478 
DR. PLAYFAIR ON THE NITROPRUSSIDES, 
immediately produces the most intense purple coloration with a soluble sulphide*. 
The action of nitric acid on the pounded salt is similar, but much more violent than 
that experienced with the solution. Nitric oxide is at first evolved, but it soon ceases 
if the mixture be kept cool, and it is followed by the copious escape of cyanogen gas, 
accompanied by hydrocyanic acid, and a gas of peculiar pungency, apparently 
hydrated cyanic acid ; more or less nitrogen and carbonic acid are also found in the 
escaping gases. The dark red solution remaining after the action, deposits, on 
cooling, abundance of nitrate of potash, and, under the most favourable circumstances, 
about 5 per cent, of a peculiar white substance, afterwards to be described. The 
red-coloured solution now precipitates protosalts of iron of a dark blue colour, or if 
it has been heated for a short time, or even stood in the cold for some days, of a dark 
green, and sometimes of a slate colour. A dark green precipitate is also produced 
on the addition of salts of copper. The same precipitates are obtained from the 
neutralized as from the acid solution. Such were the preliminary observations made 
on repeating Dobereiner’s experiment. 
One important fact was observed in this preliminary trial, viz. that nitric oxide 
disappeared during the action, and in fact only occurred when the transformation 
was so violent as to escape control. This gas was therefore probably one important 
cause of the change, and it therefore became necessary to examine its action on the 
cyanides, as a more simple means of eliciting its mode of action. 
3. The first obvious experiment was to ascertain whether cyanide of potassium 
charged with nitric oxide would produce prussides exerting the remarkable colouring 
action on the sulphides. Nitric oxide is in fact readily absorbed by cyanide of potas- 
sium, the solution becoming red-coloured and depositing a black substance resem- 
bling paracyanogen. This red-coloured solution did not of itself give any colour 
when mixed with a sulphide. It was now converted into a prusside by the addition 
of protosulphate of iron. The resulting prusside was now found to strike a magnifi- 
cent purple colour with a soluble sulphide. The same coloration was obtained when 
a prusside was made from common cyanide of potassium added to a solution of pro- 
tosulphate of iron, through which nitric oxide had been passed. It was obvious from 
these experiments that nitric oxide was one of the great causes of the change experi- 
enced by the prusside. 
4. The action of nitric oxide on the prussides themselves was now examined. It 
was found that nitric oxide could be passed through a solution of ferrocyanide of 
potassium without producing any sensible change. But when the prusside was mixed 
with sufficient acid to take up its alkaline base, it was now found that nitric oxide 
was freely absorbed by this mixture when heated, though not in the cold ; and that 
* The intensity and beauty of this coloration render the nitroprussides the most sensible of all tests for the 
presence of the minutest trace of a soluble sulphide. The presence of quantities insensible to ordinary tests is 
at once strongly exhibited by the use of this colouring agent. 
