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ing a due supply of air, within and without, burns very beautifully when a proper glass is placed over the burner. These burners, when very carefully regulated, consume about three cubical feet of gas per hour, and give light equal to that of six wax candles; but it is requisite, on account of carelessness and mismanagement, to allow four cubical feet to each burner per hour. The bat's wing burner should not consume more than three cubic feet per hour. Besides the different varieties of coal, some of which, as has been hinted, are much preferable to others, and coal tar, a useful gas may be procured from o of other substances; and in the laboratory of the Royal Institution, the retort is often fed with waste paper, saw-dust, pieces of wood, &c., and the gas is consumed for a variety of purposes where oil was formerly employed. e following are the results of some experiments upon these subjects, compared with the produce from coal. 1. The retort was charged with four pounds of coal. The quantity of gas amounted, after havung passed the purifiers, to twenty cubic feet. The coke remaining in the retort weighed 2 lb. 8-7 ozs. The heating power of the gas flame was compared with that of a wax candle, by ascertaining the time required by each to raise two ounces of water, in a thin copper vessel, from 55° to 212°. The flames were made as similar in dimensions as possible, and so placed that their joints just touched the bottom of the vessel. The heating Power of the candle being assumed as - 1, that of the coal gas flame was - 1:5. , 2. Four pounds of the dried wood of the common willow yielded sixteen cubical feet of gas, and fourteen ounces of charcoal remained in the retort. The gas burned with a very pale blue flame, and was unfit for the purpose of illumination, and contained no olefiant gas. 3. Four pounds of the wood of the mountain ash afforded fifteen cubical feet and a half of gas, and thirteen ounces and a half of charcoal. The flame was very pale and blue. 4. Four pounds of white birch wood gave fourteen cubical feet of gas, and twelve ounces of charcoal. The flame similar to 2 and 3. 5. Four pounds of hazel wood yielded thirteen cubical feet and a half of gas, and twelve ounces and a half of charcoal. Its heating power was ~ 1:2. It burned with a better flame than 2, 3, and 4, but the intensity was not sufficient for any useful purpose of illumination. 6. Four pounds of writing paper gave eighteen cubical feet of gas, and the remaining charcoal, which beautifully retained the form and texture of the paper, weighed eleven ounces and a half. The heating power of the gas was - 1-6. It burned with a flame nearly approaching in illuminating power to that of coal gas. These experiments, along with others which it is thought unnecessary to notice, prove that the gas from woods is not fit for the purposes of illumination, although, as evolved during the production of charcoal, it may conveniently be consumed in the laboratory as a source of heat. We may now describe the retort oven for ge

nerating gas on a large scale. Fig. 1, plate I. GAS LIGHT, exhibits a longitudinal section, and fig. 1. shows the front elevation of the oven, built about ten feet above the ground, upon piers or arches, which saves brick-work, and allows a stage or platform to be erected in front of the fire Fo of the ovens. Between the back part of the ovens and the wall of the building in which they are erected is left an empty space of a few inches to prevent the heat of the oven being communicated to the wall. The whole interior of the oven, as well as the horizontal flue which passes underneath the crown of it, near the upper tier of retorts, is lined with fire-bricks. The uppermost part or crown of the arch is constructed of large fire-bricks, of such a shape as will allow to flatten the upper part of the arch as much as possible, in order to contract the space between the two upper retorts and the crown of the arch of the oven. R, IR, fig. 2 and 3, are cylindrical retorts, placed horizontally in the oven, the lower series are either supported by a large fire-brick, placed, edgeways underneath the retort, or by means of a stout wrought iron pillar, as shown in the design. The two upper retorts are supported by wrought iron straps, T, T, T. The straps pass through the brick-work of the upper part of the oven, as shown in the designs, and they are secured with screws and nuts to an iron bearing bar, the extremities of which are supported by the outer walls of the oven. Each retort is furnished at the extremity opposite to the mouthiece with a short projecting piece or tail let into the brick-work of the oven. M, fig. 4, shows the mouth-piece of the retort with its cross bar and hand screw; and fig. 5 shows the mouth-piece drawn to a larger scale. E is the hand-screw, with its cross or bearing bar D, which passes through the projecting arms C, C. The lid of the mouth-piece has a conical edge, so that it fits close when put into its place by means of the hand-screw E. F, fig. 6, is the fire-place, with the ash-pit E of the oven. The door of the ash-pit is provided with three slits covered within by a register slide, to regulate the admission of air as occasion may require. The fire passes freely and uniformly round all the retorts, and the whole cavity of the oven acquires an equable temperature, which it retains, if the workman takes care to admit as little air as possible through the register door of the ash-pit, when the upper part of the arch, or crown of the oven has acquired a bright cherry red heat. The liquid substances, namely the tar and ammoniacal fluid, collect in the hydraulic main H, which is furnished with a perpendicular diaphragm or partition plate to cause a certain quantity of the liquid deposited in it to accumulate to a certain height, and thus to seal the perpendicular pipe P. The liquid cannot flow out of the horizontal pipe H, till it rises to the level of the diaphragm. K, fig. 7, is the discharging pipe, connected with the upper part of the horizontal main H: it serves to convey away the gaseous and liquid products from the hydraulic main H. By means of this pipe the tar and ammoniacal fluids are jo into any convenient reservoir, called

the tar cistern, which is so air-tight, and from this vessel the liquid may be drawn off by means of a pipe or stop-cock. The extremity of the pipe which communicates with the liquid is bent downwards, so that no air can enter the vessel. It is essential that the condensation of the vaporous fluids should be fully completed before they reach the tar vessel. To effect this, there is usually allowed a considerable distance to intervene between the discharging pipe, K, and the reservoir destined to receive the condensable products; or the pipe is made to pass through a vessel containing water, called the condenser, which acts in a similar manner as the refrigeratory of a common still. It is obvious that it is immaterial how the condensation of the vaporous fluid is effected; it is essential, however, that the condensation should be complete before the liquid tar and ammoniacal fluid reach the reservoir destined to receive these products. The gaseous fluid, which accompanies the condensable products, is then made to pass into the lime machine, in order to be deprived, by means of quicklime and water, of the portion of sulphureted hydrogen and carbonic acid gas which was combined with the gas. And, when this has been accomplished, the purified gas is conveyed into the gas-holder, where it is stored up for use. In some establishments, the hydraulic main is furnished with two discharging pipes; the one carries away the condensable fluid, into which the perpendicular pipes, P, fig. 8, dip, whilst the other serves to convey away the gaseous fluids to a condenser, in order to deposit the vaporous portion of condensable liquid it may contain, and from thence the gas passes into the purifying apparatus, or lime machine. X, fig. 9, is a small screw plug, which, when opened, restores the equilibrium of the air within and without the retort previous to the lid being taken off, to prevent the loud report which otherwise . when the lid or cover of the retort is suddenly removed, To avoid these explosive reports, which had become a nuisance to the neighbourhood of gas works, the practice of gradually withdrawing the lid of the retort, and, at the same time, presenting a lighted torch, has been adopted at some works, which fully remedies the evil. The quantity of gas to be obtained from coal varies according to the coalemployed and the manner in whichitis treated; the quality also depends on the mode of applying the heat. Taking it for granted that the most advantageous method of decomposing it is followed, the quantity from the different kinds of coal varies. In stating the proportions, therefore, we can come only at an average conclusion. Mr. Peckston, in his work on Coal-gas, states, that a chaldron of Newcastle Wall's End coal will yield 10,000 feet, supposing it decomposed under the most advantageous circumstances; 2 cwt. will, therefore, yield about 750 feet. At Edinburgh, 2 cwt. of Parrot-coal yield, on an average, 860 feet of gas. According to Mr. Neilson, engineer, Glasgow, 2 cwt. of Lesmahago coal will produce 1008 cubic feet of gas, allowing four and a half each pound. Mr. Russell, of London, has

stated the quantity from Newcastle coal to be the same, four feet and a half per pound. Mr. Dewy, in a paper in the Annals of Philosophy, asserts, that at Liverpool, Mr. King considers it good economy to procure 7000 feet from a ton of Wigan Orral coal, making it only 700 feet from 2 cwt., a very little more than three feet per pound. He has stated also, that, at Glasgow, 1200 feet are procured from 2 cwt. of cannelcoal, which is considerably above that mentioned by Mr. Neilson. From these various statements, the general conclusion has been drawn, that 2 cwt. of good coal ought to yield about 1000 feet of gas. With respect to the quantity to be obtained from oil, this must, of course, also depend on the nature of the oil, and the manner of decomposing it. Mr. Ricardomentions, that, from repeated trials in various oil-gas establishments, it has been ascertained that onegallon produces 100 cubic feet. From the experiments of Mr. Brande, and Mr. Faraday, it appears that the same quantity affords from 100 to 110 feet. In some instances it has been known to amount to about 120; but in these cases, it was not good, the additional quantity having been derived from substances put into the retort. At Leith, a gallon of whale oil affords from ninety-eight to 108 cubic feet; and the same quantity of palm-oil, from ninety-seven to 114. It may be considered a fair estimate to obtain 100 feet from each gallon, presuming, of course, that the oil is decomposed under the most favorable circumstances, so as to get a gas possessing the greatest illuminating power; for on this every thing depends. From experiments performed on a small scale, and from trials made at Leith, Dr. Fyfe found, that if the oil be allowed to flow into a retort brought just to a red heat, there is comparatively little gas, but a great deal of volatile oil. When the retort is brought to an intense heat, lamp-black is formed in considerable quantity; so that, in both of these ways, there is a great loss. When the retort is at a full red heat, the oil seems to undergo decomposition most easily, and to give off the largest proportion of good gas. In conducting the decomposition of coal, the evolution of the gas is far from being, with regard to quantity, uniform, during different periods of the distillatory process. The formation of the gas is more rapid in the beginning of the process, and gradually slackens as the operation proceeds. The gas also differs in its chemical constitution, at different periods of the process; although, in the case of large supplies, this difference is of little consequence after the gas is purified in the usual manner. The former consideration, however, has given rise to various modes of operating, of which it will be proper to take some notice. It must be obvious o, in proportion as the mass of coal in the retort becomes carbonised or converted into coke, the exterior surface becomes a gradually increasing obstacle to the action of the heat upon the interior or central part of the coal remaining to be decomposed. The heat required on that account must be more intense, and kept up to purpose; and the extrication of gas becomes slower and slower, as the operation proceeds. The loss occasioned by this rapid diminution of the means employed, is serious in every point of view, in regard both to the quantity of fuel used and time wasted, but it is unavoidable in the operation of decomposing coal in masses or layers from five to ten inches in thickness, and must be a great drawback on the vakie of the gas-light discovery. The loss of fuel, it is obvious, must be just in proportion to the quantity of carbonised matter, or coke, which is kept hot to no purpose, awaiting the decomposition of that portion of coal which it is the very means of protecting from becoming undecom

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operation, for the evolution of 115 cubic feet of gas, is required in the eighth hour for the production of no more than forty-two cubic feet, being a decrease in effect of nearly two-thirds. When larger retorts are employed for decomposing coal, in masses from five to ten inches in thickness, the loss of heat is in a much greater ratio. In the hope of remedying, in some measure, the evils thus distinctly ascertained to arise from the undue thickness of the masses of coal subjected to the distillatory process, there have not been wanting manufacturers who have had recourse to experiments on a large scale, to ascertain with certainty whether they might not be gainers by suffering the distillatory process, when the retorts are charged with two bushels of coal, to proceed only for the space of six hours, instead of eight. But the result of these experiments has shown satisfactorily, that it is more profitable to keep up the distillatory process for a period of eight hours, with the retorts fully charged, than to abridge the operation by terminating it at the end of six hours. Others, again, have imagined, that it would be more economical to decompose a less quantity of coal at once, or to decrease the thickness of the stratum of coal in the cylindrical or in any of the before-named retorts; but then, again, serious difficulties occur in the practice. The more frequent charging of the retorts and luting on the covers, which such a mode of o: requires, occasions a prodigious waste of fuel, time, and labor. A greater number of retorts, and more workmen, must likewise be employed, in order to produce the requisite quantity of gas daily, which the manufacturer is called upon to supply; more space of ground is required, and more dead capital must be sunk in the establishment. The more frequent and sudden alterations of temperature which the retorts necessarily suffer, by the more frequent introduction of cold coal, renders them extremely liable to become injured; and it is almost impossible to maintain a number of retorts, thus worked, at a uniform temperature. One of the best purposes to which the tar produced in the distillation of coal can be applied, is to the production of gas, which yields in the proportion of about eighteen cubic feet from each pound, and of an excellent quality for illumination. The following is an account of Mr. Clegg's apparatus for its decomposition, and which appears to answer better than any yet devised :- A, plate I., fig. 10, is a tar cistern., B, a cock by which it is drawn off. As a o small stream of tar is apt to stop, by its stiffness, a larger quantity than is wanted is allowed to run into E, upon the edge of the dividing plate C, adjusted by the screw D: the excess runs off by a waste pipe into any proper vessel, while a due portion trickles through E into F, and runs down G, G, into H, where, when the tar has reached the level I, it is conducted into the retort K, L, M, the return of gas being prevented by the immersion of the end of the tube G, G, into the tar in the vessel H. The retort, resembling a bent pipe or syphon, is so inserted in a proper flue, that the ends K, M, provided with lids or

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