Iodide of Ammonium. Symbol, NH{4}I. Atomic weight, 144. This salt may be prepared by adding carbonate of ammonia to iodide of iron, but more easily by the following process:--A strong solution of hydrosulphate of ammonia is first made, by passing sulphuretted hydrogen gas into liquor ammoniæ To this liquid iodine is added until the whole of the sulphuret of ammonium has been converted into iodide. When this point is reached, the solution at once colors brown from solution of free iodine. On the first addition of the iodine, an escape of sulphuretted hydrogen gas and a dense deposit of sulphur take place. After the decomposition of the hydrosulphate of ammonia is complete, a portion of hydriodic acid--formed by the mutual reaction of sulphuretted hydrogen and iodine--attacks any carbonate of ammonia which may be present, and causes an effervescence. The effervescence being over, the liquid is still acid to test-paper, from excess of hydriodic acid; it is to be cautiously neutralized with ammonia, and evaporated by the heat of a water-bath to the crystallizing point. The crystals should be thoroughly dried over a dish of sulphuric acid, and then sealed in small tubes containing each about half a drachm of the salt; by this means it will be preserved colorless. Iodide of ammonium is very soluble in alcohol, but it is not advisable to keep it in solution, from the rapidity with which it decomposes and becomes brown. The most common impurity of commercial iodide of ammonium is sulphate of ammonia; it is detected by its sparing insolubility in alcohol. Iodide of Cadmium. Symbol, CdI. Atomic weight, 182. This salt is formed by heating filings of metallic cadmium with iodine, or by mixing the two together with addition of water. It is useful in iodizing collodion intended for keeping, since it does not become brown from liberation of free iodine with the same rapidity as the alkaline iodides. Iodide of cadmium is very soluble both in alcohol and water; the solution yielding on evaporation large six-sided tables of a pearly lustre, which are permanent in the air. The crystalline form of this salt is a sufficient criterion of its purity. Iodide of Iron. Symbol, FeI. Atomic weight, 154.