Chapter XIV (2)

In its physical _properties_ carbonic oxide resembles nitrogen; this is explained by the equality of their molecular weights. The absence of colour and smell, the low temperature of the absolute boiling point, -140° (nitrogen, -146°), the property of solidifying at -200° (nitrogen, -202°), the boiling point of -190° (nitrogen, -203°), and the slight solubility (Chapter I., Note 30), of carbonic oxide are almost the same as in those of nitrogen. The chemical properties of both gases are, however, very different, and in these carbonic oxide resembles hydrogen. Carbonic oxide burns with a blue flame, giving 2 volumes of carbonic anhydride from 2 volumes of carbonic oxide, just as 2 volumes of hydrogen give 2 volumes of aqueous vapour. It explodes with oxygen, in the eudiometer, like hydrogen.[29] When breathed it acts as a strong poison, being absorbed by the blood;[30] this explains the action of charcoal fumes, the products of the incomplete combustion of charcoal and other carbonaceous fuels. Owing to its faculty of combining with oxygen, carbonic oxide acts as a powerful reducing agent, taking up the oxygen from many compounds at a red heat, and being itself transformed into carbonic anhydride. The reducing action of carbonic oxide, however, is (like that of hydrogen, Chapter II.) naturally confined to those oxides which easily part with their oxygen--as, for instance, copper oxide--whilst the oxides of magnesium or potassium are not reduced. Metallic iron itself is capable of reducing carbonic anhydride to carbonic oxide, just as it liberates the hydrogen from water. Copper, which does not decompose water, does not decompose carbonic oxide. If a platinum wire heated to 300°, or spongy platinum at the ordinary temperature, be plunged into a mixture of carbonic oxide and oxygen, or of hydrogen and oxygen, the mixture explodes. These reactions are very similar to those peculiar to hydrogen. The following important distinction, however, exists between them--namely: the molecule of hydrogen is composed of H_{2}, a group of elements divisible into two like parts, whilst, as the molecule of carbonic oxide, CO, contains unlike atoms of carbon and oxygen, in none of its reactions of combination can it give two molecules of matter containing its elements. This is particularly noticeable in the action of chlorine on hydrogen and on carbonic oxide respectively; with the former chlorine forms hydrogen chloride, and with the latter it produces the so-called carbonyl chloride, COCl_{2}: that is to say, the molecule of hydrogen, H_{2}, under the action of chlorine divides, forming two molecules of hydrochloric acid, whilst the molecule of carbonic oxide enters in its entirety into the molecule of carbonyl chloride. This characterises the so-called _diatomic_ or _bivalent_ reactions of radicles or _residues_. H is a monatomic residue or radicle, like K, Cl, and others, whilst carbonic oxide, CO, is an indivisible (undecomposable) bivalent radicle, equivalent to H_{2} and not to H, and therefore combining with X_{2} and interchangeable with H_{2}. This distinction is evident from the annexed comparison:

HH, hydrogen. CO, carbonic oxide.
HCl, hydrochloric acid. COCl_{2}, carbonyl chloride.
HKO, potash. CO(KO)_{2}, potassium carbonate.
HNH_{2}, ammonia. CO(NH_{2})_{2}, urea.
HCH_{3}, methane. CO(CH_{3})_{2}, acetone.
HHO, water. CO(HO)_{2}, carbonic acid.

[29] It is remarkable that, according to the investigations of Dixon,
perfectly dry carbonic oxide does not explode with oxygen when a
spark of low intensity is used, but an explosion takes place if
there is the slightest admixture of moisture. L. Meyer, however,
showed that sparks of an electric discharge of considerable
intensity produce an explosion. N. N. Beketoff demonstrated that
combustion proceeds and spreads slowly unless there be perfect
dryness. I think that this may he explained by the fact that water
with carbonic oxide gives carbonic anhydride and hydrogen, but
hydrogen with oxygen gives hydrogen peroxide (Chapter VII.), which
with carbonic oxide forms carbonic anhydride and water. The water,
therefore, is renewed, and again serves the same purpose. But it
may be that here it is necessary to acknowledge a simple contact
influence. After Dixon had shown the influence of traces of
moisture upon the reaction CO + O, many researches were made of a
similar nature. The fullest investigation into the influence of
moisture upon the course of many chemical reactions was made by
Baker in 1894. He showed that with perfect dryness, many chemical
transformations (for example, the formation of ozone from oxygen,
the decomposition of AgO, KClO_{3} under the action of heat, &c.)
proceeds in exactly the same manner as in the presence of
moisture; but that in many cases traces of moisture have an
evident influence. We may mention the following instances: (1) Dry
SO_{3} does not act upon dry CaO or CuO; (2) perfectly dry
sal-ammoniac does not give NH_{3} with dry CaO, but simply
volatilises; (3) dry NO and O do not react; (4) perfectly dry
NH_{3} and HCl do not combine; (5) perfectly dry sal-ammoniac does
not dissociate at 350° (Chapter VII., Note 15 bis); and (6)
perfectly dry chlorine does not act upon metals, &c.

[30] Carbonic oxide is very rapid in its action, because it is absorbed
by the blood in the same way as oxygen. In addition to this, the
absorption spectrum of the blood changes so that by the help of
blood it is easy to detect the slightest traces of carbonic oxide
in the air. M. A. Kapoustin found that linseed oil and therefore
oil paints, are capable of giving off carbonic oxide while drying
(absorbing oxygen).

Such monatmic (univalent) residues, X, as H, Cl, Na, NO_{2}, NH_{4}, CH_{3}, CO_{2}H (carboxyl), OH, and others, in accordance with the law of substitution, combine together, forming compounds, XX'; and with oxygen, or in general with diatomic (bivalent) residues, Y--for instance, O, CO, CH_{2}, S, Ca, &c. forming compounds XX´Y; but diatomic residues, Y, sometimes capable of existing separately may combine together, forming YY´ and with X_{2} or XX´, as we see from the transition of CO into CO_{2} and COCl_{2}. This combining power of carbonic oxide appears in many of its reactions. Thus it is very easily absorbed by cuprous chloride, CuCl, dissolved in fuming hydrochloric acid, forming a crystalline compound, COCu_{2}Cl_{2},2H_{2}O, decomposable by water; it combines directly with potassium (at 90°), forming (KCO)_{_n_}[31] with platinum dichloride, PtCl_{2}, with chlorine, Cl_{2}, &c.

[31] The molecule of metallic potassium (Scott, 1887), like that of
mercury, contains only one atom, and it is probably in virtue of
this that the molecules CO and K combine together. But as in the
majority of cases potassium acts as a univalent radicle, the
polymeride K_{2}C_{2}O_{2} is formed, and probably
K_{10}C_{10}O_{10}, because products containing C_{10} are formed
by the action of hydrochloric acid. The black mass formed by the
combination of carbonic oxide with potassium explodes with great
ease, and oxidises in the air. Although Brodie, Lerch, and Joannis
(who obtained it in 1873 in a colourless form by means of NH_{3}K,
described in Chapter VI., Note 14) have greatly extended our
knowledge of this compound, much still remains unexplained. It
probably exists in various polymeric and isomeric forms, having
the composition (KCO)_{_n_} and (NaCO)_{_n_}.

But the most remarkable compounds are (1) the compound of CO with metallic nickel, a colourless volatile liquid, Ni(CO)_{4}, obtained by L. Mond (described in Chapter XXII.) and (2) the compounds of carbonic oxide with the alkalis, for instance with potassium or barium hydroxide, &c.--although it is not directly absorbed by them, as it has no acid properties. Berthelot (1861) showed that potash in the presence of water is capable of absorbing carbonic oxide, but the absorption takes place slowly, little by little, and it is only after being heated for many hours that the whole of the carbonic oxide is absorbed by the potash. The salt CHKO_{2} is obtained by this absorption; it corresponds with an acid found in nature--namely, the simplest organic (carboxylic) acid, _formic acid_, CH_{2}O_{2}. It can be extracted from the potassium salt by means of distillation with dilute sulphuric acid, just as nitric acid is prepared from sodium nitrate. The same acid is found in ants and in nettles (when the stings of the nettles puncture the skin they break, and the corrosive formic acid enters into the body); it is also obtained during the action of oxidising agents on many organic substances; it is formed from oxalic acid, and under many conditions splits up into carbonic oxide and water. In the formation of formic acid from carbonic oxide we observe an example of the synthesis of organic compounds, such as are now very numerous, and are treated of in detail in works on organic chemistry.

Formic acid, H(CHO_{2}), carbonic acid, HO(CHO_{2}), and oxalic acid, (CHO_{2})_{2}, are the simple organic or carboxylic acids, R(CHO_{2}) corresponding with HH and HOH. Commencing with carbonic oxide, CO, the formation of carboxylic acids is clearly seen from the fact that CO is capable of combining with X_{2}, that is of forming COX_{2}. If, for instance, one X is an aqueous residue, OH (hydroxyl), and the other X is hydrogen, then the simplest organic acid--formic acid, H(COOH)--is obtained. As all hydrocarbons (Chapter VIII.) correspond with the simplest, CH_{4}, so all organic acids may be considered to proceed from formic acid.

In a similar way it is easy to explain the relation to other compounds of carbon of those compounds which contain nitrogen. By way of an example, we will take one of the carboxyl acids, R(CO_{2}H), where R is a hydrocarbon radicle (residue). Such an acid, like all others, will give by combination with NH_{3} an ammoniacal salt, R(CO_{2}NH_{4}). This salt contains the elements for the formation of two molecules of water, and under suitable conditions by the action of bodies capable of taking it up, water may in fact be separated from R(CO_{2}NH_{4}), forming by the loss of one molecule of water, _amides_, RCONH_{2}, and by the loss of two molecules of water, _nitriles_, RCN, otherwise known as _cyanogen compounds_ or _cyanides_.[32] If all the carboxyl acids are united not only by many common reactions but also by a mutual conversion into each other (an instance of which we saw above in the conversion of oxalic acid into formic and carbonic acids) one would expect the same for all the cyanogen compounds also. The common character of their reactions, and the reciprocity of their transformation, were long ago observed by Gay-Lussac, who recognised a common group or radicle (residue) cyanogen, CN, in all of them. The simplest compounds are _hydrocyanic_ or _prussic acid_, HCN, cyanic acid, OHCN, and free cyanogen, (CN)_{2}, which correspond to the three simplest carboxyl acids: formic, HCO_{2}H, carbonic, OHCO_{2}H, and oxalic, (CO_{2}H)_{2}. Cyanogen, like carboxyl, is evidently a monatomic residue and acid, similar to chlorine. As regards the amides RCONH_{2}, corresponding to the carboxyl acids, they contain the ammoniacal residue NH_{2}, and form a numerous class of organic compounds met with in nature and obtained in many ways,[33] but not distinguished by such characteristic peculiarities as the cyanogen compounds.

[32] The connection of the cyanogen compounds with the rest of the
hydrocarbons by means of carboxyl was enunciated by me, about the
year 1860, at the first Annual Meeting of the Russian Naturalists.

[33] Thus, for instance, _oxamide_, or the amide of oxalic acid,
(CNH_{2}O)_{2}, is obtained in the form of an insoluble
precipitate on adding a solution of ammonia to an alcoholic
solution of ethyl oxalate, (CO_{2}C_{2}H_{5})_{2}, which is formed
by the action of oxalic acid on alcohol: (CHO_{2})_{2} +
2(C_{2}H_{5})OH = 2HOH + (CO_{2}C_{2}H_{5})_{2}. As the nearest
derivatives of ammonia, the amides treated with alkalis yield
ammonia and form the salt of the acid. The nitriles do not,
however, give similar reactions so readily. The majority of amides
corresponding to acids have a composition RNH_{2}, and therefore
recombine with water with great ease even when simply boiled with
it, and with still greater facility in presence of acids or
alkalis. Under the action of alkalis the amides naturally give off
ammonia, through the combination of water with the amide, when a
salt of the acid from which the amide was derived is formed:
RNH_{2} + KHO = RKO + NH_{3}.

The same reaction takes place with acids, only an ammoniacal salt
of the acid is of course formed whilst the acid held in the amide
is liberated: RNH_{2} + HCl + H_{2}O = RHO + NH_{4}Cl.

Thus in the majority of cases amides easily pass into ammoniacal
salts, but they differ essentially from them. No ammoniacal salt
sublimes or volatilises unchanged, and generally when heated it
gives off water and yields an amide, whilst many amides volatilise
without alteration and frequently are volatile crystalline
substances which may be easily sublimed. Such, for instance, are
the amides of benzoic, formic, and many other organic acids.

The reactions and properties of the amides and nitriles of the organic acids are described in detail in books on organic chemistry; we will here only touch upon the simplest of them, and to clearly explain the derivative compounds will first consider the ammoniacal salts and amides of carbonic acid.

As carbonic acid is bibasic, its ammonium salts ought to have the following composition: _acid carbonate of ammonium_, H(NH_{4})CO_{3}, and _normal carbonate_, (NH_{4})_{2}CO_{3}; they represent compounds of one or two molecules of ammonia with carbonic acid. The acid salt appears in the form of a non-odoriferous and (when tested with litmus) neutral substance, soluble at the ordinary temperature in six parts of water, insoluble in alcohol, and obtainable in a crystalline form either without water of crystallisation or with various proportions of it. If an aqueous solution of ammonia be saturated with an excess of carbonic anhydride, and then evaporated over sulphuric acid in the bell jar of an air-pump, crystals of this salt are separated. Solutions of all other ammonium carbonates, when evaporated under the air-pump, yield crystals of this salt. A solution of this salt, even at the ordinary temperature, gives off carbonic anhydride, as do all the acid salts of carbonic acid (for instance, NaHCO_{3}), and at 38° the separation of carbonic anhydride takes place with great rapidity. _On losing carbonic anhydride_ and water, the acid salt is converted into the normal salt, 2(NH_{4})HCO_{3} = H_{2}O + CO_{2} + (NH_{4})2CO_{3}; the latter, however, decomposes in solution, and can therefore only be obtained in crystals, (NH_{4})_{2}CO_{3},H_{2}O, at low temperatures, and from solutions containing _an excess of ammonia_ as the product of dissociation of this salt: (NH_{4})_{2}CO_{3} = NH_{3} + (NH_{4})HCO_{3}. But the normal salt,[34] according to the general type, is capable of decomposing _with separation of water_, and forming _ammonium carbamate_, NH_{4}O(CONH_{2}) = (NH_{4})_{2}CO_{3}-H_{2}O; this still further complicates the chemical transformations of the carbonates of ammonium. It is in fact evident that, by changing the ratios of water, ammonia, and carbonic acid, various intermediate salts will be formed containing mixtures or combinations of those mentioned above. Thus the ordinary commercial _carbonate of ammonia_ is obtained by heating a mixture of chalk and sulphate of ammonia (Chapter VI.), or sal-ammoniac, 2NH_{4}Cl + CaCO_{3} = CaCl_{2} + (NH_{4})_{2}CO_{3}. The normal salt, however, through loss of part of the ammonia, partly forms the acid salt, and, partly through loss of water, forms carbamate, and most frequently presents the composition NH_{4}O(CONH_{2}) + 2OH(CO_{2}NH_{4}) = 4NH_{3} + 3CO_{2} + 2H_{2}O. This salt, in parting under various conditions with ammonia, carbonic anhydride, and water, does not present a constant composition, and ought rather to be regarded as a mixture of acid salt and amide salt. The latter must be recognised as entering into the composition of the ordinary carbonate of ammonia, because it contains less water than is required for the normal or acid salt;[35] but on being dissolved in water this salt gives a mixture of acid and normal salts.

[34] The acid salt, (NH_{4})HCO_{3}, on losing water ought to form the
_carbamic acid_, OH(CNH_{2}O); but it is not formed, which is
accounted for by the instability of the acid salt itself. Carbonic
anhydride is given off and ammonia is produced, which gives
ammonium carbamate.

[35] In the normal salt, 2NH_{3} + CO_{2} + H_{2}O, in the acid salt,
NH_{3} + CO_{2} + H_{2}O, but in the commercial salt only 2H_{2}O
to 3CO_{2}.

Each of the two ammoniacal salts of carbonic acid has its corresponding amide. That of the acid salt should be acid, if the water given off takes up the hydrogen of the ammonia, as it should according to the common type of formation of the amides, so that OHCONH_{2}, or _carbamic acid_, is formed from OHCO_{3}NH_{4}. This acid is not known in a free state, but its corresponding ammoniacal salt or _ammonium carbamate_ is known. The latter is easily and immediately formed by mixing 2 volumes of _dry_ ammonia with 1 volume of dry carbonic anhydride, 2NH_{3} + CO_{2} = NH_{4}O(CONH_{2}); it is a solid substance, smells strongly of ammonia, attracts moisture from the air, and decomposes completely at 60°. The fact of this decomposition may be proved[36] by the density of its vapour, which = 13 (H = 1); this exactly corresponds with the density of a mixture of 2 volumes of ammonia and 1 volume of carbonic anhydride. It is easily understood that such a combination will take place with any ammonium carbonate under the action of salts which take up the water--for instance, sodium or potassium carbonate[37]--as in an anhydrous state ammonia and carbonic anhydride only form one compound, CO_{2}2NH_{3}.[38] As the normal ammonium carbonate contains two ammonias, and as the amides are formed with the separation of water at the expense of the hydrogen of the ammonias, so this salt has its symmetrical amide, CO(NH_{2})_{2}. This must be termed carbamide. It is identical with urea, CN_{2}H_{4}O, which, contained in the urine (about 2 per cent. in human urine), is for the higher animals (especially the carnivorous) the ordinary product of excretion[39] and oxidation of the nitrogenous substances found in the organism. If ammonium carbamate be heated to 140° (in a sealed tube, Bazaroff), or if carbonyl chloride, COCl_{2}, be treated with ammonia (Natanson), urea will be obtained, which shows its direct connection with carbonic acid--that is, the presence of carbonic acid and ammonia in it. From this it will be understood how urea during the putrefaction of urine is converted into ammonium carbonate, CN_{2}H_{4}O + H_{2}O = CO_{2} + 2NH_{3}.

[36] Naumann determined the following dissociation tensions of
the vapour of ammonium carbamate (in millimetres of mercury):--

-10° 0° +10° 20° 30° 40° 50° 60°
5 12 30 62 124 248 470 770

Horstmann and Isambert studied the tensions corresponding to
excess of NH_{3} or CO_{2}, and found, as might have been
expected, that with such excess the mass of the salt formed (in a
solid state) increases and the decomposition (transition into
vapour) decreases.

[37] Calcium chloride enters into double decomposition with ammonium
carbamate. Acids (for instance, sulphuric) take up ammonia, and
set free carbonic anhydride; whilst alkalis (such as potash) take
up carbonic anhydride and set free ammonia, and therefore, in this
case for removing water only sodium or potassium carbonate can be
taken. An aqueous solution of ammonium carbamate does not entirely
precipitate a solution of CaCl_{2}, probably because calcium
carbamate is soluble in water, and all the (NH_{3})_{2}CO_{2} is
not converted by dissolving into the normal salt,
(NH_{4}O)_{2}CO_{3}.

[38] It must be imagined that the reaction takes place at first between
equal volumes (Chapter VII.); but then carbamic acid,
HO(CNH_{2}O), is produced, which, as an acid, immediately combines
with the ammonia, forming NH_{4}O(CNH_{2}O).

[39] Urea is undoubtedly a product of the oxidation of complex
nitrogenous matters (albumin) of the animal body. It is found in
the blood. It is absorbed from the blood by the kidneys. A man
excretes about 30 grams of urea per day. As a derivative of
carbonic anhydride, into which it is readily converted, urea is in
a sense a product of oxidation.

Thus urea, both by its origin and decomposition, is an amide of carbonic acid. Representing as it does ammonia (two molecules) in which hydrogen (two atoms) is replaced by the bivalent radicle of carbonic acid, urea retains the property of ammonia of entering into combination, with acids (thus nitric acid forms CN_{2}H_{4}O,HNO_{3}), with bases (for instance, with mercury oxide), and with salts (such as sodium chloride, ammonium chloride), but containing an acid residue it has no alkaline properties. It is soluble in water without change, but at a red heat loses ammonia and forms _cyanic acid_, CNHO,[39 bis] which is a nitrile of carbonic acid--that is to say, is a cyanogen compound, corresponding to the acid ammonium carbonate, OH(CNH_{4}O_{2}), which on parting with 2H_{2}O ought to form cyanic acid, CNOH. Liquid cyanic acid, exceedingly unstable at the ordinary temperatures, gives its stable solid polymer cyanuric acid, O_{3}H_{3}C_{3}N_{3}. Both have the same composition, and they pass one into another at different temperatures. If crystals of cyanuric acid be heated to a temperature, _t_°, then the vapour tension, _p_, in millimetres of mercury (Troost and Hautefeuille) will be:

_t._ 160°, 170°, 200°, 250°, 300°, 350°
_p._ 56, 68, 130, 220, 430, 1,200

The vapour contains cyanic acid, and, if it be rapidly cooled, it condenses into a mobile volatile liquid (specific gravity at 0° = 1·14). If the liquid cyanic acid be gradually heated, it passes into a new amorphous polymeride (cyamelide), which, on being heated, like cyanuric acid, forms vapours of cyanic acid. If these fumes are heated above 150° they pass directly into cyanuric acid. Thus at a temperature of 350°, the pressure does not rise above 1,200 mm. on the addition of vapours of cyanic acid, because the whole excess is transformed into cyanuric acid. Hence, the above-mentioned figures give the tension of dissociation of cyanuric acid, or the greatest pressure which the vapours of HOCN are able to attain at a given temperature, whilst at a greater pressure, or by the introduction of a larger mass of the substance into a given volume, the whole of the excess is converted into cyanuric acid. The properties of cyanic acid which we have described were principally observed by Wöhler, and clearly show the _faculty of polymerisation of cyanogen compounds_. This is observed in many other cyanogen derivatives, and is to be regarded as the consequence of the above-mentioned explanation of their nature. All cyanogen compounds are ammonium salts, R(CNH_{4}O_{2}), deprived of water, 2H_{2}O; therefore the molecules, RCN, ought to possess the faculty of combining with two molecules of water or with other molecules in exchange for it (for instance, with H_{2}S, or HCl, or 2H_{2}, &c.), and are therefore capable of combining together. The combination of molecules of the same kind to form more complex ones is what is meant by polymerisation.[40]

[39 bis] Its polymer, C_{3}N_{3}H_{3}O_{3}, is formed together
with it. Cyanic acid is a very unstable, easily changeable liquid,
while cyanuric acid is a crystalline solid which is very stable at
the ordinary temperature.

[40] Just as the aldehydes (such as C_{2}H_{4}O) are alcohols (like
C_{2}H_{6}O) which have lost hydrogen and are also capable of
entering into combination with many substances, and of
polymerising, forming slightly volatile polymerides, which
depolymerise on heating. Although there are also many similar
phenomena (for instance, the transformation of yellow into red
phosphorus, the transition of cinnamene into metacinnamene, &c.)
of polymerisation, in no other case are they so clearly and simply
expressed as in cyanic acid. The details relating to this must be
sought for in treatises on organic and theoretical chemistry. If
we touch on certain sides of this question it is principally with
the view of showing the phenomenon of polymerisation by typical
examples, for it is of more frequent occurrence than was formerly
supposed among compounds of several elements.

Besidea being a substance very prone to form polymerides, cyanic acid presents many other features of interest, expounded in greater detail in organic chemistry. However we may mention here the production of the cyanates by the oxidation of the metallic cyanides. Potassium cyanate, KCNO, is most often obtained in this way. Solutions of cyanates by the addition of sulphuric acid yield cyanic acid, which, however, immediately decomposes: CNHO + H_{2}O = CO_{2} + NH_{3}. A solution of ammonium cyanate, CN(NH_{4})O, behaves in the same manner, but only in the cold. On being heated it completely changes because it is transformed into urea. The composition of both substances is identical, CN_{2}H_{4}O, but the structure, or disposition of, and connection between, the elements is different: in the ammonium cyanate one atom of nitrogen exists in the form of cyanogen, CN--that is, united with carbon--and the other as ammonium, NH_{4}, but, as cyanic acid contains the hydroxyl radicle of carbonic acid, OH(CN), the ammonium in this salt is united with oxygen. The composition of this salt is best expressed by supposing one atom of the hydrogen in water to be replaced by ammonium and the other by cyanogen--_i.e._ that its composition is not symmetrical--whilst in urea both the nitrogen atoms are symmetrically and uniformly disposed as regards the radicle CO of carbonic acid: CO(NH_{2})_{2}. For this reason, urea is much more stable than ammonium cyanate, and therefore the latter, on being slightly heated in solution, is converted into urea. This remarkable isomeric transformation was discovered by Wöhler in 1828.[41] Formamide, HCONH_{2}, and _hydrocyanic acid_, HCN, as a nitrile, correspond with formic acid, HCOOH, and therefore ammonium formate, HCOONH_{4}, and formamide, when acted on by heat and by substances which take up water (phosphoric anhydride) form hydrocyanic acid, HCN, whilst, under many conditions (for instance, on combining with hydrochloric acid in presence of water), this hydrocyanic acid forms formic acid and ammonia. Although containing hydrogen in the presence of two acid-forming elements--namely, carbon and nitrogen[42]--hydrocyanic acid does not give an acid reaction with litmus (cyanic acid has very marked acid properties); _but it forms salts_, _MCN_, thus presenting the properties of a feeble acid, and for this reason is called an _acid_. The small amount of energy which it has is shown by the fact that the cyanides of the alkali metals--for instance, potassium cyanide (KHO + HCN = H_{2}0 + KCN) in solution have a strongly alkaline reaction.[43] If ammonia be passed over charcoal at a red heat, especially in the presence of an alkali, or if gaseous nitrogen be passed through a mixture of charcoal and an alkali (especially potash, KHO), and also if a mixture of nitrogenous organic substances and alkali be heated to a red heat, in all these cases the alkali metal combines with the carbon and nitrogen, forming a metallic cyanide, MCN--for example, KCN.[43 bis] Potassium cyanide is much used in the arts, and is obtained, as above stated, under many circumstances--as, for instance, in iron smelting, especially with the assistance of wood charcoal, the ash of which contains much potash. The nitrogen of the air, the alkali of the ash, and the charcoal are brought into contact at a high temperature during iron smelting, and therefore, under these conditions, a considerable quantity of potassium cyanide is formed. In practice it is not usual to prepare potassium cyanide directly, but a peculiar compound of it containing potassium, iron, and cyanogen. This compound is potassium ferrocyanide, and is also known as _yellow prussiate of potash_. This saline substance (_see_