Chapter XXII: ) has the composition K{4}FeC{6}N{6} + 2H{2}O. The name

of cyanogen ([Greek: kuanos]) is derived from the property which this yellow prussiate possesses of forming, with a solution of a ferric salt, FeX_{3}, the familiar pigment Prussian blue. The yellow prussiate is manufactured on a large scale, and is generally used as the source of the other cyanogen compounds.

[41] It has an important historical interest, more especially as at
that time such an easy preparation of substances occurring in
organisms without the aid of organic life was quite unexpected,
for they were supposed to be formed under the influence of the
forces acting in organisms, and without the latter their formation
was considered impossible. And in addition to destroying this
illusion, the easy transition of NH_{4}OCN into CO(NH_{2})_{2} is
the best example of the passage of one system of equilibrium of
atoms into another more stable system.

[42] If ammonia and methane (marsh gas) do not show any acid
properties, that is in all probability due to the presence of a
large amount of hydrogen in both; but in hydrocyanic acid one atom
of hydrogen is under the influence of two acid-forming elements.
Acetylene, C_{2}H_{2}, which contains but little hydrogen,
presents acid properties in certain respects, for its hydrogen is
easily replaced by metals. Hydronitrous acid, HN_{3}, which
contains little hydrogen, also has the properties of an acid.

[43] Solutions of cyanides--for instance, those of potassium or
barium--are decomposed by carbonic acid. Even the carbonic
anhydride of the air acts in a similar way, and for this reason
these solutions do not keep, because, in the first place, free
hydrocyanic acid itself decomposes and polymerises, and, in the
second place, with alkaline liquids it forms ammonia and formic
acid. Hydrocyanic acid does not liberate carbonic anhydride from
solutions of sodium or potassium carbonates. But a mixture of
solutions of potassium carbonate and hydrocyanic acid yields
carbonic anhydride on the addition of oxides like zinc oxide,
mercuric oxide, &c. This is due to the great inclination which the
cyanides exhibit of forming double salts. For instance,
ZnK_{2}(CN)_{4} is formed, which is a soluble double salt.

[43 bis] The conversion of the atmospheric nitrogen into cyanogen
compounds, although possible, has not yet been carried out on a
large scale, and one of the problems for future research should be
the discovery of a practical and economical means of converting
the atmospheric nitrogen into metallic cyanides, not only because
potassium cyanide has found a vast and important use for the
extraction of gold from even the poorest ores, but more especially
because the cyanides furnish the means for effecting the synthesis
of many complex carbon compounds, and the nitrogen contained in
cyanogen easily passes into other forms of combination such as
ammonia, which is of great importance in agriculture.

If four parts of yellow prussiate be mixed with eight parts of water and three parts of sulphuric acid, and the mixture be heated, it decomposes, volatile hydrocyanic acid separating. This was obtained for the first time by Scheele in 1782, but it was only known to him in solution. In 1809 Ittner prepared anhydrous prussic acid, and in 1815 Gay-Lussac finally settled its properties and showed that it contains only hydrogen, carbon, and nitrogen, CNH. If the distillate (a weak solution of HCN) be redistilled, and the first part collected, the anhydrous acid may be prepared from this stronger solution. In order to do this, pieces of calcium chloride are added to the concentrated solution, when the anhydrous acid floats as a separate layer, because it is not soluble in an aqueous solution of calcium chloride. If this layer be then distilled over a new portion of calcium chloride at the lowest temperature possible, the prussic acid may be obtained completely free from water. It is, however, necessary to use the greatest caution in work of this kind, because prussic acid, besides being extremely poisonous, is exceedingly volatile.[44]

[44] The mixture of the vapours of water and hydrocyanic acid, evolved
on heating yellow prussiate with sulphuric acid, may be passed
directly through vessels or tubes filled with calcium chloride.
These tubes must be cooled, because, in the first place,
hydrocyanic acid easily changes on being heated, and, in the
second place, the calcium chloride when warm would absorb less
water. The mixture of hydrocyanic acid and aqueous vapour on
passing over a long layer of calcium chloride gives up water, and
hydrocyanic acid alone remains in the vapour. It ought to be
cooled as carefully as possible in order to bring it into a liquid
condition. The method which Gay-Lussac employed for obtaining pure
hydrocyanic acid consisted in the action of hydrochloric acid gas
on mercuric cyanide. The latter may he obtained in a pure state if
a solution of yellow prussiate be boiled with a solution of
mercuric nitrate, filtered, and crystallised by cooling; the
mercuric cyanide is then obtained in the form of colourless
crystals, Hg(CN)_{2}.

If a strong solution of hydrochloric acid be poured upon these
crystals, and the mixture of vapours evolved, consisting of
aqueous vapour, hydrochloric acid, and hydrocyanic acid, be passed
through a tube containing, first, marble (for absorbing the
hydrochloric acid), and then lumps of calcium chloride, on cooling
the hydrocyanic acid will be condensed. In order to obtain the
latter in an anhydrous form, the decomposition of heated mercury
cyanide by hydrogen sulphide may be made use of. Here the sulphur
and cyanogen change places, and hydrocyanic acid and mercury
sulphide are formed: Hg(CN)_{2} + H_{2}S = 2HCN + HgS.

Anhydrous prussic acid is a very mobile and volatile liquid; its specific gravity is 0·697 at 18°; at lower temperatures, especially when mixed with a small quantity of water, it easily congeals; it boils at 26°, and therefore very easily evaporates, and at ordinary temperatures may be regarded as a gas. An insignificant amount, when inhaled or brought into contact with the skin, causes death. It is soluble in all proportions in water, alcohol, and ether: weak aqueous solutions are used in medicine.[45]

[45] A weak (up to 2 p.c.) aqueous solution of hydrocyanic acid is
obtained by the distillation of certain vegetable substances. The
so-called laurel water in particular enjoys considerable notoriety
from its containing hydrocyanic acid. It is obtained by the
steeping and distillation of laurel leaves. A similar kind of
water is formed by the infusion and distillation of bitter
almonds. It is well known that bitter almonds are poisonous, and
have a peculiar characteristic taste. This bitter taste is due to
the presence of a certain substance called amygdalin, which can be
extracted by alcohol. This amygdalin decomposes in an infusion of
bruised almonds, forming the so-called bitter almond oil, glucose,
and hydrocyanic acid:

C_{10}H_{27}NO_{11} + H_{2}O = C_{7}H_{6}O + CNH + 2C_{6}H_{12}O_{6}
Amygdalin in Water Bitter Hydrocyanic Glucose
bitter almonds almond acid
oil

If after this the infusion of bitter almonds be distilled with
water, the hydrocyanic acid and the volatile bitter almond oil are
carried over with the aqueous vapour. The oil is insoluble in
water, or only sparingly soluble, while the hydrocyanic acid
remains as an aqueous solution. Bitter almond water is similar to
laurel water, and is used like the former in medicine, naturally
only in small quantities because any considerable amount has
poisonous effects. Perfectly pure anhydrous hydrocyanic acid keeps
without change, just like the weak solutions, but the strong
solutions only keep in the presence of other acids. In the
presence of many admixtures these solutions easily give a brown
polymeric substance, which is also formed in a solution of
potassium cyanide.

The salts MCN--for instance, potassium, sodium, ammonium--as well as the salts M´´(CN)_{2}--for example, barium, calcium, mercury--are soluble in water, but the cyanides of manganese, zinc, lead, and many others are insoluble in water. They form double salts with potassium cyanide and similar metallic cyanides, an example of which we will consider in a further description of the yellow prussiate. Not only are some of the double salts remarkable for their constancy and comparative stability, but so also are the soluble salt HgC_{2}N_{2}, the insoluble silver cyanide AgCN, and even potassium cyanide in the absence of water. The last salt,[46] when fused, acts as a reducing agent with its elements K and C, and oxidises when fused with lead oxide, forming potassium cyanate, KOCN, which establishes the connection between HCN and OHCN--that is, between the nitriles of formic and carbonic acids--and this connection is the same as that between the acids themselves, since formic acid, on oxidation, yields carbonic acid. Free cyanogen, (CN)_{2} or CNCN, corresponds to hydrocyanic acid in the same manner as free chlorine, Cl_{2} or ClCl, corresponds to hydrochloric acid. This composition, judging from what has been already stated, exactly expresses that of the nitrile of oxalic acid, and, as a matter of fact, oxalate of ammonia and the amide corresponding with it (oxamide, Note 33), on being heated with phosphoric anhydride, which takes up the water, yield _cyanogen_, (CN)_{2}. This substance is also produced by simply heating some of the metallic cyanides. Mercuric cyanide is particularly adapted for this purpose, because it is easily obtained in a pure state and is then very stable. If mercuric cyanide be heated, it decomposes, in like manner to mercury oxide, into metallic mercury and cyanogen: HgC_{2}N_{2} = Hg + C_{2}N_{2}.[47] When cyanogen is formed, part of it always polymerises into a dark brown insoluble substance called _paracyanogen_, capable of forming cyanogen when heated to redness.[48] Cyanogen is a colourless, poisonous gas, with a peculiar smell and easily condensed by cooling into a colourless liquid, insoluble in water and having a specific gravity of 0·86. It boils at about -21°, and therefore cyanogen may be easily condensed into a liquid by a strong freezing mixture. At -35° liquid cyanogen solidifies. The gas is soluble in water and in alcohol to a considerable extent--namely, 1 volume of water absorbs as much as 4-1/2 volumes, and alcohol 23 volumes. Cyanogen resists the action of a tolerably high temperature without decomposing, but under the action of the electric spark the carbon is separated, leaving a volume of nitrogen equal to the volume of the gas taken. As it contains carbon it burns, and the colour of the flame is reddish-violet, which is due to the presence of nitrogen, all compounds of which impart more or less of this reddish-violet hue to the flame. During the combustion of cyanogen, carbonic anhydride and nitrogen are formed. The same products are obtained in the eudiometer with oxygen or by the action of cyanogen on many oxides at a red heat.

[46] This salt will be described in Chapter XIII.

[47] For the preparation it is necessary to take completely dry mercuric
cyanide, because when heated in the presence of moisture it gives
ammonia, carbonic anhydride, and hydrocyanic acid. Instead of
mercuric cyanide, a mixture of perfectly dry yellow prussiate and
mercuric chloride may be used, then double decomposition and the
formation of mercuric cyanide take place in the retort. Silver
cyanide also disengages cyanogen, on being heated.

[47] _Paracyanogen_ is a brown substance (having the composition of
cyanogen) which is formed during the preparation of cyanogen by
all methods, and remains as a residue. Silver cyanide, on being
slightly heated, fuses, and on being further heated evolves a gas;
a considerable quantity of paracyanogen remains in the residue.
Here it is remarkable that exactly half the cyanogen becomes
gaseous, and the other half is transformed into paracyanogen.
Metallic silver will be found in the residue with the
paracyanogen; it may be extracted with mercury or nitric acid,
which does not act on paracyanogen. If paracyanogen be heated in a
vacuum it decomposes, forming cyanogen; but here the pressure _p_
for a given temperature _t_ cannot exceed a certain limit, so that
the phenomenon presents all the external appearance of a physical
transformation into vapour; but, nevertheless, it is a complete
change in the nature of the substance, though limited by the
_pressure of dissociation_, as we saw before in the transformation
of cyanuric into hydrocyanic acid, and as would be expected from
the fundamental principles of dissociation. Troost and
Hautefeuille (1868) found that for paracyanogen,

_t_ = 530° 581° 600° 635°
_p_ = 90 143 296 1,089 mm.

However, even at 550° part of the cyanogen decomposes into carbon
and nitrogen. The reverse transition of cyanogen into paracyanogen
commences at 350°, and at 600° proceeds rapidly. And if the
transition of the first kind is likened to evaporation, then the
reverse transition, or polymerisation, presents a likeness to the
transition of vapours into the solid state.

The relation of cyanogen to the metallic cyanides is seen not only in the fact that it is formed from mercuric cyanide, but also by its forming cyanide of sodium or potassium on being heated with either of those metals, the sodium or potassium taking fire in the cyanogen. On heating a mixture of hydrogen and cyanogen to 500° (Berthelot),[49] or under the action of the silent discharge (Boilleau), hydrocyanic acid is formed, so that the reciprocity of the transitions does not leave any doubt in the matter that all the nitriles of the organic acids contain cyanogen, just as all the organic acids contain carboxyl and in it the elements of carbonic anhydride. Besides the amides,[50] the nitriles (or cyanogen compounds, RCN), and nitro-compounds (containing the radicle of nitric acid, RNO_{2}), there are a great number of other substances containing at the same time carbon and nitrogen, particulars of which must be sought for in special works on organic chemistry.

[49] Cyanogen (like chlorine) is absorbed by a solution of sodium
hydroxide, sodium cyanide and cyanate being produced: C_{2}N_{2} +
2NaHO = NaCN + CNNaO + H_{2}O. But the latter salt decomposes
relatively easily, and moreover part of the cyanogen liberated by
heat from its compounds undergoes a more complex transformation.

[50] If, in general, compounds containing the radicle NH_{2} are called
amides, some of the _amines_ ought to be ranked with them; namely,
the hydrocarbons C_{_n_}H_{2_m_}, in which part of the hydrogen is
replaced by NH_{2}; for instance, methylamine, CH_{3}NH_{2},
aniline, C_{6}H_{5}NH_{2}, &c. In general the amines may be
represented as ammonia in which part or all of the hydrogen is
replaced by hydrocarbon radicles--as, for example, trimethylamine,
N(CH_{3})_{3}. They, like ammonia, combine with acids and form
crystalline salts. Analogous substances are sometimes met with in
nature, and bear the general name of _alkaloids_; such are, for
instance, quinine in cinchona bark, nicotine in tobacco, &c.