SULPHURETED HYDROGEN. 686 SULPHURIC ACID. iodine decompose sulphureted hydrogen, and this is why the odor of the latter is readily de- stroyed by bleaching-powder. Sulphureted hy- drorrpn is used in the manufacture of certain metallic sulpliides, for the purification of sul- pluiric acid, and extensively in analytical chem- istry. The sulphureted hydrogen produced in certain industrial processes is utilized by burn- ing it and transforming the resulting sulphur di- oxide either into sulphuric acid or into free sul- phur. SULPHURIC ACID, H.SO^. A well-kno™ acid compound of hydrogen, sulphur, and oxygen. Wlien pure and free from water, it is a colorless, oily liquid of specific gravity 1.84 (approximate- Iv) at 0° C. While it may readily be under- cooled, it solidifies normally at 6.79° C. (44.3° F._). At about 290° C. (554° F.) it begins to boil, breaking up into sulphur trioxide and water vapor, and as some of the latter is retained by the boiling acid, the temperature rapidly rises to about 338° C. (640° F.). It is highly'hygro- scopic, absorbing about 30 per cent, of its weight of water. It readily chars organic matter, and is poisonous, not only on account of its power- ful corrosive action, but also on account of a specific effect on the blood. Chemically it is a dibasic acid, either one or both of its hydrogen atoms being replaced ly metals, with the forma- tion, respectively, of either acid or neutral salts (sulphates). Its chemical constitution is gen- erally assumed to be represented by the formula H— "O — S — O — — O — H. By adding to it a little water, it may be caused to form the hydrate HnSO^.H^O, which crystallizes out when the mixture is cooled and may then be readily isolated from the mother-liquor by suction. another well-defined hydrate is formed, having the composition H;SO,.2H.O. The term 'sulphuric acid,' however, as tech- nically and commercially understood, seldom if ever refers to the actual monohydrate or to any of the other recognized hydrates of sulphur tri- oxide. It refers either to a series of solutions of the monohydrate in water or to a series of solutions of sulpluir trioxide in the monohydrate. ( H,SOj + SO3. ) The first series are known com- mercially as 'chamber acid' — 'oil of vitriol' — 'concentrated acid' or in terms of their specific gravity as indicated by a hydrometer scale, as 60° Baum^ acid, 144° Twaddell acid and so forth. The second series are termed 'fuming' or Woodhausen acids, and are always estimated al- kali-metrically according to the percentage of free sulphur trioxide contained. Technically, therefore, "sulphuric acid' miglit be considered to be the generic name of a series of solutions of sulphur trioxide (SO3) in water, some of which solutions are distinguished by variations of properties which occur uniformly with uniform percentage mixtures of sul|)luir trioxide and water and are therefore chemical ' hydrates of sulphur trioxide — most of which are, however,, merely solutions of convenient strength for use in the arts. The manufacture of sulphuric acid is one of the greatest chemical industries. The processes involved incliule the production of sulphur di- oxide, the transformation of this into sulphur trioxide, and the transformation of the trioxide into sulphuric acid. Of these, the oxidation of the dioxide is the most important, the dioxide it- self being readily produced by burning sulphur, DIAGRAM SHOWING CBAXIBEH PROCESS FOB MAKING SULPHURIC ACID. In the flgiirp. A is a bench of pyrites burners, niter oven. etc. The biirner-pras is conducted through pipe D to the Glover tower. E, where it meets the dilute acids and oxides of nitrogen. The fan J carries the gases througrh the pipe I to the first cliamber, K, where oxidation of the sulphur dioxide talces i)lace in presence of water vapor supplied by the steam-line shown above the chambers, thence* to the second and third chambers, M and N, through the flues I, and Ij and surface condensers L and L,. The acid drained from the bottom of each chamber and the condensers is collectH?d in the tanlv R^. The pump Sa delivers this acid to the tank Hj, over the Glover tower, or to the storage tank V, whence it goes to the tank-car V. The strong acid coming from the Glover tower is collected in cooler Q and tankBiand is delivered by the pump Sj to the tank H7 over the second Gay-Lussac tower, P. and to the storage tank U,. The gases from the last chamber, N. are conducted through the pipe I3 to the first Gay-Lussac tower, O, and thence to the second Gay-I.ussac tower. P. their flow being maintained by the fan J,. The exhausted gases pass to the atmosphere at T. The nitrous vitriol from the first Gay-Lussac tower is collected in the tank R4 and is delivered by the pump 84 to tank H over the Glover tower. The nitrous vitriol from the second Gay-Lussac tower, containing but little N2O3. is collected in the tank Kg and is delivered by the pump S3 to the tank Hg over the first Gay-Lussac tower. In different works this scheme varies somewhat In detail, but not in its essential points. Glass bottles containing commercial sulphuric acid often burst in winter, owing to the separa- tion of crystals of this hydrate, whose meltins;- point is about 8° C. (about 46° F.). and which decomposes into .sulphuric acid and water above 205° C. (about 370° F.). If pure sulphuric acid is mixed with water in the proportion of 49 parts of the former to 18 parts of the latter. mineral sulphides, or stilphureted hydrogen, with free access of air, the resulting gases, tech- nically termed 'burner-gas.' including from 5 to 8 per cent, of the dioxide (the remainder is con- stituted by the nitrogen and excessive oxygen of the air and by impurities). The two processes devised to effect the oxidation of the dioxide are alike catalytic in their action, in so far as the
