Cyanides of the Metals. 67 



precipitate for a short time in dilute nitric acid : but if the cya- 

 nide of potassium contains any chlorides or undecomposed 

 ferrocyanide, these impurities cannot be separated by this 

 means. The best method of obtaining cyanide of silver free 

 from all these impurities, is to add a solution of nitrate of sil- 

 ver to a solution of the crystals of cyanide of potassium and 

 silver. Pure cyanide of silver precipitates, although the cya- 

 nide of potassium originally used contained all the impurities 

 before enumerated. 



The cyanide of silver should not be dried at a heat exceed- 

 ing 260°, as it assumes a brownish colour when exposed to a 

 higher temperature. It also becomes brown by exposure to 

 light. 



Hydrochloric acid converts the cyanide of silver into the 

 chloride with the rapid evolution of hydrocyanic acid. 



Nitric acid has no effect upon it except when concentrated 

 and boiling. 



Sulphuric acid diluted with its own volume of water de- 

 composes the cyanide of silver when boiling, with the escape 

 of hydrocyanic acid, and the formation of sulphate of silver, 

 which crystallizes by cooling, or slight dilution with water; 

 by this means the cyanide may be separated from the chloride, 

 which is not soluble in sulphuric acid. 



Cyanide of silver is soluble in the alkaline chlorides, and in 

 the chlorides of calcium barium and magnesium when boiled, 

 though it is but slightly soluble in these latter when cold. It 

 is also soluble in ferrocyanide of potassium, forming one or 

 two distinct salts easily crystallized, but which we have not 

 yet sufficiently investigated to warrant a more particular notice 

 of them at present. It dissolves freely in hyposulphite of soda, 

 forming a crystalline compound by evaporation. 



The best solvent of cyanide of silver is cyanide of potassium, 

 requiring only one equivalent of the latter to one equivalent of 

 the former, and constituting a distinct compound. This solu- 

 tion, when evaporated, yields an abundant crop of crystals of 

 a mixed character, most generally in the form of hexagonal 

 plates, somewhat resembling chlorate of potash, often inter- 

 mixed with numerous small crystals, chiefly rhombic prisms, 

 more transparent than the former crystals, which transparency 

 they lose by drying. Their proportion to the other crystals 

 is generally very small. Many crops of crystals are ob- 

 tained without one of these rhombs being perceptible. Their 

 proportion varies according to the rapidity or slowness of 

 evaporation and variation in the preparation of the salt. 



These two forms of crystals were carefully separated and 

 dried at a temperature of 220°. A higher temperature causes 



F2 



