Coal gas is a combination of various gases, whose illuminating properties depend upon, and are exactly in proportion to, the quantity of carbon they contain. The particles of carbon raised to a white heat give the light, for the gaseous part has a feeble flame, and requires a higher temperature than solid matter, which becomes luminous at about 700° in the dark, and at from 1000° to 2000° in bright daylight. Coal gas consists of a combination of illuminants: olefiant gas, which contains 86 per cent. of carbon, carburetted hydrogen or marsh gas, which contains 75 per cent., carbonic oxide, carbonic acid gas, hydrogen, sulphuretted hydrogen, and a very small quantity of nitrogen, besides the bisulphide of carbon, and benzol, a pure hydro-carbon, consisting of 12 equivalents of carbon and 6 of hydrogen.
The poisonous quality of coal gas is owing to the carbonic oxide, which is fatal to life, and its explosive quality to carburetted hydrogen, which also is generated by decomposition of vegetable matter in stagnant pools and marshes; and in the firedamp of mines it still bears testimony to the vegetable origin of coal. That fatal gas increases in explosive force as it mixes with atmospheric air, and is at a maximum when it amounts to 12 per cent. Hydrogen, carburetted hydrogen, and carbonic oxide do not add much to the light, on account of the feeble flame of hydrogen and the small quantity of carbon they contain, but they force the chief illuminating gases out of the iron retorts in which the coal is distilled before the heat has had time to decompose them, and they also enable them to burn without smell or smoke.
Carbonic acid, bisulphide of carbon, and sulphuretted hydrogen are impurities from which coal gas is freed before it is fit for use. By passing the gas over lime, the lime absorbs both the carbonic acid and the sulphuretted hydrogen; one per cent. of carbonic acid diminishes the illuminating power six per cent., and the sulphuretted hydrogen has an abominable smell.
The bisulphide of carbon, consisting of one equivalent of carbon and two of sulphur, is got rid of by passing the gas over hot lime. The water of the lime is decomposed, and carbonic oxide and sulphuretted hydrogen are produced; but the latter may be absorbed by passing the gas again over lime, or through a mixture of sawdust and the oxide of iron. The oxide of iron decomposes the sulphuretted hydrogen, forms water and sulphide of iron, then the air restores the sulphide to oxide, and the sulphur is deposited in the mixture. After passing the gas through it till none of that impurity remains, the gas is fit for use. The test is the nitro-prusside of sodium, which the gas stains purple if any of the impurity remains.
Paraffin, already mentioned as isomeric, is a pure hydrocarbon, colourless, transparent, and of crystalline texture. It melts at a heat of 120° or 130°, burns like wax without smell or smoke, and makes beautiful candles, which give a brilliant light on account of the 86 per cent. of carbon they contain. Paraffin oil is much used for lamps; the manufacture of these two substances at Bathgate is one of the largest chemical establishments in the world.
The black fœtid gas water resulting from the distillation of coals, formerly thrown away, is so rich in the salts of ammonia, that it has become the chief source from which these materials so important in the arts are obtained.
Ammonia is well known to be a colourless gas, with an acrid pungent smell, consisting of one equivalent of nitrogen and three of hydrogen. It has an alkaline character, combining with acids, and is extremely soluble in water.
Now the gas water contains carbonate of ammonia and sulphide of ammonium, and when any acid strong enough to decompose these substances is put into the liquid, the carbonic acid and sulphuretted hydrogen being volatile are driven off, and the acid combines with the ammonia to form a salt. For example, when muriatic acid is put into the liquid, it drives off the volatile gases and combines with the ammonia in solution to form muriate of ammonia, which is dissolved in water and evaporated till it crystallises; then it is vaporized and sublimed to free it from impurities.
When ammonia and muriatic acid are separately vaporized, the two colourless transparent vapours, when mixed, combine into solid muriate of ammonia, a result so unexpected that as Mr. Playfair justly observes, it could only have been taught by experiment. About 4,000 tons of muriate of ammonia are annually made from gas water in England for soldering, and for making alum.
Sulphate of ammonia to the extent of 5,000 tons is annually made by adding oil of vitriol to the liquid. It is also used for making alum, as well as for manure; it supplies our grain with nitrogen, an important article of vegetable food. To these may be added 2,000 tons of carbonate of ammonia, so that a substance that was considered to be good for nothing yields 11,000 tons of valuable materials, but even this quantity forms only part of the enormous amount annually consumed in the manufactures of Great Britain.