GAS-LIGHTING. When coal, oil, wax, wood, or any other organic inflammable sub stance is exposed to destructive distillation in close vessels, an inflammable gas is the result. This gas is some compound of hydrogen and carbon: more or less brilliant when ignited, according to its constitution. That the gas obtainable from coal is inflammable has been known for more than 200 years ; and so long ago as 1691 Dr. Clayton made and used in flammable gas as a philosophical experiment. In 1733 Sir James Lowther brought before the notice of the Royal Society the existence of inflammable gas in one of the \Vhitehaven collieries ; and some years afterwards Dr. Watson further draw public attention to the subject.
But, although the properties of coal gas were known to so many persons, no one thought of applying it to a useful object until the year 1792, when Mr, Murdoch, an en gineer, residing at Redruth in Cornwall, erected a little gasometer and apparatus, which produced gas enough to light his own house and offices. In 1797 he erected a simi lar apparatus. in Ayrshire, where he then re sided. In the following year he was engaged to put up a gas -work at the manufactory of Boulton and Watt, at Soho. This was the first application of gas in the large way ; but, excepting in manufactories or among scientific men, it excited little attention until the year 1802, when the front of the great Soho manu factory was brilliantly illuminated with it on the occasion of the public rejoicings at the peace. Soon after this, several manufacturers, whose works required light and heat, adopted the use of gas. Mr. Winsor, a German, brought the subject forward in London, and formed a National Light and Heat Company,' in 1804, which failed. In 1807 however he lighted up Pall Mall, and this continued for some years to be the only street in London in which gas was used. In a few years after wards a Gas Company was chartered ; and London gradually became better lighted. The business of the company steadily increased; and in the year 1823 it was shown that this company alone consumed annually 20,678 chaldrons of coals, which produced on an average 680,000 cubic feet of gas every night ; this was distributed by means of 122 miles of pipe, which supplied more than 30,000 burners, giving a light equal to as many pounds of tallow candles. Several other companies were established in London. The provincial towns followed the example, and continental cities one by one availed themselves of this useful system, until at length it has become one of the best known and extensively used of. mo dern improvements.
Various substitutes for coal in gas-making have been proposed from time to time, such as resin, wood, and peat, but with little suc cess. Oil yields a brilliant gas, which is easily
manufactured ; but coal excels them all in cheapness, and is almost universally em ployed.
Although in the large way there are many practical difficulties to be surmounted in the manufacture of coal gas, the operation is easily understood; it is merely a process of distil lation. A quantity of coal is put into a retort, which is well closed, and placed upon the fire; the temperature is raised to redness, which decomposes the coal, and drives the gas re sulting from the decomposition through a pipe leading from the retort to the receptacle pre pared for it. A mass of coke 'remains, of greater bulk though less weight, than the coal first put in. Such is the theory of gas-making ; the manner of putting it into practice remains to be described.
The retorts now in use are generally long cast-iron vessels, semi-cylindrical in shape, with the flat side placed lowermost, and each holding from two to three bushels of coals. They are placed in ovens, in groups of three, five, seven, or more ; and their mouths, where the coal is put in, stand out in front of the ovens. Just behind the mouth of the retort a pipe leads from it perpendicularly several feet; then, taking a sudden turn, it descends again, and enters a much larger pipe techni cally called the hydraulic main, which runs through the whole building, and receives the gas produced from all the retorts. This great main is generally about half full of the tar and water which leave the coal with the heated gas, and rise with it in the state of vapour, but are condensed by the coldness of the main. Into this mixture the end of the pipe dips, and is thus closed against a return of gas, which would take place if the supply should slacken. The gas is now made ; but it is very impure, being mixed with water, tar, sulphuretted hydrogen, and other impurities. The tar and water ere easily got rid of, little more being required for this purpose than to cool the gas and to allow the deposit to run off. Tho separation of the sulphuretted hy drogen can onlybe effected by some substance for which it has a chemical attraction, but which has no influence on coal gas. Such a substance is lime. The lime is used by being mixed up with water into a thin pasty mass, which is placed in a cylindrical vessel, and is constantly stirred by machinery. The gas as it comes from the condenser passes into the lime, and comes from it partially purified ; it then enters another purifier, made and fur nished precisely in a similar way; after that another; and often a fourth, in large works. When it leaves the last vessel, it may be con sidered pore.