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gravity of 0.925. When heated with ammonia it yields guanidine, and on boiling with alcoholic potash it yields potassium carbonate.

Chlorcarbonic ester, Cl·CO·OC2H5, is formed by the addition of well-cooled absolute alcohol to phosgene (carbonyl chloride). It is a pungent-smelling liquid, which fumes strongly on exposure to air. It boils at 93.1°C., and has a specific gravity of 1.144 (15°C.). When heated with ammonia it yields urethane. Sodium amalgam converts it into formic acid; whilst with alcohol it yields the normal carbonic ester. It is easily broken down by many substances (aluminium chloride, zinc chloride, &c.) into ethyl chloride and carbon dioxide.

Percarbonates.—Barium percarbonate, BaCO4, is obtained by passing an excess of carbon dioxide into water containing barium peroxide in suspension; it is fairly stable, and yields hydrogen peroxide when treated with acids (E. Merck, Abs. J.C.S., 1907, ii. p. 859). Sodium percarbonates of the formulae Na2CO4, Na2C2O6, Na2CO5, NaHCO4 (two isomers) are obtained by the action of gaseous or solid carbon dioxide on the peroxides Na2O2, Na2O3, NaHO2 (two isomers) in the presence of water at a low temperature (R. Wolffenstein and E. Peltner, Ber., 1908, 41, pp. 275, 280). Potassium percarbonate, K2C2O6, is obtained in the electrolysis of potassium carbonate at −10 to −15°.

CARBON BISULPHIDE, CS2, a chemical product first discovered in 1796 by W. A. Lampadius, who obtained it by heating a mixture of charcoal and pyrites. It may be more conveniently prepared by passing the vapour of sulphur over red hot charcoal, the uncondensed gases so produced being led into a tower containing plates over which a vegetable oil is allowed to flow in order to absorb any carbon bisulphide vapour, and then into a second tower containing lime, which absorbs any sulphuretted hydrogen. The crude product is very impure and possesses an offensive smell; it may be purified by forcing a fine spray of lime water through the liquid until the escaping water is quite clear, the washed bisulphide being then mixed with a little colourless oil and distilled at a low temperature. For further methods of purification see J. Singer (Journ. of Soc. Chem. Ind., 1889, p. 93), Th. Sidot (Jahresb., 1869, p. 243), E. Allary (Bull. de la Soc. Chim., 1881, 35, p. 491), E. Obach (Jour. prak. Chem., 1882 (2), 26, p. 282).

When perfectly pure, carbon bisulphide is a colourless, somewhat pleasant smelling, highly refractive liquid, of specific gravity 1.2661 (18°/4°) (J. W. Brühl) or 1.29215 (0°/4°) (T. E. Thorpe). It boils at 46.04° C. (T. E. Thorpe, Journ. Chem. Soc., 1880, 37, p. 364). Its critical temperature is 277.7° C., and its critical pressure is 78.1 atmos. (J. Dewar, Chem. News, 1885, 51, p. 27). It solidifies at about −116°C., and liquefies again at about −110°C. (K. Olszewski, Jahresb., 1883, p. 75). It is a mono-molecular liquid (W. Ramsay and J. Shields, Jour. Chem. Soc., 1893, 63, p. 1089). It is very volatile, the vapour being heavy and very inflammable. It burns with a pale blue flame to form carbon dioxide and sulphur dioxide. It is almost insoluble in water, but mixes in all proportions with absolute alcohol, ether, benzene and various oils. It is a good solvent for sulphur, phosphorus, wax, iodine, &c. It dissociates when heated to a sufficiently high temperature. A mixture of carbon bisulphide vapour and nitric oxide burns with a very intense blue-coloured flame, which is very rich in the violet or actinic rays. When heated with water in a sealed tube to 150° C. it yields carbon dioxide and sulphuretted hydrogen. Zinc and hydrochloric acid reduce it to tri-thioformaldehyde (CH2S)3 (A. Girard, Comptes rendus, 1856, 43, p. 396). When passed through a red-hot tube with chlorine it yields carbon tetrachloride and sulphur chloride (H. Kolbe). Potassium, when heated, burns in the vapour of carbon bisulphide, forming potassium sulphide and liberating carbon. In contact with chlorine monoxide it forms carbonyl chloride and thionyl chloride (P. Schützenberger, Ber., 1869, 2, p. 219). When passed with carbon dioxide through a red-hot tube it yields carbon oxysulphide, COS (C. Winkler), and when passed over sodamide it yields ammonium thiocyanate. A mixture of carbon bisulphide vapour and sulphuretted hydrogen, when passed over heated copper, gives, amongst other products, some methane.

Carbon bisulphide slowly oxidizes on exposure to air, but by the action of potassium permanganate or chromic acid it is readily oxidized to carbon dioxide and sulphuric acid. By the action of aqueous alkalis, carbon bisulphide is converted into a mixture of an alkaline carbonate and an alkaline thiocarbonate (J. Berzelius, Pogg. Ann., 1825, 6, p. 444), 6KHO + 3CS2 = K2CO3 + 2K2CS3 + 3H2O; on the other hand, an alcoholic solution of a caustic alkali converts it into a xanthate (A. Vogel, Jahresb., 1853, p. 643),

CS2 + KHO + R·OH = H2O + RO·CS·SK.

Aqueous and alcoholic solutions of ammonia convert carbon bisulphide into ammonium dithiocarbamate, which readily breaks down into ammonium thiocyanate and sulphuretted hydrogen (A. W. Hofmann),

CS2 + 2NH3 → NH2·CSS·NH4 → H2S + NH4CNS.

Carbon bisulphide combines with primary amines to form alkyl dithiocarbamates, which when heated lose sulphuretted hydrogen and leave a residue of a dialkyl thio-urea,

CS2 + 2R·NH2 → R·NH·CSS·NH3R → CS(NHR)2 + H2S;

or if the aqueous solution of the dithiocarbamate be boiled with mercuric chloride or silver nitrate solution, a mustard oil (q.v.) is formed,

 R·NH·CSS·NH3R + HgCl2 → Hg(R·NH·CSS)2 → 2RNCS + HgS + H2S.

Carbon bisulphide is used as a solvent for caoutchouc, for extracting essential oils, as a germicide, and as an insecticide.

Carbon monosulphide, CS, is formed when a silent electric discharge is passed through a mixture of carbon bisulphide vapour and hydrogen or carbon monoxide (S. M. Losanitsch and M. Z. Jovitschitsch, Ber., 1897, 30. p. 135).

CARBONDALE, a city of Lackawanna county, Pennsylvania, U.S.A., on the Lackawanna river, 16 m. N.E. of Scranton. Pop. (1890) 10,833; (1900) 13,536, of whom 2553 were foreign-born; (1910 census) 17,040. Carbondale is served by the Erie, the Delaware & Hudson (which has machine shops here), and the New York, Ontario & Western railways. The city lies near the upper end of the Lackawanna valley, and the scenery of the surrounding mountains makes it a summer resort of some importance. It has a public library, a small park, an emergency hospital and the Carbondale city private hospital. Carbondale is situated in one of the richest anthracite coal regions of the state, and its principal interest is in coal. Among its manufactures are foundry and machine shop products, sheet-iron, silk, glass, thermometers and hydrometers, bobbins and refrigerating machines. The value of the city’s factory products increased from $1,146,181 in 1900 to $2,315,695 in 1905, or 102%. The settlement of the place began in 1824 with the opening of the coal mines, and Carbondale was chartered as a city in 1851.

CARBONIC ACID, in chemistry, properly H2CO3, the acid assumed to be formed when carbon dioxide is dissolved in water; its salts are termed carbonates. The name is also given to the neutral carbon dioxide from its power of forming salts with oxides, and on account of the acid nature of its solution; and, although not systematic, this use is very common.

CARBONIFEROUS SYSTEM, in geology, the whole of the great series of stratified rocks and associated volcanic rocks which occur above the Devonian or Old Red Sandstone and below the Permian or Triassic systems, belonging to the Carboniferous period. The name was first applied by W. D. Conybeare in 1821 to the coal-bearing strata of England and Wales, including the related grits and limestones immediately beneath them. The term is a relic of that early period in the history of stratigraphy when each group of strata was supposed to be distinguished by some peculiar lithological character. In this case the carbonaceous beds—coal-seams—naturally appealed most strongly to the imagination, and the name is a good one, notwithstanding the fact that coal-seams occupy but a small fraction of the total thickness of the Carboniferous system; and although subsequent investigations have demonstrated the existence of coal in other geological formations, in none of these does it play so prominent a part. The stratified rocks of this system include marine limestones, shales and sandstones; estuarine, lagoonal and fresh-water shales, sandstones and marls with beds of coal, oil-bearing rocks, gypsum and salt.

In many parts of the world there is no sharp line of demarcation between the Devonian and the Carboniferous rocks; neither can the fossil faunas and floras be clearly separated at any well-defined line; this is true in Britain, Belgium, Russia, Westphalia and parts of North America. Again, at the summit of the Carboniferous series, both the rocks and their fossil contents merge gradually into those of the succeeding Permian