■,' :i .rv. a New Class of Salts. 201 



blue became insoluble and was thrown on a filter and washed. 

 It was obvious from this experiment that there must be a 

 potassium salt in combination with the prussian blue, because 

 the quantity of iron salt added was quite insufficient to unite 

 with the iron and cyanogen of the radical. This idea was 

 confirmed by finding that 2 equivs. of sulphate of copper were 

 required to effect the precipitation, which 1 equiv. of sulphate 

 of iron had effectually done. To separate the potassium salt 

 present in the latter case, the precipitated prussian blue, after 

 being washed with cold water, was mixed with water and 

 boiled. The whole was now thrown on a filter, and a solu- 

 tion of a fine ruby-red colour passed through. This solution 

 gave a salmon-coloured precipitate with a protosalt of iron. 

 This precipitate does not readily occur in an acid liquid, and 

 hence the addition of the iron salt to the original oxidized 

 solution does not effect a complete precipitation, the filtrate 

 from it being yellow from dissolved nitroprusside of iron. 

 There being always some nitroprusside of iron along with the 

 prussian blue, the simple treatment with hot water does not 

 wholly oeconomise the products, as it only separates the salt of 

 potassium. The mixture may therefore be decomposed by 

 caustic potash, which, added in sufficient quantity, forms per- 

 oxide of iron, and ferrocyanide instead of ferridcyanide, — 



(Fe^Cy6+3K) + KO + 2FeO=2(FeCya + 2K)-f-Fe2 03. 



The ferrocyanide may now be separated from the nitroprus- 

 side, either by precipitation by alcohol, or by the addition of 

 nitrate of lead. These plans were not however so advantage- 

 ous as the simple means of separation given above. That 

 method was followed for some time until the examination of 

 the nitroprussides threw some light on their properties and 

 composition ; it was then found that a process yielding a much 

 larger product of the new compound might be invented. The 

 following study was therefore made of the products arising 

 from the oxidation of the prussides by nitric acid. The 

 knowledge thus obtained led, as was expected, to a verj' oeco- 

 nomical and simple means of obtaining the nitroprussides in 

 large quantities. 



6. As nitric oxide was one of the most important means of 

 producing the conversion of prussides into nitroprussides, it 

 was necessary to operate so as to prevent its escape. This 

 was done by keeping the mixture of acid and prusside well- 

 cooled at the first part of the action. Nitric oxide is almost 

 always evolved at first, but it soon diminishes to nothing as 

 the action proceeds. A copious evolution of gas takes place. 

 The escaping gas burns with the characteristic purple flame 



