404 PRINCIPLES OF CHEMISTRY 



already stated, exactly expresses that of the nitrile of oxalic acid, and, 

 as a matter of fact, oxalate of ammonia and the amide corresponding 

 with it (oxamide), on being heated with phosphoric anhydride, which 

 takes up the water, yield cyanogen. This substance is also produced 

 by simply heating some of the metallic cyanides. Mercuric cyanide 

 is particularly adapted for this purpose, because it is easily obtained in 

 a pure state and is very stable in a separate form. 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 an odorous, colourless, poisonous 

 gas, easily condensed by cooling into a colourless liquid, insoluble in 

 water and having a specific gravity of O86. It boils at about 21, 

 and therefore cyanogen may be easily condensed into a liquid by a 

 strong freezing mixture. At - 35 liquid cyanogen solidifies. This gas 

 is soluble in water and in alcohol to a considerable extent namely, 1 

 volume of water absorbs as much as 4J volumes, and alcohol 23 volumes. 

 Cyanogen resists the action of heat without decomposing, but, under 

 the action of the electric spark, the carbon is separated, leaving a 



47 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 mercuric 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, formed 

 during the preparation of cyanogen by all methods, and remaining as a residue. Silver 

 cyanide, on being slightly heated, fuses, and on being further heated evolves a gas ; in 

 the residue a considerable quantity of paracyanogen is formed. 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 exceeds the determined magnitude, so that the 

 phenomenon presents all the external appearance of a physical transformation into 

 vapour ; but, nevertheless, it is a complete change in the nature of the substance, but 

 limited by the pressure of dissociation, as we saw before in the transformation of cyanuric 

 acid into hydrocyanic, and as ought to be expected from the fundamental principles of 

 dissociation. Troost and Hautefeuille (1868) found that for paracyanogen 



t = 530 581 600 (535 



p = 90 143 290 1089 mm. 



However, already 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 state. 



