}. The sulphuric acid, owing to its affinity for water,
decomposes the oxalic acid, as represented in the equation

C_{2}H_{2}O_{4} + (H_{2}SO_{4}) = (H_{2}SO_{4}) + H_{2}O + CO_{2} + CO.

~Properties.~ Carbon monoxide is a light, colorless, almost odorless gas,
very difficult to liquefy. Chemically it is very active, combining
directly with a great many substances. It has a great affinity for
oxygen and is therefore combustible and a good reducing agent. Thus, if
carbon monoxide is passed over hot copper oxide, the copper is reduced
to the metallic state:

CuO + CO = Cu + CO_{2}.

When inhaled it combines with the red coloring matter of the blood and
in this way prevents the absorption of oxygen, so that even a small
quantity of the gas may prove fatal.

[Illustration: Fig. 61]

~The reducing power of carbon monoxide.~ Fig. 61 illustrates a
method of showing the reducing power of carbon monoxide. The
gas is generated by gently heating 7 or 8 g. of oxalic acid
with 25 cc. of concentrated sulphuric acid in a 200 cc. flask
A. The bottle B contains a solution of sodium hydroxide,
which removes the carbon dioxide formed along with the
monoxide. C contains a solution of calcium hydroxide to show
that the carbon dioxide is completely removed. E is a
hard-glass tube containing 1 or 2 g. of copper oxide, which is
heated by a burner. The black copper oxide is reduced to
reddish metallic copper by the carbon monoxide, which is
thereby changed to carbon dioxide. The presence of the carbon
dioxide is shown by the precipitate in the calcium hydroxide
solution in D. Any unchanged carbon monoxide is collected
over water in F.

~Carbon disulphide~ (CS_{2}). Just as carbon combines with oxygen to form
carbon dioxide, so it combines with sulphur to form carbon disulphide
(CS_{2}). This compound has been described in the chapter on sulphur.

~Hydrocyanic acid~ (_prussic acid_)(HCN). Under the proper conditions
carbon unites with nitrogen and hydrogen to form the acid HCN, called
hydrocyanic acid. It is a weak, volatile acid, and is therefore easily
prepared by treating its salts with sulphuric acid:

KCN + H_{2}SO_{4} = KHSO_{4} + HCN.

It is most familiar as a gas, though it condenses to a colorless liquid
boiling at 26 deg.. It has a peculiar odor, suggesting bitter almonds, and
is extremely poisonous either when inhaled or when taken into