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 CN2H4O,HNO3), 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,48 which is a nitrile of carbonic acid—that is to say, is a cyanogen compound, corresponding to the acid ammonium carbonate, OH(CNH4O2), which on parting with 2H2O ought to form cyanic acid, CNOH.
Liquid cyanic acid, exceedingly unstable at the ordinary temperatures, gives its stable solid polymer cyanuric acid, O3H3C3N3. 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(CNH4O2), deprived of water, 2H2O; 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 H2S, or HCl, or 2H2, &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.49
Besides 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