1410 PKINCIPLES OF CHEMISTRY 



cyanogen compound, corresponding to the acid ammonium carbonate, 

 OH(CNH 4 O 2 ), which on parting with 2H 2 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 N3. 

 Both have the same composition, and they pass one into another at 

 jdifferent temperatures. If crystals of cyanuric acid be heated to a tem- 

 perature, t , then the vapour tension, /?, 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 vapoiir contains cyanic acid, and, if it be rapidly cooled, it con- 

 denses into a mobile volatile liquid (specific gravity at 0= T14). If 

 the liquid cyanic acid be gradually heated, it passes into a new amor- 

 phous 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 Wohler, and clearly show the faculty of polymerisation of 

 cyanogen compounds. This is observed in many other cyanogen deriva- 

 tives, 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, 

 RON, 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 HC1, or 2H 2 , &c.), and are therefore capable of combining to- 

 gether. The combination of molecules of the same kind to form more 

 complex ones is what is meant by polymerisation. 40 



40 Just as the aldehydes (such as C 2 H 4 O) are alcohols (like C 2 H 6 0) 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 trans- 

 formation 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. 



