ssium ferrocyanide with a large excess
of concentrated sulphuric acid, K4Fe(CN)6 + 6H2SO4 + 6H2O = 2K2SO4 +
FeSO4 + 3(NH4)2SO4 + 6CO. It is a colourless, odourless gas of
specific gravity 0.967 (air = 1). It is one of the most difficultly
liquefiable gases, its critical temperature being -139.5 deg. C., and
its critical pressure 35.5 atmos. The liquid boils at -190 deg. C.,
and solidifies at -211 deg.C. (L.P. Cailletet, _Comptes rendus_, 1884,
99, p. 706). It is only very slightly soluble in water. It burns with
a characteristic pale blue flame to form carbon dioxide. It is very
poisonous, uniting with the haemoglobin of the blood to form
carbonyl-haemoglobin. It is a powerful reducing agent, especially at
high temperatures. It is rapidly absorbed by an ammoniacal or acid
(hydrochloric acid) solution of cuprous chloride. It unites directly
with chlorine, forming carbonyl chloride or phosgene (see below), and
with nickel and iron to form nickel and iron carbonyls (see NICKEL and
IRON). It also combines directly with potassium hydride to form
potassium formate (see FORMIC ACID). The volume composition of carbon
monoxide is established by exploding a mixture of the gas with oxygen,
two volumes of the gas combining with one volume of oxygen to form two
volumes of carbon dioxide. This fact, coupled with the determination
of the vapour density of the gas, establishes the molecular formula
CO.

_Carbon dioxide_, CO2, is a gas first distinguished from air by van
Helmont (1577-1644), who observed that it was formed in fermentation
processes and during combustion, and gave to it the name _gas
sylvestre_. J. Black (_Edin. Phys. and Lit. Essays_, 1755) showed that
it was a constituent of the carbonated alkalis and called it "fixed
air." T.O. Bergman, in 1774, pointed out its acid character, and A.L.
Lavoisier (1781-1788) first proved it to be an oxide of carbon by
burning carbon in the oxygen obtained from the decomposition of
mercuric oxide. It is a regular constituent of the atmosphere, and is
found in many spring waters and in volcanic gases; it also occurs in
the uncombined condition at the Grotto del Cane (Naples) and in the
Poison Valley (Java). It is a constituent of the minerals cerussite,
malachite, azurite, spathic iron ore, calamine, strontianite,
witherite, calcite aragonite, limestone, &c. It may be prepared by
burning carbon in excess of air or ox