198 Dr. Play fair on the Nitroprussides, 



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* remarked that previous 

 to the complete decomposition of the prussides, a strong cofFee- 

 coloured liquid was produced, which, after neutralization, pre- 

 cipitated protosalts of iron of a dark blue colour. Gmelinf) 

 to whom chemistry was already indebted for important disco- 

 veries 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 J of Oakenshaw, without 

 his being aware that it had already been remarked by Gmelin. 

 Campbell §, 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 amethystine 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 exami- 

 nation 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 dis- 

 solved in water causes a precipitation of sulphur and the pro- 

 duction of various colours, from a pink to a violet or blue 

 shade. When the acid liquid is neutralized with potash, it 

 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 



* Schw. J. xxvi. p. 305. 



t ^nn. Pharm. vol. xxviii. p. 67» and Memoirs of Chem. Soc. vol. i. p. 41. 



X Unpublished Letter. § Handbiich, vol. i. p. 1G7. 



II Turner's Chemistry, p. 343. ij Phil. Mag. vol. xvii. 194. 

 *♦ The intensity and beauty o? this coloration render the nitroprussides 



the most sensible of idl tests for the presence of the minutest trace of a 

 soluble sulphide. The presence of quantities insensible to oi-dinary tests is 

 at once strongly exhibited by the use of this colouring agent. 



