398 PRINCIPLES OF CHEMISTRY 



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,O=CO 2 + 2NH 3 . 



Thus urea, both by its origin and reactions, is an amide of carbonic 

 acid. Representing as it does ammonia (two molecules) in. which the 

 hydrogen (two atoms) is replaced by the bivalent radicle of carbonic acid, 

 urea retains the property of ammonia of entering into combination, 

 with acids (for instance, with nitric acid, CISr 2 H 4 O,HNO 3 ), with bases 

 (for instance, with mercury oxide), and with salts (for instance, sodium 

 chloride, ammonium chloride), but it has no alkaline properties. It is 

 soluble in water without change, but at a red heat loses ammonia and 

 forms cyanic acid, CNHO, and its polymeride, cyanuric acid, C 3 N 3 H 3 O 3 . 

 The first is a nitrile of carbonic acid that is to say, is a cyanogen 

 compound. This conformity is evident from the fact that the acid 

 ammonium carbonate, OH(CNH 4 O. 2 ), on parting with 2H 2 O, ought 

 to form cyanic acid, CNOH. There is a relation of direct poly- 

 merisation between cyanuric acid (a solid, crystalline, and very stable 

 substance) and cyanic acid (a liquid, very unstable substance, easily 

 changeable in several ways). Both have the same composition, and 

 they pass one into another at different temperatures. If crystals of 

 cyanuric acid be heated to a temperature, , 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, 1200. 



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 directly pass into cyanuric acid. Thus, at a tempera- 

 ture of 350, the pressure does not rise above 1200 mm. on the addi- 

 tion of vapours of cyanic acid, because the whole excess is transformed 

 into cyanuric acid. Therefore, "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 large mass of matter 

 into the 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 Wohler, and clearly show the faculty of 

 polymerism of cyanogen compounds. This is observed in many other 



