The first description given to the public of an ap paratus coal gas for useful purpose., appears to hove-been in 1808, by Mr Samuel Clegg, in a papentransmitted to the Society of Arts. He therein describes a retort similar to ffig..2 in form, and protected from the immediate-action of the fire by an interposed curved plate of iron. This cyliu drioal-figure has been generally adopted at the gas works in. the metropolis, and those of many provin cial towns. Elliptical ones, similar to fig. 6, have, however, lately been introduced into the Westminster works ; and, though much superior to the former, are yet less perfect than that shown in fig. 8, a con struction which has been extensively used since the year"1808 in the northern.manufacturing districts of England, and in Scotland.
Retorts, having for their vertical section a square or parallelogram, are also used (figs. 9 and 10), and these, as well as those of the cylindrical and elliptical shape, are occasionally placed so that two or three more are heated from••he same fire ; or a number of retorts, arranged with fires common to all. Figs. 11, 12, 1304 exhibit varieties of these ; all, however, are liable to the objection that, when •any retort be comes useless, those connected with it require to be stopped and disused during the time the faulty one is replacing. Constructions have should admit.of these repairs without causing inter ruption ; but there is no great appearance of prac ticability in any plan for this purpose hitherto made public. When a number of retorts have to be heat ed from the same fire, any thing like regularity of temperature is difficult to obtain, on account of the different distances at which they must be placed from the source of heat ; and, on the whole, except the circumstance of saving a little room, the plan has nothing to recommend it.
The fuel necessary to decompose any given quan tity of coal may generally be stated at about one third of its weight. There are statements published where less than one-fourth is mentioned as sufficient ; but, where the retorts are to be kept at a proper, or bright red heat, so as to be capable of producing the largest and best supply of gas, less than one-third cannot be reckoned upon with certainty ; and, as inferior coal is frequently used for fuel, the propor tion may, in such cases, amount to, or even exceed, one-half the weight of the coal to be distilled.
It would much exceed our limits to notice every variety of retort which has been proposed. It may
yet, however, be proper to notice another device for which the last person named obtained a patent, three to four years since. An outline of this machine, com prising an horizontal and vertical section, is drawn in fig. 15; a, a, a, a, is a flat cylindrical vessel of iron, having an aperture, b, at one side, and door to close it. In the centre is a vertical spindle, c, carrying a num ber of horizontal arms, d d; e is a representation of a pan or vessel for containing the coal, and with one of which each arm, d, is furnished. About one third part of the lower and upper surface of the ves sel, a a, is exposed to the action of a fire, f, and kept red hot; the flues, g, conducting the flame to the chimney, in_ the manner shown in the figure; and the pans of coal (being, by means of the aper ture k introduced upon the arms d d) are brought in succession over the heated part, and there lower ed, so that they may rest upon the red hot surface. The distillation is thus carried on, and, when com pleted, the axle is turned partially round, and ano ther pan, or set of pans, exposed to heat in the like manner. This is a complex and expensive ap paratus ; and, inasmuch as the coal is not brought into immediate contact with the red hot surface, it must necessarily be inferior in performance . to the elliptical or other forms, figs. 6, 7,.8 ; and,.though there are statements before, the public which re present the results in a very advantageous light, these must be received with .caution. It is men tioned,• that, by the use of this retort, 16,000 cu bic feet of. gas, and, upwards, may be obtained from one London chaldron of Newcastle coals, being 590 cubic feet to the cwt. ; and that the same weight of coal distilled in a cylindrical .retort will not give more than 370 cubic feet to the cwt) By using flat elliptical retorts, however, and expos. ing the coal in very thin layers, upwards of 600 cu bic feet have been obtained ; and, for general prac tice, where the coal is of good quality, 500 to 55Q cubic feet may be calculated upon as the product of each cwt.
The quantity of gas which one chakkon of fordshire coal yielded, when distilled in the retort last described (fig. 15), was, according to Mr Ac cum, .1L,000 cubic feet,.or about 400 to the cwt. This falls much short of .what has already been stated as the product of the best coal from that dbl., trict.