h 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
RbHC_4H_40_6 and CsHC_4H_40_6 have been employed, as have also the alums
(see above). The double chloride of caesium and antimony 3CsCl.2SbCl_3 (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 deg.C. and 500 deg.C. It is a silvery white
metal which burns on heating in air. It melts at 26 deg. to 27 deg.C. and has a
specific gravity of 1.88 (15 deg.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).

_Caesium hydroxide_, Cs(OH)_2, 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,
is obtained by the direct action of chlorine on caesium, or by solution of
the hydroxide in hydrochloric acid. It forms small cubes which melt at a
red heat and volatilize readily. It deliquesces in moist air. Many double
chlorides are known, and may be prepared by mixing solutions of the two
components in the requisite proportions. The _bromide_, CsBr, and _iodide_,
CsI, resemble the corresponding potassium salts. Many trihaloid salts of
caesium are also known, such as CsBr_3, CsClBr_2, CsI_3, CsBrI_2, CsBr_2I,
&c. (H.L. Wells and S.L. Penfield, _Zeit. fur anorg. Chem._, 1892, i, p.
85). _Caesium sulphate_, Cs_2SO_4, may be prepared by dissolving the
hydroxide or carbonate in