516 Mr. R. C. Campbell on the Ferrocyanides. 



dissolved in water, shows likewise the same reaction, although 

 containing no cyanate of potash. 



I have already stated, that the investigation undertaken with 

 the view of isolating the colouring substance in these experi- 

 ments, was without success ; nevertheless some observations 

 were made which seem to stand in relation to this subject. 



It has often been remarked, that, on adding an acid to the 

 solution of the heated double salt, a minute quantity of a red- 

 dish powder is precipitated. This powder contains iron ; it 

 is decolorized by carbonate of potash, but the colour is not 

 reproduced by exposure to the sun. The solution from 

 which this powder has been precipitated, on being saturated 

 with alkali, and exposed to the sun's rays, is found not to have 

 lost the colouring property. I suspect that this red powder 

 is the same as is observed to be precipitated from some speci- 

 mens of commercial prussiate of potash, on the addition of 

 strong acetic acid. Prussiate of potash is sometimes observed 

 in commerce of a darker tint than usual, and it is this variety 

 which frequently shows the above-mentioned reaction. 



If a solution of cyanide of ammonium be added to a solu- 

 tion of acetate of copper, a compound of copper and cyanogen 

 is precipitated, not analogous in its composition to the cyanide 

 from which it is precipitated. The supernatant solution be- 

 comes pink-coloured for a few seconds. In order to see if this 

 reaction result from the action of the free cyanogen on the 

 cyanide of ammonium, a stream of this gas was sent through 

 a solution of the salt. It became first yellow, and then red, 

 but a different red from that mentioned in the reaction with 

 the copper salt. 



Professor Liebig pointed out to me another instance of a 

 fugitive pink colour occurring in a cyanogen compound, which 

 Prof. Gmelin of Heidelberg nas mentioned in his System of 

 Chemistry. When strong nitric acid is added to pounded 

 yellow prussiate of potash, and slightly heated, much cyanogen 

 gas is evolved; when the heat is not too strongly applied, 

 neither prussic acid nor nitrous acid are remarked. The 

 mass becomes black, and is quite soluble in water: if there 

 be added to this solution, first, an excess of potash, and 

 then some drops of sulphuret of potassium, the solution be- 

 comes beautifully pink-coloured, disappearing after the lapse 

 of a few seconds. 



It differs from the pink colour observed as occurring in the 

 solution of the heated double salt when exposed to the sun, in 

 its action to sulphuretted hydrogen. This gas, or an alkaline 

 sulphuret, instantly destroys the colour resulting from the 



