32 MR JOHNSTON ON PARACYANOGEN 



I. — Preparation of Paracyanogen. 



1. When pure dry bicyanide of mercury is heated in close vessels, it gives off 

 metallic mercury and a gas, which is wholly absorbed by solution of caustic pot- 

 ash. This gas is pure cyanogen. The pure dry salt gives off along with it no ap- 

 preciable quantity of any other gas. 



2. In all cases, however, when the pure bicj'^anide is wholly decomposed, 

 there remains in the retort a greater or less quantity of a black matter resembling 

 charcoal, sometimes in the form of powder, light, porous, and void of lustre : at 

 others more dense and coherent, and when it has been in contact with the sides 

 of the retort exhibiting a shining metallic lustre. 



3. Since pure bicyanide of mercury consists wholly of cyanogen and mercury, 

 and since, during the decomposition by heat, pure cyanogen and pure mercury 

 are alone given off, the black residue, when freed from metalUc mercury, can con- 

 tain only carbon and nitrogen, in the same proportion in which they enter into 

 the constitution of cyanogen. It must either be a new body having the same ele- 

 mentary constitution as cyanogen, or it must be a mixture of two or more sub- 

 stances which taken together have such a constitution. 



4. This black substance did not escape the notice of Gay Lussac in his re- 

 searches upon cyanogen. He recognised in it the presence of nitrogen, as many 

 succeeding chemists have done, but he supposed the greater part to be carbon de- 

 rived from the decomposition of a portion of the cyanogen. This opinion is still 

 generally entertained. 



5. Seven or eight years have now elapsed since, in a paper which I had the 

 honour of reading before this Society, and which was afterwards published in 

 Brewster's Edinburgh Journal of Science for 1829, p. 75, 1 endeavoured to shew that 

 this black matter was not a mere mixture of two or more substances, but was in 

 reality a new body, which, though differing so remarkably in physical and chemical 

 properties, yet contained the same elements as cyanogen, and combined together 

 in precisely the same proportion. Chemists, however, were not prepared at that 

 time for the reception of so extraordinary an opinion. The only case of isome- 

 rism then clearly made out, was that of the cyanic and fuhninic acids analyzed by 

 LiEBiG, and even over that case the researches of Edmund Davy stiU threw some 

 doubt. It was not surprising, therefore, that the result at which I had arrived 

 should be regarded with a suspicion, which the many striking examples of isome- 

 rism since discovered has not yet wholly removed. 



6. Dr Thomson, in his System of Chemistry, has objected that I have not shewn 

 the absence of hydrogen in the black matter ; but, as the dry bicyanide contains 

 no hydrogen, it is obvious that none can be present in any residue it may leave. 

 LiEBiG having prepared and analyzed a portion of this substance, concluded that 



