ygen, by the direct decomposition
of many carbonates by heat, and by the decomposition of carbonates
with mineral acids, M2CO8 + 2HCl = 2MCl + H2O + CO2. It is also
formed in ordinary fermentation processes, in the combustion of all
carbon compounds (oil, gas, candles, coal, &c.), and in the process of
respiration.

It is a colourless gas, possessing a faint pungent smell and a
slightly acid taste. It does not burn, and does not support ordinary
combustion, but the alkali metals and magnesium, if strongly heated,
will continue to burn in the gas with formation of oxides and
liberation of carbon. Its specific gravity is 1.529 (air = 1). It is
readily condensed, passing into the liquid condition at 0 deg. C.
under a pressure of 35 atmospheres. Its critical temperature is 31.35
deg. C., and its critical pressure is 72.9 atmos. The liquid boils at
-78.2 deg. C. (l atmo.), and by rapid evaporation can be made to
solidify to a snow-white solid which melts at -65 deg. C.(see LIQUID
GASES). Carbon dioxide is moderately soluble in water, its coefficient
of solubility at 0 deg. C. being 1.7977 (R. Bunsen). It is still more
soluble in alcohol. The solution of the gas in water shows a faintly
acid reaction and is supposed to contain _carbonic acid_, H2CO3. The
gas is rapidly absorbed by solutions of the caustic alkalis, with the
production of alkaline carbonates (q.v.), and it combines readily with
potassium hydride to form potassium formate. It unites directly with
ammonia gas to form ammonium carbamate, NH2COONH4. It may be readily
recognized by the white precipitate which it forms when passed through
lime or baryta water. Carbon dioxide dissociates, when strongly
heated, into carbon monoxide and oxygen, the reaction being a balanced
action; the extent of dissociation for varying temperatures and
pressures has been calculated by H. Le Chateller (_Zeit. Phys. Chem._,
1888, 2, p. 782; see H. Sainte-Claire Deville, _Comptes rendus_, 1863,
56, p. 195 et seq.). The volume composition of carbon dioxide is
determined by burning carbon in oxygen, when it is found that the
volume of carbon dioxide formed is the same as that of the oxygen
required for its production, hence carbon dioxide contains its own
volume of oxygen. Carbon dioxide finds industrial application in the
preparation of soda by the Solvay process, in the sugar industry, in
the manufacture of mineral wa