272 Dr. Playfair on the 'Nitroprussides, 



shows that some change has resulted in their composition, 

 for the iron or electro-negative metal is now in greater than 

 atomic proportion to the electro-positive metal. The pro- 

 portion of carbon is also somewhat different. Still the dif- 

 ference in composition is not very considerable, although 

 decidedly marked ; it is not however sufficient to cause any 

 obvious alteration in their general properties. In fact there 

 is an attached impurity, probably a cyanide of iron, which 

 cannot now be removed by crystallization, precipitation, di- 

 gestion with nitric acid, or any of the ordinary means of pu- 

 rification. This impurity, if it be one, remains so obstinately 

 attached that all methods of purification have quite failed to 

 remove it. This circumstance, before it was understood, had 

 thrown the greatest difficulties in thewayof the inquiry,andpro- 

 tracted it to a most tedious length by preventing the attainment 

 of accordant results. It is to prevent the like inconvenience 

 to those who repeat these experiments that this section of the 

 paper is specially devoted. Attention has previously been 

 drawn to the fact, that the nitroprussides form chemical com- 

 pounds with the cyanides of iron. This seems to be a case 

 of the same kind, but of more ultimate union. The impurity 

 or chemically attached cyanide in this case appears to be 

 FeCy^, or perhaps FeCy + H Cy, judging from analysis only, 

 for its separation has not been accomplished. The proportion 

 in which it is present is very small, generally only 2(FeCy^) 

 to 7 equivs. of a nitroprusside, or if it be a chemical com- 

 pound, 7{Fe^Cyi2 3j^io + 5R) + Fe2Cy'*. Still as the cry- 

 stalline form and all the properties of the nitroprussides re- 

 main unchanged, we can scarcely view its presence in any 

 other light than as an impurity. Several of the nitroprussides, 

 viz. nitroprussic acid and the nitroprussides of ammonium and 

 calcium, have not yet been obtained free from this impurity, 

 and are therefore described in this section. 



Nitroprtissic Acid. 



19. The mode of preparation of this acid has been already 

 described at page 209. It is however most readily prepared 

 from nitroprusside of silver by adding to it as much hydro- 

 chloric acid as suffices to form chloride of silver with the silver 

 in the salt. The dark red solution thus obtained soon evolves 

 hydrocyanic acid, even in the cold, and after a time prusside 

 of potassium indicates the presence of iron in solution. If the 

 solution be heated, it deposits abundance of a brown precipi- 

 tate resembling oxide of iron. When the latter is separated 

 by filtration, and the solution is evaporated in vacuo over sul- 



