a Neia Class of Salts. 199 



found in the escaping gases. The dark red solution remain- 

 ing after the action, deposits, on cooling, abundance of nitrate 

 of potash, and, under the most favourable circumstances, about 

 5 per cent, of a peculiar lakite substance, afterwards to be de- 

 scribed. The red-coloured solution now precipitates proto- 

 salts of iron of a dark blue colour, or if it has been heated for 

 a short time, or even stood in the cold for some days, of a dark 

 green, and sometimes of a slate colour. A dark green preci- 

 pitate is also produced on the addition of salts of copper. The 

 same precipitates are obtained from the neutralized as from 

 the acid solution. Such were the preliminary observations 

 made on repeating Dcibereiner's experiment. 



One important fact was observed in this preliminary trial, 

 viz. that nitric oxide disappeared during the action, and in 

 fact only occurred when the transformation was so violent as 

 to escape control. This gas was therefore probably one im- 

 portant cause of the change, and it therefore became necessary 

 to examine its action on the cyanides, as a more simple means 

 of eliciting its mode of action. 



3. The first obvious experiment was to ascertain whether 

 cyanide of potassium charged with nitric oxide would produce 

 prussides exerting the remarkable colouring action on the sul- 

 phides. Nitric oxide is in fact readily absorbed by cyanide 

 of potassium, the solution becoming red-coloured and depo- 

 siting a black substance resembling paracyanogen. This red- 

 coloured solution did not of itself give any colour when mixed 

 with a sulphide. It was now converted into a prusside by the 

 addition of protosulphate of iron. The resulting prusside was 

 now found to strike a magnificent purple colour with a soluble 

 sulphide. The same coloration was obtained when a prusside 

 was made from common cyanide of potassium added to a solu- 

 tion of protosulphate of iron, through which nitric oxide had 

 been passed. It was obvious from these experiments that 

 nitric oxide was one of the great causes of the change experi- 

 enced by the prusside. 



4. The action of nitric oxide on the prussides themselves 

 was now examined. It was found that nitric oxide could be 

 passed through a solution of ferrocyanide of potassium with- 

 out producing any sensible change. But when the prusside 

 was mixed with sufficient acid to take up its alkaline base, it 

 was now found that nitric oxide was freely absorbed by this 

 mixture when heated, though not in the cold ; and that the 

 resulting liquid exhibited the strong coloration of sulphides. 

 Ferrocyanide of lead, or any other ferrocyanide, gave, when 

 mixed with strong acids, a similar result. It was therefore 

 obvious that the peculiar compound might be obtained from 



