AMINES AND AMIDES. CYANOGEN COMPOUNDS. 361 



Mercuric cyanide, Hydrargyri cyanidum, Hg-(CN) 2 . A white 

 crystalline salt, obtained by dissolving mercuric oxide in hydrocyanic 

 acid ; it is soluble in water and alcohol and evolves cyanogen when 

 heated. 



Analytical reactions for hydrocyanic acid. 

 (Potassium cyanide, KCN, may be used.) 



1. Hydrocyanic acid, or soluble cyanides, give with silver nitrate 

 a white precipitate of silver cyanide, which is sparingly soluble in 

 ammonia, soluble in alkali cyanides or thiosulphates, but insoluble 

 in diluted nitric acid. Concentrated nitric acid dissolves it with 

 decomposition : 



HCN + AgN0 3 = AgCN + HNO 3 . 



2. Hydrocyanic acid mixed with ammonium hydric sulphide and 

 evaporated to dryness forms sulphocyanic acid, which, upon being 

 slightly acidulated with hydrochloric acid, gives with ferric chloride 

 a blood-red color of ferric sulphocyanate. (Excess of ammonium 

 sulphide must be avoided.) 



3. Hydrocyanic acid, or soluble cyanides, give, when mixed with 

 ferrous and ferric salts and potassium hydroxide, a greenish precipi- 

 tate, which, upon being dissolved in hydrochloric acid, forms a pre- 

 cipitate of Prussian blue, Fe 4 (FeC 6 N 6 ) 3 . This reaction depends on 

 the formation of potassium ferrocyanide by the action of the cyanogen 

 upon both the potassium of the potassium hydroxide and the iron of 

 the ferrous salt. In alkaline solutions, the blue precipitate does not 

 form, for which reason hydrochloric acid is added. 



4. Hydrocyanic acid heated with dilute solution of picric acid gives 

 a deep-red color on cooling. 



In cases of poisoning, the matter under examination is distilled (if neces- 

 sary after the addition of water) from a retort connected with a cooler. To 

 the distilled liquid the above tests are applied. If the substance under ex- 

 amination should have an alkaline or neutral reaction, the addition of some 

 sulphuric acid may be necessary in order to liberate the hydrocyanic acid. 

 The objectionable feature to this acidifying is the fact that non-poisonoua 

 potassium ferrocyanide might be present, which upon the addition of sulphuric 

 acid would liberate hydrocyanic acid. In cases where the addition of an acid 

 becomes necessary, a preliminary examination should, therefore, decide 

 whether or not ferro- or ferricyanides are present. 



Antidotes. Hydrocyanic acid is a powerful poison both when inhaled or 

 swallowed in the form of the acid or of soluble cyanides. As an antidote is 

 recommended a mixture of ferrous sulphate and ferric chloride with either 

 sodium carbonate or magnesia. The action of this mixture is explained in 



