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§ 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.

It is distinctly a blood poison, the gas uniting with the alkali of the blood, and the sulphide thus produced partly decomposing again in the lung and breathed out as SH₂. Lehmann[60] has studied the effects on animals; an atmosphere containing from 1 to 3 per thousand of SH₂ kills rabbits and cats within ten minutes; the symptoms are mainly convulsions and great dyspnœa. An atmosphere containing from 0·4 to 0·8 per thousand produces a local irritating action on the mucous membranes of the respiratory tract, and death follows from an inflammatory œdema of the lung preceded by convulsions; there is also a paralysis of the nervous centres. Lehmann has recorded the case of three men who breathed 0·2 per thousand of SH₂: within from five to eight minutes there was intense irritation of the eyes, nose, and throat, and after thirty minutes they were unable to bear the atmosphere any longer. Air containing 0·5 per thousand of SH₂ is, according to Lehmann, the utmost amount that can be breathed; this amount causes in half an hour smarting of the eyes, nasal catarrh, dyspnœa, cough, palpitation, shivering, great muscular weakness, headache and faintness with cold sweats. 0·7 to 0·8 per thousand is dangerous to human life, and from 1 to 1·5 per thousand destroys life rapidly. The symptoms may occur some little time after the withdrawal of the person from the poisonous atmosphere; for example, Cahn records the case of a student who prepared SH₂ in a laboratory and was exposed to the gas for two hours; he then went home to dinner and the symptoms first commenced in more than an hour after the first breathing of pure air. Taylor[61] records an unusual case of poisoning in 1857 at Cleator Moor. Some cottages had been built upon iron slag, the slag contained sulphides of calcium and iron; a heavy storm of rain washed through the slag and considerable volumes of SH₂ with, no doubt, other gases diffused during the night through the cottages and killed three adults and three children.

[60] K. B. Lehmann, Arch. f. Hygiene, Bd. xiv., 1892, 135.

[61] Principles and Practice of Medl. Jurisp., vol. ii. 122.

§ 49. Post-mortem Appearances.—The so-called apoplectic form of SH₂ poisoning, in which the sufferer dies within a minute or two, shows no special change. The most frequent change in slower poisoning is, according to Lehmann, œdema of the lungs. A green colour of the face and of the whole body is sometimes present, but not constant. A spectroscopic examination of the blood may also not lead to any conclusion, the more especially as the spectrum of sulphur methæmoglobin may occur in any putrid blood. The pupils in some cases have been found dilated; in others not so.

Chronic poisoning.—Chronic poisoning by SH₂ is of considerable interest in a public health point of view. The symptoms appear to be conjunctivitis, headache, dyspepsia and anæmia. A predisposition to boils has also been noted.

§ 50. Detection.—Both ammonium and hydric sulphides blacken silver and filter-paper moistened with acetate of lead solution. To test for hydric sulphide in air a known quantity may be aspirated through a little solution of lead acetate. To estimate the quantity a decinormal solution of iodine in potassium iodide[62] solution is used, and its exact strength determined by d.n. sodic hyposulphite solution[63]; the hyposulphite is run in from a burette into a known volume, e.g., 50 c.c., of the d.n. iodine solution, until the yellow colour is almost gone; then a drop or two of fresh starch solution is added and the hyposulphite run in carefully, drop by drop, until the blue colour of the starch disappears. If now a known volume of air is drawn through 50 c.c. of the d.n. iodine solution, the reaction I₂ + SH₂ = 2HI + S will take place, and for every 127 parts of iodine which have been converted into hydriodic acid 17 parts by weight of SH₂ will be necessary; hence on titrating the 50 c.c. of d.n. iodine solution, through which air containing SH₂ has been passed, less hyposulphite will be used than on the previous occasion, each c.c. of the hyposulphite solution being equal to 1·11 c.c. or to 1·7 mgrm. of SH₂.

[62] 12·7 grms. of iodine, 16·6 grms. of potassium iodide, dissolved in a litre of water.

[63] 24·8 grms. of sodic hyposulphite, dissolved in a litre of water.