forming the Nitroprussides. 13 



are the following : — The solution will become of a deeper yellow 

 colour, which gradually passes to a brown, acquiring a faint 

 alkaline reaction, and evolving a trace of ammonia ; some per- 

 oxide of iron precipitates ; and if the mixture be exposed for a 

 sufficient time, no ferrocyanide of potassium can be detected in 

 the solution, but in its stead is found the ferridcyanide of potas- 

 sium, together with a small quantity of the nitroprusside of the 

 same base. 



These changes take place very slowly, and it was not till after 

 several weeks that any nitroprusside could be detected in the 

 solution. The action of light also seems to be necessary to effect 

 this latter change ; for in some of the same solution kept ex- 

 cluded from the light for a much longer time, no nitroprusside 

 could be detected, though the other changes noticed had to a 

 slight extent taken place. 



I may also remark, that when the ferridcyanide of potassium 

 (red prussiate of potash) is substituted for the ferrocyanide of 

 potassium in this process, as also in the two former, nitroprus- 

 sides will be formed with even greater facility. This is what 

 might have been expected ; for in each case using the ferrocya- 

 nide, that salt seems first to be converted, more or less com- 

 pletely, into the ferridcyanide, and this latter gradually into the 

 nitroprusside. 



Dr. Playfair's process of forming the nitroprussides, viz. by 

 the action of nitric acid on the ferrocyanides, particularly that 

 of potassium, affords those compounds in larger quantity and 

 with far greater facility than any of the methods just referred to, 

 which are, however, interesting in a chemical point of view, as 

 they show that those very remarkable compounds may be formed 

 under a variety of different circumstances, and may hereafter 

 tend in some degree to elucidate the theory of their formation, 

 which is not at present very clearly understood. 



In the Number of this Journal for last May, vol. v. p. 330, 

 will be found a new test which I proposed for nitric acid and the 

 nitrates, which is founded on the formation of nitroprussides by 

 the action of the nitric acid (if present in the substance under 

 examination) on a little of the ferrocyanide of potassium added 

 to the mixture, and treated in the way there described. The 

 facts which I have just referred to in the present paper, viz. that 

 nitroprussides may be formed by the action of other chemical 

 agents besides nitric acid on the ferrocyanides, would seem at 

 first sight to render that test rather ambiguous ; but the circum- 

 stances under which the nitroprussides are produced where nitric 

 acid is the active agent, are different from those where they are 

 formed by the combined action of hydrochloric acid and a chlo- 

 rate on the ferrocyanides. In the former, those salts are imme- 



