HYDROCYANIC ACID. 987 



particularly under the influence of light, ammonia being formed 

 with a brown precipitate. This decomposition is prevented by 

 a slight admixture of any other acid. When potassium is 

 heated in the vapour of hydrocyanic acid, cyanide of potassium 

 is formed, and the hydrogen of the acid is disengaged as gas. 

 When its vapour is passed over ignited quicklime or barytes, a 

 mixture of cyanide of the metal and cyanate of its oxide is 

 formed, and pure hydrogen disengaged. Chlorine decomposes 

 hydrocyanic acid, and forms hydrochloric acid and chloride of 

 cyanogen. 



The aqueous solution of hydrocyanic acid may be prepared 

 by precipitating potash from cyanide of potassium, by means of 

 tartaric acid, and may in this way be obtained at once of a de- 

 terminate strength and in a good condition for preservation 

 (page 452). But to prepare this acid in considerable quantity 

 the following process may be followed, which is that of Geiger 

 with the proportions modified. Eight parts of ferrocyanide of 

 potassium with 7 parts of oil of vitriol diluted with 36 parts of 

 water, are slowly distilled nearly to dryness, the product being 

 transmitted through a Liebig's condenser, and collected in a 

 receiver containing at first 8 parts of water. The condensed 

 liquid contains very uniformly 1.62 parts of hydrocyanic acid, 

 which corresponds with one half of the quantity of cyanogen 

 in the salt, the other half of the cyanogen remaining in the 

 residuary insoluble ferrocyanide ; the latter is white or yellow- 

 ish-white when pure, but is often coloured by prussian blue. 

 In this reaction 2 equivalents of ferrocyanide of potassium are 

 decomposed by 6 equivalents of sulphuric acid, which liberate 

 3 equivalents of hydrocyanic acid. The residuary products are 

 an insoluble ferrocyanide of potassium and iron, K,FeH-FeCy 3 

 (Everitt), and 3 equivalents of bisulphate of potash ; or : 



2(K 2 + FeCy 3 ) and 6(HO.SO 3 )=K.Fe + FeCy 3 and 3(HO,SO 3 

 + KO,SO 3 ) and 3HCy. 



If ferrocyanide of potassium be viewed as a double cyanide 

 of iron and potassium, FeCyH- 2 KCy, then the decomposition 

 in the foregoing process is limited to the cyanide of potassium 

 of the salt, of which cyanide 3 atoms are decomposed out of 



