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the crude substance mixed with earthy impurities, is heated in earthen pots (Figure 14); the sulphur distils over in the form of vapor, which is condensed in similar pots placed outside the furnace. The sulphur thus obtained is refined or purified by a second distillation.

If the vapor of sulphur is quickly cooled below its melting-point, it solidifies in a crystalline powder, called

flowers of sulphur; just as watery vapor, when cooled below the freezing point of water, becomes snow. If

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sulphur is gently heated, it melts, and may be cast into sticks, and is then known as brimstone, or roll sulphur.

45. Properties of Sulphur.-Sulphur is a yellow solid, and exists in three allotropic forms (14). The first is that in which it crystallizes in nature. The other two are obtained by melting sulphur. If melted sulphur be allowed to cool slowly, it crystallizes in long, transparent needle-shaped crystals, which are quite different in form from the natural crystals, and have a specific

gravity of 1.98, while the natural crystals have a specific gravity of 2.07. These transparent crystals become opaque, after an exposure

of a few days to the air, owing to their breaking up into several crystals of the natural and permanent form. The third allotropic form of sulphur is obtained by pouring melted sulphur at 450° into cold water. It then forms a soft, tenacious mass, resembling india-rubber, and has a specific gravity of 1.96. This form of sulphur is not permanent. In a few hours it returns to its ordinary brittle state. These peculiar modifications are also apparent when sulphur is heated. It begins to melt at 239°, and forms an amber-colored limpid liquid; as the temperature rises, it begins to thicken, and to become dark-colored, so that at about 450° it can scarcely be poured from the vessel ; heated still higher, it again becomes fluid, and remains as a dark thin liquid, till the temperature rises to 836°, when it begins to boil, and gives off a red vapor.

Sulphur is an inflammable substance, and when heated in the air or oxygen burns with a bluish flame, and with the suffocating odor so well known in the burning of a common lucifer match. It is insoluble in water and in most liquids, but dissolves in carbonic bisulphide (50).

46. Sulphurous Anhydride (Sulphurous Acid).This compound, SO2, as we have seen (11), is formed when sulphur is burned in oxygen, or in the air, which is a mixture of oxygen and other gases. The suffocating odor which we perceive when a common friction match is lighted is caused by SOZ.

The most important property of So, is its power of bleaching silk, woollen, straw, etc. It is largely used for this purpose in the arts.

This property may be illustrated by holding a red rose or other flower in a jar of SO2. The flower quickly loses its color.

47. Sulphuric Acid. - Sulphuric acid, or oil of vitriol, is the most important and most powerful of all the acids. It has a very strong attraction for water, and when the two are mixed, intense heat is developed. Hence caution is necessary in mixing them, especially in thick glass vessels. The acid should always be poured into the water, not the water into the acid. It rapidly chars all organic matter, and is a valuable drying agent.

Sulphuric acid was first prepared by distilling green vitriol, or ferrous sulphate, whence its name, oil of vitriol. In this form it is sometimes known as Nordhausen acid, from the name of the city in Saxony where it was first manufactured. It is a fuming liquid, denser than the ordinary acid, and is a mixture of this acid with the sulphuric anhydride.

This method has long been superseded by a more convenient one, which depends upon the fact that, though SO, does not combine with free oxygen and water to form sulphuric acid, it is capable of taking up the oxygen when the latter is united with nitrogen in the form of nitrous anhydride, N,Oz. Thus:

SO, +1,0 +0,0,= H,SO, + 2NO. Sulphurous anhydride, water, and nitrous anhydride, yield sulphuric acid and nitric oxide.

The NO formed in this decomposition takes up another atom of oxygen from the air, becoming 1,0g, and this is again able to convert a second molecule of SO2, with water, H,O, into sulphuric acid, H,SO4; being a second time reduced to NO, and ready again to take up another atom of oxygen from the air. Hence it is clear that the NO acts simply as a carrier of oxygen between the air and the SO,; and a very small quantity of it is therefore able to convert a very large quantity of sulphurous anhydride, water, and nitrous anhydride, iņto sulphuric acid.

This process is conducted on a large scale in chambers made of sheet-lead, which are often of a capacity of 50,000 or 100,000 cubic feet.

The sulphurous anhydride is obtained either by burning sulphur in a current of air, or by roasting a mineral called iron pyrites (ferric bisulphide, Fes), and is led, together with air, into the chamber. The N, O, is got from sodic nitrate (soda saltpetre), which is decomposed either by the heat of the burning sulphur, or in a separate furnace. Jets of steam are also blown into the chamber, and a thorough draft is maintained by a high chimney. The sulphuric acid, as it forms, falls to the floor of the chamber, whence it is continually drawn off. It is then heated, first in open leaden pans, and then in vessels of glass or platinum (as lead is attacked by the strong acid), until the excess of water is driven off, and a sufficiently concentrated acid is obtained.

So extensively is this acid used, that the quantity manufactured in South Lancashire (England) alone exceeds 3,000 tons a week.

This method of making sulphuric acid may be illustrated, on a small scale, by dipping a shaving or cloth in nitric acid, and then holding it in a jar of sulphurous anhydride, with a little water at the bottom of the jar. Red fumes at once appear, which show that the nitric acid has lost some of its oxygen, having given it up to the SO,, which thus becomes Sog. If the experiment be repeated a number of times, the water at the bottom of the jar will be found to contain sulphuric acid.

48. Sulphuric Anhydride. - If fuming Nordhausen acid be gently heated in a glass retort, which is connected with a receiver, kept cool by ice, white fumes will pass over and solidify into a white silky fibrous

This solid is sulphuric anhydride. It has no acid properties, and can be moulded in the fingers with

mass.

out charring the skin. When thrown on water, the heat emitted is so intense that it hisses like red-hot iron.

49. Hydric Sulphide, Sulphuretted Hydragen, or Hydrosulphuric Acid. Sulphur combines with hydrogen, forming hydric sulphide, H,S, a compound analogous in composition to water. It is a gas of a very offensive odor, and very poisonous. It reddens litmuspaper, and is therefore an acid. It is best prepared by the action of dilute sulphuric acid on ferrous sulphide, FeS. Thus :

Fe$+H,SO,= H,S + FeSO4 The hydrogen and the iron change places, forming hydric sulphide and ferrous sulphate (sulphate of iron, or green vitriol).

This gas is an invaluable agent in chemical analysis. 50. Carbonic Bisulphide. - If the vapor of sulphur be passed over red-hot charcoal, a volatile compound, CS,, is formed. It is seen to be analogous to carbonic acid, CO2. It is a very inflammable, colorless liquid, of an offensive odor, and is called carbonic bisulphide (or bisulphide of carbon). It is of great importance in the arts, since it dissolves gums, caoutchouc, sulphur, and phosphorus.

51. The Oxygen Group. — The two compounds, H S and CS2, are analogous in composition to H,0 and CO2, and it is true in general that for every oxide there is a corresponding sulphide. Thus we have K,O, potassic oxide (potassa), and K,S, potassic sulphide (sulphide of potassium); Bao, baric oxide (baryta), and BaS, baric sulphide (sulphide of barium); Al,0g, aluminic oxide (alumina), and Al,Sz, aluminic sulphide (sulphide of aluminium); and so on.

Oxygen and sulphur, then, belong to the same group of elements, which may be called the oxygen group. To

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