§ 46. =Detection of Free Chlorine.=--The usual method of detection is to prepare a solution of iodide of potassium and starch and to soak strips of filter-paper in this solution. Such a strip, when moistened and submitted to a chlorine atmosphere, is at once turned blue, because chlorine displaces iodine from its combination with potassium. Litmus-paper, indigo blue or other vegetable colours are at once bleached. To estimate the amount of chlorine a known volume of the air is drawn through a solution of potassium iodide, and the amount of iodine set free, determined by titration with sodic hyposulphite, as detailed at p. 74. III.--Hydric Sulphide (Sulphuretted Hydrogen). § 47. Hydric sulphide, SH₂, is a colourless transparent gas of sp. gravity 1·178. It burns with a blue flame, forming water and sulphur dioxide, and is soluble in water; water absorbing about three volumes at ordinary temperatures. It is decomposed by either chlorine gas or sulphur dioxide. It is a common gas as a constituent of the air of sewers or cesspools, and emanates from moist slag or moist earth containing pyrites or metallic sulphides; it also occurs whenever albuminous matter putrefies; hence it is a common constituent of the emanations from corpses of either man or animals. It has a peculiar and intense odour, generally compared to that of rotten eggs; this is really not a good comparison, for it is comparing the gas with itself, rotten eggs always producing SH₂; it is often associated with ammonium sulphide. § 48. =Effects.=--Pure hydric sulphide is never met with out of the chemist’s laboratory, in which it is a common reagent either as a gas or in solution; so that the few cases of poisoning by the pure gas, or rather the pure gas mixed with ordinary air, have been confined to laboratories. The greater number of cases have occurred accidentally to men working in sewers, or cleaning out cesspools and the like. In small quantities it is always present in the air of towns, as shown by the blackening of any silver ornament not kept bright by frequent use.