COMPOUNDS OF CAKBON WITH OXYGEN AND NITKOGEN 415 



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 6*86. It boils at about 21, and therefore cyanogen may be easily 

 condensed into a liquid by a strong freezing jmixture. 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 muclj as 

 4J 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 



4r 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 ine'rcuric cyanide, a mixture of perfectly dry yellow prus* 

 siate 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. 



48 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 pheno 

 menon 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 = 580 581 600 635 



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 etate. 



