208 Dr. Playfair on the Nib'oprussides, 



Reagents. Behaviour of the nitroprusside. 



Proto- and persalts of tin No change. 



Salts of zinc Light salmon-colourerl precipitate. 



Salts of copper Light green precipitate. 



Salts of nickel Dirty white precipitate. 



Salts of cobalt Flesh-coloured precipitate. 



Protosalts of iron Salmon-coloured precipitate. 



Persalts of iron No change. 



Caustic alkalies T^"'"" ^^^ red-coloured solutions of an 



I. orange colour. 



The beautiful colour immediately produced on the addition 

 of a soluble sulphide, is a most marked character of the nitro- 

 prussides. This purple coloration is most intense, and enables 

 the detection of the most minute quantity of either reagent. 

 As a test for the presence of sulphides it is wonderfully useful, 

 enabling minute quantities of them to be found in circum- 

 stances where the ordinary means of testing altogether fails to 

 denote their presence. This purple coloration is however 

 only transitory, the compound soon breaking up into various 

 substances, among which, hydrocyanic acid, ammonia, ni- 

 trogen, oxide of iron, a ferrocyanide, a sulphocyanide and a 

 hyponitrite may be recognized. 



The soluble nitroprussides are decomposed when sulphu- 

 retted hydrogen is passed through them, oxide of iron, prus- 

 sian blue, sulphur, a ferrocyanide, and a peculiar sulphur 

 compound being among the products of decomposition. 



The alkalies decompose the soluble nitroprussides when 

 their solutions are mixed together and boiled. The products 

 of the transformation in this case are oxide of iron, nitrogen, 

 a ferrocyanide and a hyponitrite. An excess of ammonia, 

 even in the cold, gradually decomposes the nitroprussides, 

 nitrogen gas being evolved, and a peculiar uncrystallizable 

 black compound remains as the result of the decomposition. 



Sulphurous acid, the sulphites and hyposulphites exert no 

 apparent action on the nitroprussides. They are however 

 wholly decomposed by boiling them with concentrated sul- 

 phuric acid ; during this decomposition, the peculiar purple 

 colour due to sulphides is observed. 



Chlorine does not produce any change when passed through 

 solutions of the nitroprussides. 



Prussian blue dissolves in an excess of some of the nitro- 

 prussides, forming a beautiful blue solution ; when the prussian 

 blue is in excess, it is able, under certain circumstances (see 

 § 5), to remove the soluble nitroprusside from solution, but 

 it again yields it up to boiling though not to cold water. 



Some of the nitroprussides are very permanent and suffer 

 no change in solution, either by exposure to the air or by the 



