rus. When perfectly dry, phosphorus inflames at a temperature of 60°.

Why is the phenomenon called "Will-o'-the-Wisp" produced?

Because of the phosphuretted hydrogen gas in stagnant waters and marshy grounds; its origin being probably in the decomposition of animal substances. The peculiar odour of fishes, when putrifying, arises from the emission of this gas.

COAL GAS.

Why does coal, subjected in close vessels to a red heat, produce gas?

Because the carbon and bitumen, of which the coal consists, thus become volatilized; and hydrogen, holding carbon in solution, is the result: this gas, combining with the oxygen of the atmosphere, produces combustion and flame. These gaseous products contain also more or less sulphuretted hydrogen, and carbonic oxide and acid. Dr. Henry conceives that gas to have the greatest illuminating power, which, in a given volume, consumes the largest quantity of oxygen.

Dr. Clayton seems to have been the first who performed this experiment, with the view of artificial illumination; though its application to economical purposes was unaccountably neglected for about sixty years. At length, Mr. Murdoch, of the Soho Foundry, instituted a series of judicious experiments on the extrication of gas from the ignited coal; and succeeded in establishing one of the most capital improvements which the arts of life have ever derived from philosophical research and sagacity.-Ure.

The coal is placed in oblong cast-iron cylinders or retorts, which are ranged in furnaces, to keep them at a red heat, and all the volatile products are conveyed by a common tube into a condensing vessel, kept cold by immersion in water; and in which, the water, tar,

pitch, ammoniacal and other condensable vapours, are retained.-Brande.

The production of hydrogen gas in a tobacco-pipe, by filling the bowl with powdered coal, then luting it over, and placing it in a fire,-is well known; but even more familiar are the alternate bursting out and extinction of those burning jets of pitchy vapour, which, as Dr. Arnott aptly observes, "contribute to render a common fire an object so lively, and of such agreeable contemplation in the winter evenings."

Why was gas adopted in cotton-mills soon after its invention ?

Because of the peculiar softness, clearness, and unvarying intensity of its light. Its being free from the inconvenience and danger resulting from the sparks and frequent snuffing of candles, is a circumstance of material importance, tending to diminish the hazard of fire, and lessening the high insurance premium on cotton-mills.

Mr. Brande illustrates the economy of gas illumination, by examining the value of the products of distillation of a chaldron of coals, the average cost of which may be considered as £2. It should afford

Chaldron of Coke, at 20s.

£1 5 0 24 Gallons of Tar and Ammoniacal Liquor at ld. 0 2 0 1200 cubic feet of Gas, at 13s. per 100 C. F.

7 16 0

£9 3 0

The history and economy of gas-lighting have been copiously illustrated in several volumes exclusively devoted to the subject; as well as by the experimenting skill of some eminent chemists, as Messrs. Henry, Brande, Ure, Accum, and others. From one of these works, aided by the Reports of the late Sir William Congreve, we learn that in the year 1814 there was only one gasometer in Peter-street, Westminster, of 14,000 cubic feet, belonging to the Chartered Gas Light Company, then the only company established in

London. In 1827 there were four great companies, having, altogether, gasometers at work capable of containing in the whole 917,940 cubic feet of gas, supplied by 1,315 retorts, and these consuming 33,000 chaldrons of coal in the year, producing 41,000 chaldrons of coke; the whole quantity of gas generated annually being upwards of 397,000,000 cubic feet; by which 61,203 private, and 7,268 public or street, lamps are lighted, in the metropolis. In addition to these great companies, there were several private establishments, whose operations are not included in the foregoing statements; for, it appears that where more than fifty lights are required, a coal-gas apparatus will be found profitable. Thus, the gas for the office of the Morning Chronicle newspaper is made on the premises.

According to Mr. Murdoch's statement, presented to the Royal Society, 2,500 feet of gas were generated from 7 cwt.-784 lb. of cannel coal. This is nearly 3 cubic feet for every pound of coal, and indicates judicious management. The price of the best Wigan cannel is 13 d. per cwt. delivered at Manchester; or about 8s. for the 7 cwt. About one-third of the above quantity of good common coal, at 10s. per ton, is required for fuel to heat the retorts. Nearly two-thirds of the weight of the coal remain in the retort, in the form of coke, which is sold on the spot at 1s. 4d. per cwt. The quantity of tar produced from each ton of cannel coal, is from 11 to 12 ale gallons. This tar is now extensively used as paint for out-buildings, &c.; and the ammoniacal liquor, also a result of the process, is turned to still more advantageous account, in the manufacture of carbonate of ammonia; (see p. 119) so that nothing is lost.

Why is it necessary that the cylinders or retorts should be redhot?

Because the gas may be produced instantly the coals are introduced. If, on the other hand, coal be put into a coal retort, and slowly exposed to heat, its bitu

men is merely volatilized, in the state of condensable tar. Little gas, and that of inferior illuminating power, is then produced.

66

Why does coke burn without smoke?

Because it is the distilled coal remaining in the retorts after the above process, and is consequently freed from all gases and vapours. Dr. Arnott observes, that a pound of coke produces nearly as much heat as a pound of coal; but we must remember that a pound of coal gives only three-quarters of a pound of coke, although the latter is more bulky than the former."

Why are the gaseous products passed through quicklime and water?

Because the sulphuretted hydrogen, and carbonic oxide and acid, may thus be absorbed; while the carburetted hydrogen gases are transmitted sufficiently pure for use into gasometers, whence the pipes issue for the supply of streets, houses, &c.

Why is a gasometer so called?

Because it enables the manufacturers of gas to mete, or measure, with accuracy, the quantity of gas expended, in proportion to the number of jets or burners made use of. The essential conditions of any apparatus for this purpose, are, that the pressure on the gas, while passing through the measurer, shall at all times be uniform: and that it shall register truly when that pressure is very small, and when the current of gas is very feeble.

The gas is conveyed or laid on to the premises of the consumers, by pipes called mains, which, in London, in the year 1824, extended 900 miles. Other methods, have, however, been adopted for its conveyance; and, in 1827, there was at Manchester an itinerant gasometer, capable of holding more than 1000 cubic feet of gas; this being mounted upon wheels, and filled with gas from the gasometer at the works, was transported about the streets of Manchester, and delivered into the gas-holders of the Company's customers,

Why is it important that the holes in the gas-burner be not too large?

will

Because, if this point be not attended to, the gas be incompletely burnt. Increased length of flame makes imperfect combustion, and diminished intensity of light.

Mr. Accum gives the following interesting statement of gas-burners. "An argand burner, which measures in the upper rim half an inch in diameter between the holes from which the gas issues, when furnished with five apertures, 1-25th part of an inch in diameter, consumes two cubic feet of gas in an hour, when the gas flame is 14 inch high. The illuminating power of this burner is equal to three tallow candles, eight in the pound. An argand burner, three-fourths of an inch in diameter, as above, and perforated with holes 1-30th of an inch in diameter (probably fifteen in number) consumes three cubic feet of gas in an hour, when the flame is 24 inches high; giving the light of four candles, eight to the pound. And an argand burner, seven-eighths of an inch diameter, as above, perforated with eighteen holes, 1-32nd of an inch diameter, consumes, when the flame is three inches high, four cubic feet of gas per hour, producing the light of six tallow candles, eight to the pound. The height of the glass chimney should never be less than five inches.

Why is cannel coal best adapted for distillation? Because it contains most bitumen, and least sulphur. A pound of good cannel coal, properly treated in a small apparatus, will yield five cubic feet of gas; equivalent in illuminating power to a mould candle, six in the pound. On a great scale, however, 34 cubic feet of good gas are all that should be expected from one pound of coal. A gas jet, which consumes half a cubic foot per hour, affords a steady light equal to that of the above candle.

Why does carburetted hydrogen give out, during its combustion, much more light than the subcarburetted?