display of the new lights; and they were made to constitute a principal feature in that exhibition."

In the course of the years 1803 and 1804, the Lyceum Theatre, in London, was lighted with gas, under the direction of Mr. Winsor; and in 1804 and 1805, Mr. Murdoch had an opportunity of carrying his plans into effect on a larger scale, by means of the apparatus erected under his superintendence in the extensive cotton-mills of Messrs. Philipps and Son, of Manchester.

The French have claimed the priority in the exhibition of gas-light; but the earliest display of it in that country did not take place till 1802, when it was exhibited in Paris. It has already been shown that Mr. Murdoch's exhibition of it was earlier, by several years.

From the first lighting up of Boulton and Watts's Soho Foundry by gas, in 1802, to the close of 1822, a period of only twenty years, so rapidly had the discovery proceeded, and so high was the appreciation of it by the public, that, by the report of Sir William Congreve, it appears that the capital vested in the gas-works of the metropolis alone amounted to one million sterling, while the pipes connected with the various establishments, embraced an extent of upwards of one hundred and fifty miles. In the course of a few years after it was first introduced it was, indeed, adopted by all the principal towns in the kingdom, for lighting streets, as well as shops and public buildings. Into private dwellings, through the careless and imperfect way in which the service-pipes were at first fitted up, and which occasioned annoyances, it was more slowly received. But as a better knowledge of its management has been acquired, it has come into more general use, till, now, gas is employed in every quarter where the means of obtaining it are within reach.

The apparatus for the production and purification of coalgas, consist, in the first place, of the retorts, or vessels for decomposing by heat the coal from which the gas is to be

procured; secondly, of the dip-pipes and condensing main, employed to conduct the gas into vessels, where it is removed from the tar and other gross products that come over the gas and tend to impair the brilliancy of the light; thirdly, of the purifying apparatus, for abstracting the sulphuretted hydrogen, carbonic acid, &c., by which the gas is contaminated; and lastly, of the gasometer, or gas-holder, with its tank, into which the gas is finally received in a purified state.

The retorts are usually formed of cast iron, and are commonly of a cylindrical shape. They are fixed in brick-work, with furnaces beneath them. The fuel required for carbonizing a given quantity of coal, that is, for separating the gaseous matter from it, is in general about two-fifths of its weight. The bright red heat is the most favourable to the process. The quality of the gas, yielded by coal, varies greatly at different periods of the heating operation. If the coal has not been previously well dried, scarcely any other than aqueous vapours and carbonic acid will, at first, be given off; these will be succeeded by the gases required for use, light carburetted hydrogen and olefiant gas, together with sulphuretted hydrogen; and these will gradually diminish in quantity till towards the close of the process, when almost the only products will be carbonic oxide and hydrogen. The time which elapses from the period at which the retorts are charged, or fitted, to the moment when they are drawn, or emptied of the residuary carbon, or cinder, varies with the kind of coal used: cannel coal, which is easily decomposed, requires but three and a half or four hours, while Newcastle coal takes six. The quantity of gas also varies with the quality of coal: this cannel coal yields 430 cubic feet of gas per hundredweight; Newcastle coal, about 370 feet.

The dip-pipes are bent pipes from which the gas ascends out of the retorts, as it is produced, into the condensing main, a large cast-iron pipe placed in a horizontal position, and sup

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ported by columns in front of the brickwork which contains the retorts. The tar, aqueous vapour, and oleaginous matter, which ascend with the gas from the retort, are left by it in the condensing main; though it is difficult to prevent small portions of these substances from escaping with it.

The further purification of the gas has now to be effected; and after passing from the condensing main, it is conveyed by pipes into other apparatus for perfecting its purification. Olefiant gas and light carburetted hydrogen, the two compounds of carbon and hydrogen, which it is sought to obtain, are mixed with several deleterious substances which the coal yields, together with them, during its destructive distillation by heat. In small quantity, carbonic oxide, nitrogen, and hydrogen, come off; but in larger, carbonic acid and sulphuretted hydrogen. These last two are the most objectionable for impurity, and can, fortunately, be more easily separated from the gas than the others.

Quick-lime, being a substance with which carbonic acid and sulphuretted hydrogen will easily unite, is employed, in one form or other, in all gas establishments, for the last step of the purifying process to which coal-gas is submitted to render it fit for combustion. Sometimes the cream of lime, or lime slaked with a little more than the usual quantity of water, is employed, and then it is necessary to agitate the liquid so as to assist the gas to come into contact with fresh portions of the lime; and in other establishments dry lime is used. In very large establishments, the gas is forced in succession through a series of vessels stored with lime, in order to purify it thoroughly; in others, an apparatus is affixed for supplying fresh lime as often as the old material has become so saturated with the deleterious gases as to be unfit for use. Being freed from impurity, it is next conveyed into the large vessel in which it is stored up for use.

The gasometer is an inverted cylindrical cup, of which the

diameter is about double the depth. It is constructed of sheet iron, well rivetted at the joints, and kept in shape by stays and braces of cast or bar iron. The sheets of iron are made to overlap at the joints, and a slip of canvas well-besmeared with white-lead is interposed between the lappings, to secure perfect tightness. The gasometer is suspended in a tank containing water, by a chain and counterpoise, over pullies. As the gasometer, when immersed, suffers a loss of weight equal to that of the portion of fluid it displaces, arrangement has to be made to counteract the varying pressure resulting from the different depths to which it is immersed, or the gas in it will be expelled at different times with varying force. It is easy, however, to calculate this force, and provide against it.

Under the bottom of the tank in which the gasometer floats, the gas is introduced and conducted off by pipes. As these pipes are usually below the level of those in the street with which they communicate, they are, apt to be filled up with condensed water, which passes off in a vaporous state with the gas. Vessels for receiving the condensed water are, therefore, connected with the entrance and exit pipes, and so contrived that the accumulated water can be easily removed.

The transmission of the gas for use, from the gasometer, is through the main and service pipes-the size of the former being relative to the united sizes of the latter; that is, the sum of the areas of the sections of main-pipes being equal to the sum of the areas of the sections of branch or service pipes supplied. The supply of gas to the main-pipe is regulated by the "governor," a piece of mechanism consisting of a rod and valve placed between them and the pipe by which the gas enters the gasometer. The main-pipes are usually of cast iron; the sections of about three yards in length, being joined by sockets, which are caulked with gasket, and soldered with lead. Water in vapour carried off by the gas will be condensed in the main-pipes, and, therefore, in laying them, it is necessary

between London and Woolwich, the reviewer adds-"We should as soon expect the people of Woolwich to suffer themselves to be fired off upon one of Congreve's ricochet rockets, as trust themselves to the mercy of a machine going at such a rate." In two-and-twenty years afterwards, trains running at more than double this speed had become of daily occurrence, and nearly quadruple the speed which so alarmed the reviewer had been attained with perfect safety.