From what has been stated in the preceding pages it becomes obvious, that a substance yielding an artificial light may be obtained from common coal in immense quantities. The attempt to derive advantage from so valuable a discovery is surely no idle speculation. Let us therefore now consider to what objects of public and private utility this mode of procuring light may be applied with effect. It is obvious that coal-gas may be preserved in a reservoir for any length of time and that it may be conveyed by means of tubes to any distance flowing equably and regularly like water. Those, indeed, who have not seen the contrivance will find it difficult to imagine with what ease it is managed. The gas may be distributed through an infinity of ramifications of tubes with the utmost facility. Near the termination of each of the tubes through which it flows, it is confined by a valve or stop-cock, upon turning which, when required to be lighted, it flows out in an equable stream and ascends by its specific levity. There is nothing to indicate its presence; no noise at the opening of the stop-cock or valve--no disturbance in the transparency of the atmosphere--it instantly bursts on the approach of a lighted taper, into a brilliant, noiseless, steady and beautiful flame. Its purity is attested by its not blacking or soiling in the least degree the metallic orifice from which it issues, nor even a sheet of white paper, or polished surface brought in contact with it. There is no escape of combustible matter unconsumed, which is so great a nuisance in all our common lights. The products of the combustion are water and carbonic acid gas[21]. The accurate and elegant experiments of Dr. W. HENRY have shewn in the most satisfactory manner, that considerably less carbonic acid is produced by the flame of coal-gas, than by that of oil, tallow, or wax[22], which sufficiently refutes the absurd notions that have been circulated respecting the pernicious effects of gas-lights. But if the gas from Newcastle coal is badly prepared, or not deprived of the portion of sulphuretted hydrogen, which it usually contains, it then emits fiery sparks and produces a portion of sulphureous acid by virtue of the union of the oxygen of the air with the sulphur dissolved in the gas, the consequence of which is, a suffocating odour, which is particularly observable in the higher stratum of the air of apartments in which the gas is burnt. Such gas likewise tarnishes all metallic bodies--it discolours the paintings effected with metallic oxids, and always produces a suffocating odour very noxious to health. It is freed from the sulphuretted hydrogen and may be rendered fit for illumination by passing it repeatedly through very dilute solutions of sub-acetate of lead, green sulphate of iron, quicklime and water, or hyper-oxymuriate of lime.