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the tar cistern, which is perfectly 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 sul phureted 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 happens 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 coal employed and the manner in which it is 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. Ricardo mentions, that, from repeated trials in various oil-gas establishments, it has been ascertained that one gallon 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 that, 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 value 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 undecomposed.

A striking exemplification of this statement will be seen in the following table, exhibiting the result of the progressive produce of coal gas, obtainable, in a given time, by means of cylindrical and parallelopiped retorts.

Experiment with one Cylindrical Retort, containing two bushels of coal.

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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 operating 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 sufficiently 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

mouth-pieces N, O, may be easy of access, and one above another: the lower branch L,M, may be placed almost horizontally, and the upper should form with it an angle of about ten degrees. The retort being made red hot, the tar will be decomposed, and the gas, and some other products, will flow from the end M, by the pipe P, into the vessel Q, in which is a partition plate R, fig. 11, extending about half way down, and allowing the heavy products to accumulate for a convenient time before they can interfere with the passage of the gas, which passes to the purifiers, as usual, by the pipe S, S, fig. 12, is a moveable lid for cleansing the vessel. It is not thought necessary particularly to describe the construction of the furnace, which may be varied according as circumstances require.

We may now describe the gas-metre erected by Mr. Accum at the works in the royal mint. It consists of a hollow wheel or cylinder, made of thin iron plate; revolving upon an horizontal axis, in the manner of a grind-stone; this wheel is enclosed in a cast iron air-tight cask containing water.

The cylinder, or wheel, is composed of two circular channels, 1 and 2, fig. 1 plate II. concentric to each other. The larger or outer channel, 1, is divided into three equal compartments, by partition plates, marked a, as shown in the design. The compartments are provided with hydraulic ducts or valves, made at the upper part of every partition plate a, a, a, and by means of them a communication is formed between the larger concentric channel, 1, and the outer case in which the wheel revolves. Similar valves are also placed at the foot of each partition plate, they are seen near the letters a, a, a, and by this means a communication is established, between each compartment or chamber of the larger concentric channel, 1, and the smaller interior circle, 2, of the wheel.

On inspecting the design, it will be seen that the valves are situated in opposite directions to each other; hence there can be no communication either between the inner smaller concentric channel 2, and the larger compartment of the wheel 1, nor between the latter compartment, and the exterior case, in which the wheel revolves, except through the valves a, a, a, which form the communicating ducts. It will be seen also, that these valves are carried from one chamber of the machine into another, but in opposite directions; the entry into one chamber being in the opposite direction to the hydraulic duct, placed in the other chamber. From these particulars the action of the machine will be obvious.

Let us suppose that the outer case, in which the wheel revolves, be filled with water to about an inch above the axis of the wheel, and that gas is conveyed into the interior small channel, by a pipe, passing along the axis, so as to allow the wheel to turn freely round, and that the pipe is turned up at right angles in the inner chamber, and projects a little way above the surface of the water, as shown in the design. The gas then must enter into the interior chamber of the wheel above the surface of the water, and must press against the adjacent partition; it will therefore cause the wheel to turn round, and, in consequence of this motion, the next par

tition plate will press the gas against the surface of the water, and cause it to pass through the hydraulic opening, in an equal quantity to that which is introduced into the exterior chainber. This alternate filling, and discharging, of the contents of each chamber, will take place once during every revolution of the wheel, and hence the number of times each particular chamber has been filled and emptied of gas may be known. In fact this machine performs the office of three revolving gas-holders, fixed on an horizontal axis, and moving in a cistern, which is the outer case of the machine. One gas-holder, or one compartment of the machine, is always in the act of becoming filled with gas, another is emptying its contents into the outer case, from which it passes into the reservoir, where it is to be stored up, or to the lamps, where it is to be burned, and the third compartment is stationary, or in an equilibrium. The wheel in any situation will therefore always have one of its receiving, and one of its discharging valves open, and consequently it will revolve.

Now to ascertain the quantity of gas discharged by one revolution of the wheel, we need only to know the capacity of the chambers, and add them together. Let us for example suppose, that each chamber contains 576 cubic inches, then one revolution of the wheel discharges a cubic foot of gas. To register the total number of revolutions which the wheel makes in a certain time, a train of wheel-work is connected with the axis of the metre; it consists of a pinion impelling a common train of wheelwork, composed of any number of wheels. The pinion on the axis of one wheel, acts into the circumference of the next wheel, and, the circumference of the wheel being as ten to one, it is obvious while the metre makes 1,000,000 revolutions, if the series consist of six wheels, the last wheel of the series will only have made one revolution. Each axis of the wheels is provided with a finger and dial-plate, divided into ten parts; therefore any number of revolutions may be read off at any time by inspection betwixt 10,000,000 and one. The velocity with which the metre acts, is of course in proportion to the quantity of gas passing through it. Thus suppose there is a burner or gas-lamp connected with the machine, of one foot capacity, lighted, which consumes four cubic feet of gas in an hour, the gas metre performs four revolutions per hour, and so on for every number of burners or lamps, not exceeding the number which the machine is calculated to supply.

The gas-holder, of the original construction, consists of two principal parts; first, of a cistern or reservoir of water, usually constructed of masonry, or of cast-iron plates, bolted and screwed together; and secondly, of an air-tight vessel which is closed at top and open at bottom, inverted with its open end downwards into the cistern of water. This vessel is always made of sheet-iron plates riveted together air-tight, and was suspended by a chain or chains, passing over wheels, supported by a frame work. If the common air be allowed to escape from the inner vessel, when its open end is under the edge of the water in the outer cistern, it will freely descend, and water will occupy the place of the

air; but if the avenue of the escape be stopped, and air be made to pass through the water, the suspended inverted vessel will rise to make room for the air. And, again, if the suspended vessel be counterpoised by a weight, so as to allow it to be a little heavier than the quantity of water which it displaces, it will descend, if the enter ing gas be withdrawn through an outlet made in the vessel to permit the gas to escape. But if the outlet be stopped, and air again be admitted under the vessel, it will rise again. The apparatus, therefore, is not only a reservoir for storing up the gas introduced into it, but serves to expel the gas which it contains, when required, into the pipes and mains connected with this machine. According to this construction of the apparatus, the interior inverted vessel forms strictly what is termed the gas-holder. It is suspended as already stated in the outer cistern, by a chain or chains, passing over pulleys, supported by blocks and frame work, and to the chain there is affixed a counterpoise balance, of such a relative weight as to allow the gas-holder a slow descent into the water, in order to propel the gas into the mains or vessel destined to receive it, with a very small and uniform weight.

It will be obvious that, when a gas-holder of this construction becomes immersed in the water, it loses as much of its weight as is equal to the bulk of water which it displaces; and hence to render its descent uniform, and to preserve the gas within of an invariable density, at any degree of immersion, a greater counterpoise is required as the gas-holder rises out of the water. Among various methods which have been adopted to attain this object, the ends of the chains by which the gas-holder is suspended, have been fastened in separate grooves, in the edge of a large wheel or pulley, of such a diameter, that the gas-holder rises to its full height before the wheel makes one revolution. In another groove, in the edge of the same wheel, was fixed the end of another chain, to which a balance weight was suspended. This weight was made nearly equal to the weight of the gas-holder. To equalise the density of the gas within the gas-holder, at any degree of immersion of the vessel, the weight chain was made to pass over a wheel, furnished with a spiral groove, so as to make the radii of the wheel change reciprocally with the relative weight of the gas-holder, and consequently to render the pressure of the gas-holder constant and uniform.

Another and more elegant method of obtaining a uniform elasticity of the gas within the gas-holder, and which has been more generally adopted, consists in passing the chain or chains by which the gas-holder is suspended over a pulley or wheels, and making the weight of that portion of the chain which is equal to the depth of the gas-holder, or that part of it which becomes immersed in the water, equal to onehalf of the weight of the specific gravity of the gas-holder. It is obvious that, before the purified gas can be admitted into the gas-holder, the vessel must be allowed to descend to the bottom of the exterior cistern, in order to get rid of the common air which it contains. This may be effected rapidly by opening the man-hole at the

top of the gas-holder, to cause the vessei to descend completely into the outer cistern filled with water. The man-hole is then screwed up again air-tight, and the machine is ready to receive the gas. It is obvious that the operation of opening the man-hole, for letting out the common air, requires only to be done once prior to the commencing of the working of the apparatus.

The collapsing gas-holder was contrived by Mr. Clegg, and certainly, of all the contrivances which have been invented for collecting and storing up large quantities of gas, this machine must be pronounced to be by far the most simple, economical, and efficient. The striking advantage of the revolving gas-holder is, that it enables the dimensions of the tank to be very much diminished, where the nature of the ground will not permit of a cistern of great depth being sunk, except at an extraordinary expense; but the still superior feature of the collapsing gas holder which we now come to describe, is, that it may be constructed of any required capacity, and adapted to a tank or cistern of such diminished depth, as scarcely to deserve that name. It requires a sheet of water no more than eighteen inches in height, so that it may be constructed in or upon ground of all descriptions, not only with every possible facility, but at an immense saving of expense.

Fig. 2, plate II. GAS LIGHT, exhibits a perspective view of this gas-holder. It is composed of two quadrangular side plates joined to two end plates meeting together at top in a ridge like the roof of a house. The side and end plates are united together by air-tight hinges, and the joints are covered with leather, to allow the side plates to fold together, and to open in the manner of a portfolio. The bottom edges of the gas-holder are immersed in a shallow cistern of water, to confine the gas. By the opening out or closing up of the sides and ends of the gas-holder, its internal capacity is enlarged or diminished, and this variation of capacity is effected without a deep tank of water to immerse the whole gas holder in, as required in the ordinary construction of rising and falling gas-holders. The collapsing gas-holder requires therefore only a very shallow trough of water to immerse the bottom edges of the gas-holder to prevent the escape of the gas introduced into it. The lower edges of the thin gas holder, which dip in water, are made to move in an horizontal plane or nearly so, when they are opened, so that they dip very little deeper in the water when shut or folded together, than when opened out.

For this purpose the top or ridge joints, which unite the two sides of the gas-holder, are slightly raised up when the sides close or approach together, or slightly depressed when the sides open out or recede from each other. To guide the whole gas-holder in this movement two perpendicular rods rise from the bottom of the shallow tank which pass through sockets in the ridge joints at the upper part of the gas-holder. These sockets are secured by collars of leather round the shafts or rods, to prevent the escape of the gas, and they are braced by chains proceeding from their upper extremities and fastened at the ground on each side of the tank

The weight of the gas-holder is balanced by levers bent in the form of the letter L, and placed in the inside of the gas-holder. These levers move on centre-pins fixed at the bottom of the shallow trough, which pass through the angles of the L levers. The perpendicular arms of the levers are jointed at their upper extremities to the sides of the gas-holder, nearly in the middle. At the ends of the horizontal arms of the L levers, are weights to counterbalance the weights of the gas-holder, and both sides of the gas holder are provided with these kinds of levers, which, at the same time that they balance its weight, cause the ridge joint of the machine to rise and fall, as before described, so that the under edges of the gas-holder, which are immersed in the water to confine the gas, must move in an horizontal plane instead of describing an arc of a circle as they would do if the ridge joint was a fixed centre of motion. When the gas-holder is closed, the perpendicular arms of the levers stand nearly in a perpendicular position; but when the gas-holder is opened out, the levers become inclined. And as they move upon a fixed fulcrum at their lower extremities, and are jointed to the sides of the gas-holder at their upper extremities, they allow the whole of the gas-holder to descend gradually upon the guide rods nearly in the same degree as the lower edges would rise up if the ridge joint was stable, and if the sides described an arc of a circle. It is obvious, however, that the latter movement is not very essential, but it is convenient, and necessary to make a very inconsiderable depth of water, in the trough or tank, serve the purpose intended. It may be also observed, that the sides of the collapsing gas-holder may be made to unfold or open on a fixed ridge point as a centre of motion; but it will then require a considerable depth of water in the tank to keep the lower edges of the sides and ends of the machine always beneath the surface of the water, because the sides of the gas-holder then describe an arc of a circle when they are open.

Mr. Malam has contrived an instrument which serves to exhibit upon a dial-plate the quantity of gas which passes through a tube in its progress to the burners. It is represented in fig. 3 plate II., where a is the pipe through which the gas passes that is to be measured; b an air-tight vessel, like bellows, with the upper flap rising or falling upon a joint or hinge, and constructed of leather or cloth, protected against the chemical action of the gas. From this vessel the gas escapes through the aperture c, into the outer case dd, and hence through the exit-pipe e, to the burners. The aperture c, is partially enclosed by the flat plate f suspended or swinging upon the rod g, and accommodating itself to the descent of the flap. When equal quantities of gas pass along in the direction a, f, b, c, d, e, in equal spaces of time, which is generally the case, the quantity of gas will be indicated by the clock movement shown in the upper part of the figure, provided the clock always stops with the supply of gas, and goes again when the supply commences; for effecting which, there is a particular contrivance, which shall be afterwards described. The clock movement in the cylindrical box,

supported and fixed upon legs mm, gives motion to an axle carrying a small eccentric wheel, or crank n, in order to raise the lever o, which has its fulcrum on the axle of the wheel q, and rests upon the periphery of the eccentric wheel. The lever being thus raised, a small spring catch p, attached to it, takes into the teeth of the wheel 9, and, when the lever again descends, the catch drives the wheel round a short way. Another spring r holds the wheel as the lever again rises; and, in this manner, by many revolutions of the eccentric wheel n, raising and lowering the lever o, the wheel q is driven entirely round. A pinion upon the axle of q works in the wheel s, which carries the index round a dial-plate, and thus registers the quantity of gas which has pass ed uniformly through the aperture c. Should the pressure of the gas, however, not be uniform, the flap of the vessel b will be raised or depressed accordingly, as indicated by the dotted line. When this happens, the connecting rods h, i, k, will raise or depress the lever o, so as to make it move through a greater or less arch, and consequently drive forward a greater or less number of the teeth of the wheel q. Upon the arm k is a stop t, which, when the flap of b descends and contracts the passage of the gas, will, by the connecting arms, h,i, k, be raised so high as to prevent the lever from being acted upon by the eccentric wheel during a part of its revolution; consequently, the arch described by the lever o will be smaller, and the progress of q and s diminished: but when the flap of the vessel bis raised, and enlarges the passage for the gas, then the stop t will be brought sufficiently low to enable the lever o to be acted upon by the periphery of the eccentric wheel during the whole revolution: in consequence of which, the arch described by the lever o will be greater, and the progress of the wheels q and s increased. A nut v, having a right and left screw, is employed to adjust the length of the rod k. For the purpose of stopping the clock movement, when the supply of gas is stopped, a paul lever u rises with the rod k, for the purpose of locking the eccentric wheel. In order to stop the passage of gas when the clock movement requires winding up, a pinion upon the axis of the fusee works in the dotted toothed arch w, w. The operation of winding up, carries the rack back; but, as the movement goes down, the rack advances, by which a tooth r, upon its axle, presses upon the short end of the lever y, which it raises, and causes to lift the rod k: at the same time making the rod h press down the flap of b, in order to bring the aperture e in contact with the plate f, and thus obstruct completely the passage of the gas.

Messrs. J. and P. Taylor are the first persons who have resorted to oil as a substance from which gas for illumination could be easily and cheaply prepared; and, in the construction of a convenient apparatus for the decomposition of this body, they have fully shown its numerous advantages over coal, while they have afforded the means of producing the most pure and brilliant flame from the inferior and cheap oils, which could not be used in lamps. The apparatus for the purpose is much smaller, much

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