516 
DR. PLAYFAIR ON THE NITROPRUSSIDES, 
If allowed to stand for a few hours, it deposits a little of the precipitates which it held 
in solution. After this the brown solution is found to contain neither ferrocyanide 
nor nitroprusside of sodium ; a persalt of iron is slightly deepened in colour when 
mixed with it, showing the presence of a mere trace of a sulphocyanide. When this 
reddish-brown solution is evaporated in the water-bath, it deposits oxide of iron and 
sulphur, and becomes decomposed. Evaporated in vacuo over sulphuric acid it de- 
posits, when nearly dry, black crystalline needles, but these seem to be a product of 
decomposition, and are mixed with oxide of iron and other substances ; attempts were 
therefore made to ascertain the composition of the original substance by precipitating 
its solution by metallic salts. Bichloride of mercury produces a brown precipitate, 
sulphate of copper a pinkish brown, and nitrate of silver a black precipitate. But 
these were obviously products of decomposition, for during the precipitation nitric 
oxide is abundantly evolved. This is especially the case in the precipitate with silver. 
If that precipitate, after being washed, be now mixed with a small quantity of hydro- 
chloric acid to take up the silver, sulphuretted hydrogen is evolved, protochloride of 
iron and abundance of sulphocyanic acid are now found in solution ; the first is re- 
cognized by the prussian blue formed on adding red prusside of potassium, the second 
by the blood-red colour which it strikes with perchloride of iron. When nitrate of 
silver is added to the red-brown solution, the black precipitate already alluded to 
falls down, but at the same time the supernatant liquor had a reddish brown colour ; 
on examining this it was found to contain a persalt and protosalt of iron, the dark 
coloration being due to the escaping nitric oxide. The amount of sulphur precipi- 
tated during the passage of sulphuretted hydrogen through the nitroprusside is about 
17 per cent.; the amount of ferrocyanide of sodium and of prussian blue has been 
found to vary much. 
From the difiiculty of obtaining the products of transformation in a pure state, I 
have not yet been able to make direct quantitative examinations of the various sub- 
stances formed ; it is therefore impossible to express the transformation in the form 
of an equation. From some experiments now in progress, I trust, however, to over- 
come those difficulties which have prevented the completion of this study in time for 
the presentation of this paper. 
On the Constitution of the Nitroprussides. 
31. In the preceding part of the paper the analyses of the nitroprussides led to the 
extremely complicated formula Feg C24 O3 Rj. This formula was a priori very 
improbable, and naturally led to the belief that an error in the estimation of the 
carbon forced its adoption. In fact, if 25 instead of 24 equivs. of carbon were pre- 
sent, the formula would resolve itself into the much simpler expression Fe2 Ng O R2. 
It is therefore important to review the evidence, in order to see whether the simple pro- 
portion of iron to carbon, 1 ; 5, might be derived from it. The following table exhibits 
the proportion of iron and carbon found in the analyses of the respective salts : — 
