boils at 93.1 deg.C., and has a specific gravity of 1.144 (15 deg.
C.). When heated with ammonia it yields urethane. Sodium amalgam
converts it into formic acid; whilst with alcohol it yields the normal
carbonic ester. It is easily broken down by many substances (aluminium
chloride, zinc chloride, &c.) into ethyl chloride and carbon dioxide.

_Percarbonates._--Barium percarbonate, BaCO4, is obtained by passing
an excess of carbon dioxide into water containing barium peroxide in
suspension; it is fairly stable, and yields hydrogen peroxide when
treated with acids (E. Merck, _Abs. J.C.S._, 1907, ii. p. 859). Sodium
percarbonates of the formulae Na2CO4, Na2C2O6, Na2CO5, NaHCO4 (two
isomers) are obtained by the action of gaseous or solid carbon dioxide
on the peroxides Na2O2, Na2O3, NaHO2 (two isomers) in the presence of
water at a low temperature (R. Wolffenstein and E. Peltner, _Ber._,
1908, 41, pp. 275, 280). Potassium percarbonate, K2C2O6, is obtained
in the electrolysis of potassium carbonate at -10 to -15 deg.

CARBON BISULPHIDE, CS2, a chemical product first discovered in 1796 by
W.A. Lampadius, who obtained it by heating a mixture of charcoal and
pyrites. It may be more conveniently prepared by passing the vapour of
sulphur over red hot charcoal, the uncondensed gases so produced being
led into a tower containing plates over which a vegetable oil is allowed
to flow in order to absorb any carbon bisulphide vapour, and then into a
second tower containing lime, which absorbs any sulphuretted hydrogen.
The crude product is very impure and possesses an offensive smell; it
may be purified by forcing a fine spray of lime water through the liquid
until the escaping water is quite clear, the washed bisulphide being
then mixed with a little colourless oil and distilled at a low
temperature. For further methods of purification see J. Singer (_Journ.
of Soc. Chem. Ind._, 1889, p. 93), Th. Sidot (_Jahresb._, 1869, p. 243),
E. Allary (_Bull. de la Soc. Chim._, 1881, 35, p. 491), E. Obach (_Jour.
prak. Chem._, 1882 (2), 26, p. 282).

When perfectly pure, carbon bisulphide is a colourless, somewhat
pleasant smelling, highly refractive liquid, of specific gravity 1.2661
(18 deg./4 deg.) (J.W. Bruhl) or 1.29215 (0 deg./4 deg.) (T.E. Thorpe).
It boils at 46.04 deg. C. (T.E. Thorpe, _Journ. Chem. Soc._, 1880, 37,
p. 364). Its critical temperature is 277.7 deg. C., and its critical
pressure is 78.1 at