A NEW CLASS OF SALTS. 
479 
the resulting liquid exhibited the strong coloration with sulphides. Ferrocyanide of 
lead, or any other ferrocyanide, gave, when mixed with strong acids, a similar result. 
It was therefore obvious that the peculiar compound might be obtained from pure 
hydroferrocyanic acid. The latter acid was prepared from prusside of lead by sul- 
phuretted 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(Fe Cy 3 + 2 H) +N02=2 (Fe 2 Cyg+3H)-l-2HO-f N. 
Until 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 perchloride 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 
nov/ gave a blue precipitate 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 ferro- 
cyanide of potassium (Fe2Cye+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 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 
* 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. 
3 Q 2 
