334 CONSIDERATION OF CARBON COMPOUNDS. 



By the action of hydrochloric acid upon silver cyanide : 



AgCy + HC1 = AgCl + HCy. 



By distilling potassium ferrocyanide with diluted sulphuric 

 acid: 



2(K 4 FeCy 6 ) + 6(H 2 SO 4 ) = K 2 Fe 2 Cy 6 + 6(KHSO 4 ) + 6HCy. 



Potassium Sulphuric Potassium Potassium Hydrocyanic 



ferrocyanide. acid. ferrous ferrocyanide. acid sulphate. acid. 



Pure hydrocyanic acid is (at a temperature below 26) a color- 

 less, mobile liquid, of a penetrating, characteristic odor resem- 

 bling that of bitter almonds; it boils at 26.5, and crystallizes 

 at 15. It is readily soluble in water, and a 2 per cent, solu- 

 tion is the diluted hydrocyanic acid, Acidum hydrocyanicum dilutum. 



According to the U. S. P., this diluted acid is made either by 

 the decomposition of potassium ferrocyanide by diluted sul- 

 phuric acid in a retort, the delivery tube of which passes into a 

 receiver containing a mixture of water and alcohol by which 

 the liberated gas is absorbed, this liquid being afterwards di- 

 luted with a sufficient quantity of water to make a 2 per cent, 

 solution, or it is made extemporaneously by the decomposition 

 of 6 parts of silver cyanide by 5 parts of hydrochloric acid, di- 

 luted with 55 parts of water, allowing the silver chloride to sub- 

 side and pouring off the clear liquid. 



The diluted acid has the characteristic odor of bitter almonds, 

 a slightly acid reaction, and is completely volatilized by heating. 

 Whilst the pure acid is very readily decomposed by exposure to 

 light, etc., the diluted acid is fairly stable, and is rendered more 

 so by a trace of mineral acids. 



Potassium cyanide, Potassii cyanidum, KCN = 65 (Cyanide of 

 potassium). The pure salt may be obtained by passing hydro- 

 cyanic acid into an alcoholic solution of potassium hydrate. The 

 commercial article, however, is a mixture of potassium cyanide 

 with potassium cyanate. It is obtained by fusing potassium 

 ferrocyanide with potassium carbonate in a crucible, when 

 potassium cyanide and cyanate are formed, whilst carbon di- 



