480 
DR. PLAYFAIR ON THE NITROPRUSSIDES, 
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 solution 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 nitro- 
prusside of iron along with the prussian blue, the simple treatment with hot water 
does not wholly economise the products, as it only separates the salt of potassium. 
The mixture may therefore be decomposed by caustic potash, which, added in suffi- 
cient quantity, forms peroxide of iron, and ferrocyanide instead of ferridcyanide, — 
(Fe2 Cy0-}-3K.)-J-K.O-{-2FeO=2(Fe Cy3-)-2K.)-f-Fe2 O3. 
The ferrocyanide may now be separated from the nitroprusside, either by precipita- 
tion by alcohol, or by the addition of nitrate of lead. These plans were not however 
so advantageous 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 very economical 
and simple means of obtaining the nitroprussides in large quantities. 
6. As nitric oxide was one of the most important means of producing the conver- 
sion 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 of cyanogen. 
Led through protosulphate of iron, after the first violent action has ceased, no black- 
ening is perceived, so that nitric oxide has ceased to be evolved. Led into caustic 
barytes, carbonate of barytes is precipitated, and the solution is found to contain 
cyanide of barium and cyanate of barytes. When the gas is collected ovei‘ mercury 
and potash is thrown into the tube containing it, a portion of gas still remains un- 
absorbed and is easily recognized as nitrogen. When the escaping gas is led into 
water it is dissolved in considerable quantity, and the water now smells strongly of 
cyanogen and of a peculiar pungent gas, which appears to be hydrated cyanic acid. 
The gas treated with ammonia deposits azulmic acid, and the usual products of the 
transformation of cyanogen. The following process is found best adapted for the 
