452 POTASSIUM. 



the ferrocyanide, which immediately colours the solution yellow 

 (Liebig.) 



The cyanide of potassium crystallizes in colourless cubes,, 

 which become opaque and deliquesce in damp air, and are very 

 soluble in water. It bears a red heat without decomposition in 

 close vessels, but with exposure to oxygen becomes cyanate of 

 potash (KO, Cy O). Its solution smells of hydrocyanic acid, 

 being decomposed by carbonic acid. The action of cyanide 

 of potassium upon the animal economy is equally powerful with 

 that of hydrocyanic acid, and as the dry salt may be preserved 

 in a well stopt bottle without change, it is preferable to the 

 acidj which is far from stable. Red oxide of mercury dissolves 

 freely in the solution of cyanide of potassium, cyanide of mer- 

 cury being formed and potash set free. The purity of the alka- 

 line cyanide may be ascertained from this property ; 1 2 grains 

 of the pure cyanide dissolving 20 grains of finely pulverised 

 oxide of mercury. 



Hydrocyanic acid for medical purposes is conveniently pre- 

 pared from this cyanide. 24 grains of cyanide of potassium, 

 56 grains of tartaric acid in crystals, and 1 ounce of water are 

 agitated together in a stout phial closed by a cork. The liquid 

 is afterwards separated by filtration from the precipitate of bi- 

 tartrate of potash; it contains 10 grains of hydrocyanic acid, or 

 rather more than 2 per cent (Dr. Clark). 



Sulphocyanide of potassium ; K, Cy S 2 ; 1222.207* 97->2. 

 Sulphocyanogen is a salt-radical consisting of two of sulphur 

 and one of cyanogen, which is formed on fusing the ferrocya- 

 nides with sulphur. To obtain it in combination with potas- 

 sium, the ferrocyanide of potassium, made anhydrous by heat 

 and reduced to a fine powder, is mixed with an equal weight of 

 flowers of sulphur, in a common cast iron pot (pitch pot), and 

 kept in a state of fusion for half an hour at a temperature infe- 

 rior to that at which the sulphur would boil and bubbles of gas 

 escape through the melted mass. No cyanogen is evolved or de- 

 composed, and the residuary matter is a mixture of sulpho- 

 cyanide of potassium and protosulphocyanide of iron, with the 

 excess of sulphur. Both sulphocyanides dissolve in water, and 

 give a solution which is colourless at first, but soon becomes red 

 from oxidation of the sulphocyanide of iron. To get rid of 

 the iron, carbonate of potash is added to the boiling solution, 

 so long as a precipitate of carbonate of iron falls, and the liquid 



