Iodide of iron, in a fit state for photographic use, is easily obtained by dissolving a drachm of iodine in an ounce of _proof spirit_--that is, a mixture of equal bulks of spirits of wine and water--and adding an excess of iron filings. After a few hours, a green solution is obtained without the aid of heat. The presence of metallic iron in excess prevents the liberation of iodine and deposit of peroxide of iron which would otherwise speedily occur. It is very soluble in water and alcohol, but the solution rapidly absorbs oxygen and deposits peroxide of iron; hence the importance of preserving it in contact with metallic iron, with which the separated iodine may recombine. By very careful evaporation, hydrated crystals of protoiodide may be obtained, but the composition of the solid salt usually sold under that name cannot be depended on. The _periodide_ of iron, corresponding to the perchloride, has not been examined, and it is doubtful if any such compound exists. Iodide of Potassium. Symbol, KI. Atomic weight, 166. This salt is usually formed by dissolving iodine in solution of potash until it begins to acquire a brown color; a mixture of iodide of potassium and _iodate of potash_ (KO IO{5}) is thus formed; but by evaporation and heating to redness, the latter salt parts with its oxygen, and is converted into iodide of potassium. _Properties._--It forms cubic and prismatic crystals, which should be hard, and _very slightly or not at all deliquescent_. Soluble in less than an equal weight of water at 60°; it is also soluble in alcohol, but not in ether. The proportion of iodide of potassium contained in a saturated alcoholic solution, varies with the strength of the spirit,--with common spirits of wine, sp. gr. ·836, it would be about 8 grains to the drachm; with alcohol rectified from carbonate of potash, sp. gr. ·823, 4 or 5 grains: with absolute alcohol, 1 to 2 grains. The solution of iodide of potassium is instantly colored brown by free chlorine; also very rapidly by peroxide of nitrogen; ordinary acids, however, act less quickly, hydriodic acid being first formed, and subsequently decomposing spontaneously. Iodide of potassium, as sold in the shops, is often contaminated with various impurities. The first and most remarkable is _carbonate of potash_. When a sample of iodide of potassium contains much carbonate of potash, it forms small and imperfect crystals, which are strongly alkaline to test-paper, and become moist on exposure to the air, from the deliquescent nature of the alkaline carbonate. _Sulphate of potash_ is also a common impurity; it may be detected by chloride of barium.