336 CONSIDERATION OF CARBON COMPOUNDS. 



with ferrous and ferric salts and potassium hydrate, a greenish 

 precipitate, which, upon being dissolved in hydrochloric acid, 

 forms a blue precipitate of Prussian blue, Fe 4 3FeCy 6 . This re- 

 action depends on the formation of potassium ferrocyanide by 

 the action of the cyanogen upon both the potassium of the 

 potassium hydrate 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 diluted solution of picric 

 acid gives a deep-red color on cooling. 



5. In cases of poisoning, the matter under examination is dis- 

 tilled (if necessary 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 examination 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- 

 poisonous 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 pre- 

 liminary 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 the above 

 reaction 3, the object being to convert the soluble cyanide into 

 an insoluble ferrocyanide of iron. In most cases of poisoning 

 by hydrocyanic acid, there is, however, no time for the action 

 of such an antidote, in consequence of the rapidity of the 

 action of the poison, and the treatment is chiefly directed to the 

 maintenance of respiration by artificial means. 



Cyanic acid, HCNO. and Sulphocyanic acid, HCNS, are both color- 

 less acid liquids, the salts of which are known as cyanates and 

 sulphocyanates. These salts are obtained from alkaline cyanides 

 by treating them with oxidizing agents or by boiling their solu- 



