SULPHUR GROUP.] CHEMISTRY 503 the colour may usually be restored by acids or alkalies. The reproduction of the yellow colour of new flannel, when it is washed with an alkaline soap for the first time, is an illustration of this. Sulphurous acid is also a powerful antiseptic, and is highly valuable as a disinfecting agent. Selenious acid is deposited from a hot aqueous solution on slow cooling in colourless prismatic crystals like salt petre. It closely resembles sulphurous acid in properties, and like it furnishes acid selenites, such as potassium hydro gen selenite, KHSeO 3 , and normal selenites, such as potas sium selenite, K 2 Se0 3 , but it also forms so-called liyperadd salts with the alkali metals, e.g., HKSe0 3 + H 2 SeO 3 . The selenites of alkali metals are soluble in water, but the other selenites are insoluble. Tellufous hydrate, prepared by decomposing tellurium tetrachloride with water, is a bulky precipitate, which, when dried over sulphuric acid, forms a light white earthy powder, having a bitter metallic taste. It is only slightly soluble in water, but dissolves in alkalies and alkaline carbonates. With the alkali metals tellurous acid forms three classes of salts corresponding to the three classes of selenites, of which the following are examples Acid potassium tellurite KHTe0 3 Normal potassium tellurite ..K 2 Te0 3 Hyperacid potassium tellurite. ...KHTe0 3 +H a Te0 3 With the alkaline earth metals tellurous acid forms normal salts, and also certain peculiar salts, such as BaTe^Og or BaTe0 3 + Te0 2 , and BaTe 4 O 9 or BaTe0 3 + 3Te0 2 . With the heavy metals it appears to form only normal salts. The tellurites of alkali metals are soluble in water, but those of other metals are difficultly soluble or insoluble. In its oxides, therefore, it will be evident tellurium differs widely from selenium and sulphur, but it much re sembles metals like bismuth and antimony, since it forms salts with acids. Thus, a tellurium sulphate of the compo sition Te(S0 4 ) 2 , it is stated, is formed by dissolving tellu rium in concentrated sulphuric acid. Tellurous hydrate also dissolves readily in acids ; the solution in hydrochloric acid probably contains either tellurium chloride, TeCl 4 , or a compound intermediate ia composition between tellurium chloride and tellurium hydroxide, Te(OH) 4 , formed from the latter by the partial displacement of the OH groups by chlorine ; when it is dissolved in oxy-acids, apparently salts are formed vn which the group TeO displaces hydrogen, corresponding to the antimony salts in which the radicle SbO displaces hydrogen. Sulphurous acid, on account of its tendency to form sulphuric acid, has considerable power as a reducing agent. Thus iodine and sulphurous acid, in presence of a large quantity of water, yield hydriodic and sulphuric acid I 2 + H 2 + H 2 S0 3 = 2HI + H 2 S0 4 . Sulphurous acid. Sulphuric acid. With chlorine and bromine similar reactions occur still more readily. On this account, sulphurous acid is largely employed as an "antichlor" to remove the excess of chlorine from articles bleached with bleaching powder. When solutions of sulphurous and selenious acid are mixed, the latter is reduced, especially on warming, and red amorphous selenium precipitated ; similarly, a black precipitate of tellurium is formed on warming a solution containing tellurous and sulphurous acids. But these acids also readily part with at least a portion of their oxygen. For instance, when hydrogen sulphide gas is passed into a solution of sulphurous acid, sulphur is depo sited, and the solution contains pentathionic acid 5H 2 S + 5H 2 S0 3 = 5S + H.,S 5 p 6 + 90H 2 . Hydrogen sulphide. ^l 6 Sulphur ^ f^ Water. When a solution of selenious acid is similarly treated, a precipitate is thrown down, which apparently consists chiefly of selenium sulphide, SeS 2 , mixed with a lower sulphide and free sulphur. Solutions of tellurous hydrate when thus treated furnish a brown precipitate of tellurium sulphide. Sulphur Trioxide, S0 3 . Dry sulphur dioxide gas and oxygen readily combine when the mixture is passed over gently heated platinum black or platinized pumice, forming sulphur trioxide or sulphuric anhydride, S0 3 . This compound may be ob tained from sulphuric acid by distilling it with phosphoric anhydride : H 2 S0 4 + P 2 5 = S0 3 + 2HP0 3 . Sulphuric acid. osphoric Sulphur trioxide. Metaphosphoric It is usually prepared from Nordhausen sulphuric acid, which gives off sulphur trioxide when gently heated, ordinary sulphuric acid remaining behind. Sulphur trioxide usually crystallizes in white slender needles, but it exists in two modifications. Thus, when the liquid oxide is cooled, it solidifies at 1G C. in long colourless prisms, which melt at the same temperature ; but if kept at temperatures below 25, it changes into a mass of fine white needles. This second modification gradually liquefies at temperatures above 50, and again passes into the first modification; it dissolves with extreme slowness in sulphuric acid, whereas the liquid oxide is miscible in all proportions with the acid. Liquid sulphuric anhydride undergoes very great expansion by heat, its mean co efficient of expansion between 25 and 45C. being 0027 for 1 C. ; it is quite colourless when pure. Sulphur trioxide very readily parts with one of its atoms of oxygen, convert ing phosphorus trichloride, for example, into phosphorus oxy trichl oride PC1 3 Phosphorus trichloride. S0 3 = Sulphur trioxide. SO 2 Sulphur dioxide. POC1 3 . Phosphorus oxytrichloride. This reaction takes place when the substances are merely mixed together, although a strong red heat is necessary in order to resolve the trioxide into sulphur dioxide and oxygen. When finely divided sulphur is added in small quanti ties to liquid sulphuric anhydride, drops of a deep blue colour sink to the bottom and solidify immediately. These consist of the compound S 2 O 3 . The temperature during the operation must be kept at 15 C., for if it is lower the anhydride does not remain liquid, and if higher the sub stance decomposes. When about a gramme of sulphur has been added the anhydride is poured off, and the solid re sidue freed from any that remains by a gentle heat. A bluish-green crystalline mass is thus obtained, which de composes at ordinary temperatures, giving off sulphur dioxide, and leaving sulphur ; water immediately decom poses it, forming sulphuric, sulphurous, and probably thio- sulphuric acids. The corresponding selenium compound, SeS0 3 , may be prepared in a similar manner. Sulphuric Acid, H 2 S0 4 . Of all chemical compounds this is probably the most important, on account of its numerous practical applica tions, and enormous quantities of it are now manufactured. The method employed consists essentially in oxidizing sulphurous acid by atmospheric oxygen, which is accom plished with the aid of nitric oxide gas in the following manner. Sulphur dioxide gas is prepared by burning sul phur, or iron pyrites, FeS 2 , in a properly constructed furnace, sulphur being always employed when the purest qualities of acid are required, and this gas is conveyed into a large leaden chamber, into which steam and air are also continu ously admitted. At the back of the furnace in which the
sulphur is burnt, a small vessel is placed containing sodium