This is a gaseous compound of hydrogen and iodine, corresponding in composition to the hydrochloric acid. It cannot, however, from its instability, be obtained in the same manner, since, on distilling an iodide with sulphuric acid, the hydriodic acid first formed is subsequently decomposed into iodine and hydrogen. An aqueous solution of hydriodic acid is easily prepared by adding iodine to water containing sulphuretted hydrogen gas; a decomposition takes place, and sulphur is set free; thus: HS + I = HI + S. _Properties._--Hydriodic acid is very soluble in water, yielding a strongly acid liquid. The solution, colorless at first, soon becomes brown from decomposition, and liberation of free iodine. It may be restored to its original condition by adding solution of sulphuretted hydrogen. Hydrosulphuric Acid. Symbol, HS. Atomic weighty 17. This substance, also known as sulphuretted hydrogen, is a gaseous compound of sulphur and hydrogen, analogous in composition to hydrochloric and hydriodic acids. It is usually prepared by the action of dilute sulphuric acid upon sulphuret of iron, the decomposition being similar to that involved in the preparation of the hydrogen acids generally:-- FeS + HO SO{3} = FeO SO{3} + HS. _Properties._--Cold water absorbs three times its bulk of hydrosulphuric acid, and acquires the peculiar putrid odor and poisonous qualities of the gas. The solution is faintly acid to test-paper, and becomes opalescent on keeping, from gradual separation of sulphur. It is decomposed by nitric acid, and also by chlorine and iodine. It precipitates silver from its solutions, in the form of black sulphuret of silver; also copper, mercury, lead, etc.; but iron and other metals of that class are not affected, if the liquid contains free acid. Hydrosulphuric acid is constantly employed in the chemical laboratory for these and other purposes. Hydrosulphate of Ammonia. Symbol, NH{4}S HS. Atomic weight, 51. The liquid known by this name, and formed by passing sulphuretted hydrogen gas into ammonia, is a double sulphuret of hydrogen and ammonium. In the preparation, the passage of the gas is to be continued until the solution gives no precipitate with sulphate of magnesia and smells strongly of hydrosulphuric acid. Properties,--Colorless at first, but afterwards changes to yellow, from liberation and subsequent solution of sulphur. Becomes milky on the addition of any acid. Precipitates, in the form of sulphuret, all the metals which are affected by sulphuretted hydrogen; and, in addition, those of the class to which iron, zinc, and manganese, belong. Hydrosulphate of ammonia is employed in photography to darken the negative image, and also in the preparation of iodide of ammonium; the separation of silver from hyposulphite solutions, etc. Hyposulphite of Soda. Symbol, NaO S{2}H{2} + 5 HO. Atomic weight, 125.