SULPHURETTED HYDROGEN, OR HYDROSULPHURIC ACID, H2S. Fig. 54. Experiment 1.-Put half an ounce of ferrous sulphide (FeS) and half an ounce of diluted sulphuric acid into a two-ounce bottle, and quickly stop the bottle with a cork, to which a bent glass tube is adapted. Introduce the longer limb of the tube into a bottle filled with cold water. The atmospheric air contained in the flask and tube first passes over, followed by a very offensive gas, which dis solves in the water, to which it likewise imparts its fetid odour of rotten eggs. This gas is called sulphuretted hydrogen. The following change takes place : FeSH, (SO,) = Fe (SO) + H2S. When the disengagement of the gas ceases, add some diluted sulphuric acid, that the gas may again be generated. The water is known to be saturated with the gas when, on shaking the bottle, the finger by which the opening is closed is no longer sucked in, or, more correctly speaking, pressed in. One measure of water contains about two and a half measures of gas in a saturated solution. It is put up in small well-stoppered bottles. If the air be admitted, the solution becomes turbid, owing to the oxygen of the air uniting with the hydrogen of the sulphuretted hydrogen, forming water, and the consequent liberation of the sulphur as a fine powder. If, during the evolution of the gas, the bottle of water be removed, the gas issuing from the tube can be ignited by a match; it burns with a blue flame, and its nauseous odour is no longer perceptible, but is replaced by the well-known odour of burning sulphur. Both constituents unite with the oxygen of the air, the sulphur forming sulphurous anhydride, and the hydrogen, water. The inhalation of sulphuretted hydrogen is detrimental to health; hence precautions should be taken to avoid it. When experimenting with it, it is best to do so where there is a free circulation of air. A cloth moistened with a little alcohol, and held before the mouth, is likewise a good protection. Sulphuretted hydrogen turns blue litmus-paper red; it also combines with bases, and hence it may be regarded as an acid. Experiment 2.-Drop some sulphuretted hydrogen water upon a bright silver or copper coin, and upon a piece of lead and iron. The first three metals tarnish quickly, and finally become black; they combine with the sulphur, forming dark metallic sulphides, whilst the hydrogen escapes; the iron, on the contrary, undergoes no change. Experiment 3.- Put into one test-tube a small portion of litharge, into another some ignited iron-rust, and pour upon them liquid hydrosulphuric acid. The yellow litharge, oxide of lead, becomes immediately black, an exchange of elements takes place, the hydrosulphuric acid gives its sulphur to the lead of the litharge, and receives in return the oxygen of the latter. Accordingly, lead sulphide and water are formed, and the offensive odour disappears. In the vessel containing the iron-rust, neither the colour nor the smell is changed-a proof that no chemical change has taken place. Experiment 4.-Repeat the same experiment with a small crystal of sugar of lead instead of the litharge, and some green vitriol instead of the iron-rust, together with a few drops of vinegar, these salts having been previously dissolved in a large quantity of water; the result will be the same as in the former experiment. Sugar of lead is lead acetate; the salt of lead is converted into sulphide, which subsides sooner or later as a black precipitate. When this solution is extremely diluted it is only rendered brown. Acetic acid is set free, and remains in solution. Most of the metallic sulphides are insoluble in water; hence sulphuretted hydrogen is peculiarly adapted for precipitating metals from their solution, so that they can be separated and collected by filtration. If sulphuretted hydrogen be passed through a solution of acetate or sulphate of copper, sulphide of copper will be precipitated, and can be separated by filtration from the acid. All the sulphides do not possess a black colour; sulphide of antimony has an orange-red colour; sulphide of arsenic a yellow, and sulphide of zinc a white colour. On this is partly based the application of sulphuretted hydrogen as a test; that is, as a means of detecting many metals. Wine containing lead is blackened by hydrosulphuric acid. Many metals are precipitated from their acidified solutions by the addition merely of sulphuretted hydrogen, as sulphides; for example, copper, silver, gold, lead, mercury, tin, antimony, and arsenic; and others are not precipitated until a base is added; for example, iron, zinc, manganese, cobalt, and nickel. Sulphuretted hydrogen may accordingly be used to separate one metal from another; it is, therefore, an important means of separation in analytical chemistry. Sulphuretted hydrogen has, as already mentioned, the formula HS, which indicates that it is composed of two atoms of hydrogen and one of sulphur, and the similarity of this formula to that of water, H2O, is apparent. Lead paper is used for the detection of sulphuretted hydrogen, by which it is coloured brown or black. It is made by passing strips of paper through a weak solution of sugar of lead in water. Finally, it remains to be stated that this gas occurs also in some mineral waters, as may be recognised by the smell and taste. Many of these springs-for instance, the celebrated springs of Harrogate and Aix-la-Chapelle-are resorted to by invalids, and are called sulphur springs. A rotten wooden water-pipe may convert an otherwise potable water, if it should contain calcium sulphide, into a nauseous sulphuretted water; but by clearing out the well, and laying down new pipes, the water may be rendered completely odourless and good. COMPOUNDS OF SULPHUR AND OXYGEN. Sulphur combines with oxygen in two proportions, forming the oxides SO, and SO,; they both unite with water to form acids. SO, is hence called sulphurous anhydride, and SO, sulphuric anhydride. The latter substance can only be prepared indirectly and with difficulty, whereas sulphurous anhydride is formed by the direct union of its constituents whenever sulphur is burnt in air or oxygen, as has been already mentioned (page 124). Sulphurous Anhydride, SO2, and Sulphurous Acid, H2SO,.— Sulphurous anhydride is a colourless gas of a pungent suffocating odour, well known as the smell of sulphur. One of its most characteristic properties is its power of bleaching animal and vegetable colours. Experiment 1.-Suspend a red flower, such as a rose or peony, in an inverted glass jar, or other similar vessel, and introduce a piece of ignited sulphur. It will be found that the colour of the flower is slowly changed to white. Sulphurous anhydride, unlike chlorine, does not actually destroy the colouring matter, for by dipping the flower into dilute sulphuric acid the original colour is slowly restored. Straw, wool, silk, &c., are very commonly bleached with sulphurous anhydride, but the result of its action on these materials is far from permanent; for on exposing a bleached straw hat in the sunshine it again becomes yellow, or "sun-burnt," as the effect is popularly termed; and on washing whitened flannel it resumes its original appearance. Sulphurous anhydride can be formed in a variety of ways; one of the most convenient, when a quantity is required in a tolerable state of purity, is the following: Experiment 2.-Select a thin-bottomed, well-made Florence flask, fit it with a delivery tube, and place in it half-anounce of thin copper, sheet or turnings, and two ounces of strong sulphuric acid. Apply a gentle but gradually increasing heat, and after a time sulphurous anhydride will be evolved, according to the equation: Cu+ 2H, SO = CuSO, + 2H2O+ SO2. Sulphurous anhydride is more than twice as heavy as air, so it may be collected in dry bottles by displacement; that is, by allowing the gas to pass to the bottom of a dry bottle. It will gradually displace the lighter air, just as water does when poured into a bottle, and as it is very soluble in water this mode is the most convenient. Experiment 3.-If a lighted taper is held over burning sulphur, or introduced into a vessel of sulphurous anhydride gas, it will be extinguished. It may be now readily explained how chimneys on fire are extinguished by scattering sulphur on the coals beneath; the sulphurous acid gas ascends in the chimney, and expels the atmospheric air present in it; the glowing soot is thereby deprived of the free oxygen, and is extinguished. Experiment 4.-Pour a little blue litmus solution into a bottle of sulphurous anhydride; it will be reddened, showing the acid character of the gas. Experiment 5.-Dip the end of the delivery tube from which the gas is issuing into a little water; the gas is absorbed in large quantities and imparts to the water its characteristic taste and smell. One measure of water dissolves about forty measures of sulphurous anhydride. The solution obtained is one of sulphurous acid: The salts corresponding to sulphurous acid are called sulphites: H2SO, Sulphurous acid. Na, SO3, Sodium sulphite. Experiment 6.-When the gas ceases to be absorbed by the water, substitute for the latter a solution of sodium carbonate; this likewise absorbs the gas and forms with it sodium sulphite, while the carbonic acid is liberated with effervescence. If a sulphite be treated with a strong acid it is decomposed and sulphurous anhydride is evolved. Experiment 7.-Place a few grains of sodium sulphite in a test-tube, pour upon it a little slightly diluted sulphuric acid, and gently warm. Sulphurous anhydride is given off and is readily identified by its odour: Na2SO+H2SO, Na2SO4 + H2O + SO2. = Experiment 8.-When the residue in the flask, in which the sulphurous anhydride was generated, has become cold, add water to it, and heat it gently to boiling until the residual mass is dissolved. The solution is dark and turbid from impurities contained in the metal, but after filtering it is of a beautiful blue colour, and transparent. If allowed to cool slowly, blue crystals of copper sulphate, Cu" SO, 5H2O (blue vitriol), of considerable size will be formed. Silver and mercury comport themselves like copper and may be used instead, but they are not as economical. Sulphuric anhydride, SO,, and Sulphuric Acid, H2(SO4). Sulphuric Anhydride.-Experiment 9.-Pour into a small flask, placed in a sand-bath over a tripod, half an ounce of fuming, or Nordhausen sulphuric acid, and heat it gently L |