_Chemical Properties._--Iodine belongs to the Chlorine group of elements, characterized by forming acids with Hydrogen, and combining extensively with the metals (see Chlorine). They are however comparatively indifferent to Oxygen, and also to each other. The Iodides of the alkalies and alkaline earths are soluble in water; also those of Iron, Zinc, Cadmium, etc. The Iodides of Lead, Silver, and Mercury are nearly or quite insoluble. Iodine possesses the property of forming a compound of a deep blue colour with Starch. In using this as a test, it is necessary first to liberate the Iodine (if in combination) by means of Chlorine, or Nitric Acids saturated with Peroxide of Nitrogen. The presence of Alcohol or Ether interferes to a certain extent with the result. IODIDE OF AMMONIUM. Symbol, NH{4}I. Atomic weight, 144. The preparation and properties of this salt are described at page 198, to which the reader is referred. IODIDE OF CADMIUM. Symbol, CdI. Atomic weight, 182. See page 199, for the preparation and properties of this salt. IODIDE OF IRON. Symbol, FeI. Atomic weight, 154. Iodide of Iron is prepared by digesting an excess of Iron filings with solution of Iodine in Alcohol. 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 Proto-iodide 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 colour; 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 6O°; 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 coloured brown by free Chlorine; also very rapidly by Peroxide of Nitrogen (page 86); ordinary acids however act less quickly, Hydriodic Acid being first formed, and subsequently decomposing spontaneously.