salts (see C.F. Plattner, Pog. Ann., 1846, p. 443, on the analysis of pollux and the subsequent work of F. Pisani, Comptes Rendus, 1864, 58, p. 714). Caesium is found in the mineral springs of Frankenhausen, Montecatini, di Val di Nievole, Tuscany, and Wheal Clifford near Redruth, Cornwall (W. A. Miller, Chem. News, 1864, 10, p. 181), and, associated with rubidium, at Dürkheim; it is also found in lepidolite, leucite, petalite, triphylline and in the carnallite from Stassfurt. The separation of caesium from the minerals which contain it is an exceedingly difficult and laborious process. According to R. Bunsen, the best source of rubidium and caesium salts is the residue left after extraction 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 (R. Bunsen, Ann., 1862, 122, p. 347; 1863, 125, p. 367). The platino-chlorides are reduced by hydrogen, and the caesium and rubidium chlorides extracted by water. See also A. Schrötter (Jour. prak. Chem., 1864, 93, p. 2075) and W. Heintz (Journ. prak. Chem., 1862, 87, p. 310). W. Feit and K. Kubierschky (Chem. Zeit., 1892, 16, p. 335) separate rubidium and caesium from the other alkali metals by converting them into double chlorides with stannic chloride; whilst J. Redtenbacher (Jour. prak. Chem., 1865, 94, p. 442) separates them from potassium by conversion into alums, which C. Setterberg (Ann., 1882, 211, p. 100) has shown 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 RbHC4H406 and CsHC4H406 have been employed, as have also the alums (see above). The double chloride of caesium and antimony 3CsCl·2SbCl3 (R. Godeffroy, Ber., 1874, 7, p. 375; Ann., 1876, 181, p. 176) has been used, the corresponding compound not being formed by rubidium. The metal has been obtained by electrolysis of a mixture of caesium and barium cyanides (C. Setterberg, Ann., 1882, 211, p. 100) and by heating the hydroxide with magnesium or aluminium (N. Beketoff, Chem. Centralblatt, 1889, 2, p. 245). L. Hackspill (Comptes Rendus, 1905, 141, p. 101) finds that metallic caesium can be obtained more readily by heating the chloride with metallic calcium. A special V-shaped tube is used in the operation, and the reaction commences between 400°C. and 500°C. It is a silvery white metal which burns on heating in air. It melts at 26° to 27°C. and has a specific gravity of 1.88 (15°C.).
The atomic weight of caesium has been determined by the analysis of its chloride and bromide. Richards and Archibald (Zeit. anorg. Chem., 1903, 34, p. 353) obtained 132.879 (O=16).