Action of Iodine upon Sulphides. 157 



Tartaric and citric acids decompose potassium iodide in a 

 similar manner. 



A mixture of any of these organic acids with potassium 

 iodide proves to be a powerful solvent of the natural sulphides. 

 An examination of the seventeen sulphides already mentioned 

 gave the following results : — 



BEHAVIOR WITH POTASSIUM IODIDE AND CITRIC ACID. 



Stibnite, argentite, sphalerite, chalcoeite, bornite, and ull- 

 mannite, are attacked in the cold, with liberation of sulphur- 

 etted hydrogen ; heat increases the action, — the gas coming off 

 freely. 



Galenite acts in the same manner, with formation of yellow 

 crystalline flakes of plumbic iodide. On applying heat, tiie 

 mineral is completely decomposed. 



Cinnabar is also decomposed in the cold, with liberation of 

 sulphuretted hydrogen. On boiling, the decomposition is 

 complete, and crystals of mercuric iodide deposit on cooling, — 

 provided no great excess of potassium iodide is present. 



Arsenopyrite is decomposed in the cold, but no sulphuretted 

 hydrogen appears to be set free. On boiling, the decomposition 

 proceeds rapidly. 



Pyrrhotite, chalcopyrite, and bournonite yield no sulphur- 

 etted hydrogen in the cold, but freely on heating. The latter 

 is but feebly attacked. 



Pyrite, niccolite and smaltite, give no sulphuretted hydrogen, 

 either cold or hot, though they are strongly attacked on boil- 

 ing the liquid. 



Tetrahedrite is attacked feebly in the cold ; on heating, the 

 action is increased. 



Molybdenite resists the action of this mixture of reagents. 



Similar results were observed with tartaric and oxalic acids, 

 their decomposing power varying. Cinnabar, treated with a 

 mixture of potassium iodide and oxalic acid, is decomposed 

 with remarkable facility; the reaction sets in immediately 

 and in the cold; on boiling, the decomposition continues with 



