92 NITRIDES.the sun, as shown by the spectroscope. When cyanogen is once formed, its
rate of decomposition at a lower temperature is so small that it may be pre-
pared pure in an indirect manner from its compounds and preserved undecom-
posed; cf. No. 64, Acetylene, and No. 67, Hydrogen Peroxide.
Free cyanogen gas which has the formula C 2 N 2 should not be confused with
the univalent radical CN. The radical CN forms a stable atomic grouping
which behaves in the same manner as the halogen atoms in halides, and it
bears the same relation to uncombined cyanogen as the chlorine ion, for
example, bears to free chlorine.
For the preparation of cyanogen gas, hydrogen cyanide (hydrocyanic acid)
is first prepared by Wohler's method of treating either sodium or potassium
ferrocyanide with sulphuric acid. The hydrogen cyanide, together with the
residues, is worked over into mercuric cyanide and the latter decomposed by
heat.
Hydrogen Cyanide from Potassium Ferrocyanide. Mercuric
Cyanide.2 K 4 [Fe(CN) 6 ] + 3 H 2 SO 4 = 3 K 2 SO 4 + K 2 Fe[Fe(CN) 6 ] + 6 HCN
6 HCN + 3 HgO = 3 Hg(CN) 2 + 3 H 2 O.Place a 400 to 500 c.c. flask on a Babo boiling funnel, and
through the cork, which fits its neck tightly, pass a delivery tube
leading to a condenser; fit the lower end of the condenser by means
of another cork stopper into an adapter that just dips into 100
c.c. of water in the receiving flask. Introduce into the distilling
flask 100 g. of coarsely broken potassium ferrocyanide and a
cooled mixture of 70 g. concentrated sulphuric acid and 130 c.c.
water. Distil until the residue in the flask consists of a thin
white slime ["prussic acid residue," K 2 Fe[Fe(CN) 6 ](?)]. Set
aside about 15 c.c. of the distillate. Dilute the remainder with
water to a volume of 300 to 400 c.c. and add mercuric oxide (about
75 grams), shaking well, until the solution no longer smells of
hydrocyanic acid (Caution) and shows a neutral or barely alkaline
reaction. Then pour in the reserved 15 c.c. of hydrocyanic acid
and evaporate the solution under a good hood till it crystallizes.
If necessary, purify the preparation by recrystallization. Almost
the theoretical yield of 85 to 90 grams is obtained.
In this and the following work regard should be paid to the extremely
poisonous nature of hydrocyanic (prussic) acid, and the operations
should be carried out under a well-ventilated hood in the special roomfor noxious materials, or in the open air.
Working up of the Prussic Acid Residues. Wash the residue