156 Action of Iodine upon Sulphides. 



we are confident that iodine (in excess) with water decom- 

 poses pyrite. 



Besides the natural sulphides, we examined the behavior of 

 a few minerals, belonging to various classes, with an aqueous 

 solution of iodine. Limonite, hematite, magnetite, and the 

 manganese oxides, as well as pyromorphite and calamine, are 

 not attacked even on boiling the solution. Cuprite is com- 

 pletely decomposed in the cold, with formation of a bright 

 green crystalline precipitate (CuI0 5 ?). Brucite, calcite, and 

 natrolite, are decidedly attacked on boiling. Anglesite is 

 feebly attacked in the cold, and strongly on heating; cerus- 

 site and vivianite appear to be slightly decomposed under 

 the same circumstances. None of these reactions have special 

 interest, except perhaps that of cuprite, which is quite charac- 

 teristic. 



Having continually in mind the possible application of 

 methods to the examination of minerals in the field, and rec- 

 ognizing the impracticability of carrying iodine save in glass 

 bottles, we were led to make another series of experiments 

 with a view to providing iodine in a portable state. To this 

 end, we took advantage of the ready decomposition of potas- 

 sium iodide by organic acids. That even the weakest acids are 

 able to set hydriodic acid free, in acting on the iodides of the 

 alkaline metals, has been frequently noticed,* and we add the 

 the following observations : 



When solid potassium iodide is added to strong acetic acid, 

 in the cold, hydriodic acid is set free ; and after standing for 

 some time (24-48 hours), this acid decomposes, and the free 

 iodine communicates to the solution a brownish-red color of 

 varying intensity. With free access of air, the liberation of 

 iodine begins at once, as shown by testing the mixture with 

 carbon disulphide. This action is more marked in strong 

 solutions than in weak. 



* Dr. Rudolph Boehm, in his Handbuch der Spec. Pathologie und Therapie, vol. XV, p. 

 21, quotes Struve's discovery that even hydro-sodium carbonate decomposes potassium 

 iodide in dilute aqueous solution. This power of carbonic acid has also been observed by 

 Hammerer (Virchow's Archiv, LIX, 1874), and Binz (ibid., LXII). 



