202 Dr. Playfair o?i the NitroprussideSf 



of cyanogen. Led through protosulphate of iron, after the 

 first violent action has ceased, no blackening is perceived, 

 so that nitric oxide has ceased to be evolved. Led into 

 caustic barytes, carbonate of barytes is precipitated, and 

 the solution is found to contain cyanide of barium and cyanate 

 of barytes. When the gas is collected over mercury and 

 potash is thrown into the tube containing it, a portion of gas 

 still remains unabsorbed and is easily recognized as nitrogen. 

 When the escaping gas is led into water it is dissolved in 

 considerable quantity, and the water now smells strongly of 

 cj'anogen and of a peculiar pungent gas, which appears to be 

 hydrated cyanic acid. The gas treated with ammonia deposits 

 azulmic acid, and the usual products of the transformation of 

 cyanogen. The following process is found best adapted for 

 the preparation of the nitroprusside. Nitric acid of commerce 

 is diluted with its own bulk of water, and the quantity of it 

 necessary to neutralize 53*3 grs. of carbonate of soda (1 equiv.) 

 is ascertained by the alkalimeter. This quantity denotes 

 1 equiv. of acid. 



Ferrocyanide of potassium is now reduced to powder and 

 is placed in a convenient vessel, and for every 422 grs. of the 

 salt used (that is for 1 equiv. Fe^Cy^', 4K + 6HO) 5 equivs. of 

 the acid are employed. This quantity of acid is found to 

 produce an ceconomical result, but it is very remarkable that 

 one-fifth of the quantity, or 1 equiv., is sufficient to convert a 

 large portion of the prusside into nitroprusside. This is the 

 more remarkable, because there are four available equivalents 

 of potassium, and it was to be expected that nitrate of potash 

 would be produced. This however is not the case, 1 equiv. 

 of nitric acid effecting oxidation to a considerable extent on a 

 double equivalent of yellow prusside. The five equivalents 

 of acid mentioned above are at once poured on the prusside, 

 as the cooling effect of the whole reduces the violence of the 

 action. The mixture assumes a milky appearance, but soon 

 the salt dissolves with a brownish-red colour like coffee, the 

 mixtui'e of gases already described being freely evolved. 

 When the solution is complete, it is found to contain ferrid- 

 cyanide of potassium mixed with a nitroprusside and nitrate 

 of potash. It is now removed into a bolt-head and digested 

 in the water-bath. It continues to evolve gas, and after a 

 time it no longer yields prussian blue with sulphate of iron, 

 but forms a dark green or a slate-coloured precipitate. The 

 solution is now removed from the water-bath and is allowed 

 to cool, during which abundance of nitrate of potash crystal- 

 lizes out, and always more or less of a peculiar white substance. 

 The dark coffee-coloured mother-liquor is now neutralized 



