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XXIV. On the Nitroprussides, a New Class of Salts. 
By Dr. Lyon Playfair, F.R.S., F.C.S. 
Received June 21, — Read June 21, 1849. 
1. In an inquiry into the constitution of the prussides, 1 found it necessary to 
examine into the somewhat anomalous action of nitric acid on the yellow prusside of 
potassium. This examination has led to the discovery of a singular class of com- 
pounds, which form the subject of the present memoir. 
The previous knowledge on the action of nitric acid on the prussides may be 
summed up very briefly. Thomson* examined the gases produced during the action, 
and recognized them to be nitrogen, cyanogen, nitric oxide, and carbonic acid, while 
the residue was believed to consist of pernitrate of iron and nitrate of potash. 
DoBEREiNER'f' remarked that previous to the complete decomposition of the prus- 
sides, a strong coffee-coloured liquid was produced, which, after neutralization, 
precipitated protosalts of iron of a dark blue colour. Gmelin:{;, to whom chemistry 
was already indebted for important discoveries in the prussides, observed that the 
coffee-coloured liquid noticed by Dobereiner was rendered of a magnificent purple 
or blue colour on the addition of an alkaline sulphide. The same fact was noted by 
Mr. Mercer § of Oakenshaw, without his being aware that it had already been re- 
marked by Gmelin. Campbell II, in repeating Gmelin’s experiment, threw out the 
intelligent suggestion that the purple colour might be due to the production of a 
sulphuret of nitrogen, which Gregory^ had already remarked produced an ame- 
thystine colour when mixed with an alcoholic solution of potash. Smee**, in an 
examination of the action of nitric acid on the prussides, observes that ferridcyanide 
is produced, nitric oxide being evolved. 
I am not aware of any further knowledge on this subject ; and as it is far from 
being sufficiently extended, a new examination was desirable. 
2. When dissolved ferrocyanide of potassium is digested with diluted nitric acid, a 
coffee-coloured liquid is produced, having the characters ascribed to it by Dobereiner 
and Gmelin. The addition of this acid solution to sulphide of potassium dissolved 
in water causes a precipitation of sulphur and the production of various colours, from 
a pink to a violet or blue shade. When the acid liquid is neutralized with potash, it 
* As quoted by Gmelin, Handbuch, Band iv. s. 370. f Schw. J. xxvi. p. 305. 
t Ann. Pharm. Bd. xxviii. s. 57, and Memoirs of Chem. Soc. vol. i. p. 41. 
§ Unpublished Letter. || Handbuch, B. i. s. 167. 
^ Turnee’s Chemistry, p. 343. ** Mag. xvii. 194. 
3 Q 
MDCCCXLIX. 
