A Manual of the Chemistry of the Carbon Compounds

50 TEE CHEMISTRY OF
By the inverse reaction, that is by taking the elements of water
out of ammonium oxalate and oxamide, which may be done by heating
them with phosphorus peutoxide, cyanogen gas is again formed
Potassium combines directly with cyanogen gaa under the influence
of heat, and by passing the gas into potash solution or over red-hot
potassium carbonate, a mixture of cyanide and cyanate is formed :—
C9N2 + K2CO8 * CNK + CNOK + COg
In the preparation of cyanogen a brown powder is left behind
which is a polymeric modification of the gas, and is called paraeyanogm.
By continued application of heat this body is completely
transformed into gaseous cyanogen.
HYDROGEN CYANIDE, HYDKOCYANIC OR PHUSSIC ACID CNH.
This compound is formed by replacing the metal in metallic
cyanides by hydrogen, and may be obtained by acting on potassium
cyanide with a dilute acid, or by decomposing mercuric eyanide with
hydrogen sulphide. It is, however, best prepared by heating a mixture
of 10 parts of coarsely powdered potassium ferrocyanide with a perfectly
cold mixture of 7 parts of sulphuric acid with (according to the
strength of the prussic acid required) 14—40 parts of water. The
flask must be connected with a good condenser, the lower end of
which dips under water contained in a receiver. To prepare the
anhydrous acid, the sulphuric acid is diluted with twice its weight of
water, and the vaponr after having passed a U tube which is filled
with calcium chloride, and immersed in water of 30°, is condensed in
a vessel surrounded by a freezing mixture.
Pure hydrocyanic acid is a colourless liquid, boiling at 26*5', and
solidifying at — 15° to a mass like ice. It is excessively poisonous,
a drop producing instant death when swallowed, and the inhalation
of its vapour ia likewise instantly fatal. It has a peculiar odour, and
its vapour when diluted with au" produces, when inhaled, a peculiar
irritation in the throat. It dissolves in water, alcohol, and ether, ia
all proportion^ and is, as well ns its concentrated aqueous solution,
inflammable, burning with a purple flame.
The anhydrous acid soon undergoes spontaneous decomposition;
giving off ammonia, aud leaving a brown residua It suffers the aanie
decomposition in an aqueous solution, a part, however, being converted
into iininiouiuiii formate. This decomposition is almost entirely
prevented by the presence of a small quantity of formic or a
miueral acid; but when it is mixed with strong acids, water is taken
up, and ammonia and formic acid are produced. On the other hand,
when ammonium formate is heated, it splits up into water and prussic
acid:—
NGH