200 Dr. Playfair on the Nitroprussides, 



pure hydroferrocyanic acid. The latter acid was prepared 

 from prusside of lead by sulphuretted hydrogen, the excess 

 of the latter being removed by the addition of a little more 

 lead salt. The filtered ferrocyanic acid was found to suffer 

 no change when exposed to the action of nitric oxide in the 

 cold ; but when the solution was kept in a water-bath and the 

 gas led through it, a change was observed. This, however, 

 at first merely consisted in the transformation of ferrocyanic 

 to ferridcyanic acid, — 



4(FeCy3 + 2H) + NO^=2(Fe2Cy«+3H) + 2HO + N. 

 TTntil this change was completed not the least coloration 

 took place on mixing the acid liquid with a sulphide. When, 

 however, the acid no longer gave prussian blue with perchlo- 

 ride of iron, it began to assume a red colour, continuing to 

 evolve a gas, and it now exhibited, after neutralization, the 

 peculiar coloration with sulphides. It now gave a blue pre- 

 cipitate with protosulphate of iron, like ordinary ferridcyanic 

 acid. This blue precipitate became paler in colour as the gas 

 continued to stream through the hot solution, until finally the 

 addition of the iron salt gave a precipitate of a clear salmon 

 colour. Here then was the acid* of the new compounds, and 

 its salts were obtained by neutralization with the respective 

 bases. This process was a great step in the inquiry, because 

 it enabled the distinctive characters of the nitroprussides to 

 be determined. At the same time it was not fitted to procure 

 the salts in sufficiently large quantities for examination. By 

 showing however what was to be looked for, it enabled a more 

 complete examination to be made of the products of oxidation 

 of the prussides by nitric acid, with a view to the separation 

 of the nitroprussides from the ferridcyanides, with which they 

 were obviously mixed. 



5. It was observed that the oxidized prusside required a 

 very small quantity of protosulphate of iron for its complete 

 precipitation. One double equivalent of ferrocyanide of potas- 

 sium (PVCy^' + 4K) was oxidized with 3 equivs. of nitric acid 

 diluted with its own volume of water. The dark red, almost 

 black liquid, was diluted with water and treated with a known 

 quantity of sulphate of iron dissolved in water. Prussian blue 

 was formed, but it remained in solution^ forming a dark blue 

 soluble fluid, of great beauty and intensity. When the added 

 sulphate of iron amounted to one equivalent, that is to one- 

 fourth of the potassium originally in the prusside, the prussian 



* On neutralizing this nitroprussic acid with carbonates, the resulting 

 salts were found to be accompanied by a nitrate, although the nitric oxide 

 had previously been passed through water in a washing-bottle. 



