Mr. R. C. Campbell on the Ferrocyanides. 515 



Neither cyanate of lime nor cyanate of potash, together or 

 singly, or mixed with prussiate of potash, show this reaction. 

 It is hence probable that this solution owes the above-men- 

 tioned property to the admixture of some foreign substance, 

 present probably, in a very small quantity. All attempts to 

 isolate any such substance were fruitless. 



As the shade of colour in the above solution is exactly that 

 of a solution of permanganate of potash, the solution and the 

 substances used were tested for manganese. They contained 

 none. The reactions of this solution too stand in contradiction 

 with some of those of the permanganates. The presence of fer- 

 rocyanide of potassium is essential to the production of the pink 

 in the sun's rays, but the action of the same salt on perman- 

 ganate of potash is first to reduce it to the green manganate, 

 and by further addition of the prussiate to oxide of manga- 

 nese. As the prussiate of potash of commerce often contains 

 traces of sulphocyanide of potassium, the experiments were 

 repeated with prussiate of potash, that had been washed with 

 hot alcohol, and by this means all sulphocyanide removed ; 

 but no difference was observed in the result. 



It is further clear, that this salt, even if it were present in 

 the prussiate of potash, could take no part in the above-men- 

 tioned reaction, for it gives no precipitate with a lime salt, 

 with or without the presence of prussiate of potash. An 

 alkaline state of the liquid is essential to the production of the 

 pink in the sun's rays ; so is likewise the presence of a ferro- 

 cyanide. If a solution of nitrate of copper be added to a 

 solution of the heated double salt, till all ferrocyanide be 

 removed, and then the excess of copper precipitated by car- 

 bonate of potash, the solution will have lost entirely the pro- 

 perty of becoming coloured in the sun's rays, but will recover 

 it on the addition of a few drops of a solution of yellow prus- 

 siate of potash. 



The air exercises no influence on these changes ; they take 

 place equally" distinctly in closed vessels. 



A temperature of 120° Fahr. destroys the colour, but on 

 cooling and re-exposure to the sun's rays, the colour again 

 appears. By evaporation in the rays of the sun, a pink salt 

 is obtained. The presence of cyanate of potash is not essen- 

 tial to the production of the colour ; if muriatic acid be added 

 to the solution until all cyanic acid be destroyed or removed, 

 and then supersaturated with alkali, the solution possesses the 

 colouring property as strong as before. Another proof that 

 cyanate of potash is not essential, is that the double salt of 

 ferroprussiate of lime and potash heated in closed vessels, and 



