GAS-LIGHT. (Eclairage par gas, Fr.;Gaslicht, Germ.) Dr. Clayton demonstrated, by numerous experiments in 1737 and 1738, that bituminous pit-coal, subjected to a red heat in close vessels, afforded a great deal of an air similar to the fire-damp of mines, but which burned with a brighter flame. It does not appear that this species of factitious air was ever produced from pit-coal for the purpose of artificial illumination till 1792, when Mr. William Murdoch, engineer to Messrs. Bolton and Watt, employed coal gas for lighting his house and offices, at Redruth in Cornwall. The gas was generated in an iron retort, whence it was received in a gasometer, distributed in different situations by pipes, and finally burned at small apertures which could be opened and stopped at pleasure. He moreover made this light movable, by confining the gas in portable tin-plate vessels, and burning it wherever he pleased. Between this period and 1802, Mr. Murdoch continued at intervals to make similar experiments; and upon occasion of the national illumination in the spring of the latter year, at the peace of Amiens, he lighted up part of the Soho manufactory with a public display of gas lights.The earliest application of this artificial light, on a large systematic scale, was made at Manchester; where an apparatus for lighting the great cotton mills of Messrs. Philips and Lee, was fitted up in 1804 and 1805, under the direction of Mr. Murdoch. A quantity of light, nearly equal to 3000 candles, was produced and distributed in this building. This splendid pattern has been since followed very generally in Great Britain, and more or less in many parts of the continents of Europe and America. By the year 1822, gas-lighting in London had become the business of many public companies. At the Peter-street station, for example, 300 retorts had been erected, supplying 15 gasometers, having each an average capacity of 20·626 cubic feet, but, being never quite filled, their total contents in gas might be estimated at 309,385 cubic feet. The extent of main pipes of distribution belonging to this station was then about 57 miles, with two separate mains in some of the streets. The product of gas was from 10,000 to 12,000 cubic feet from a chaldron of coals. The annual consumption of coals was therefore altogether 9282 chaldrons, affording 11,384,000 cubic feet of gas, allowing 153 retorts to be in constant daily action, upon an average of the year; and illuminating 10,660 private lamps, 2248 street lamps, and 3894 theatre lamps.At the Brick-lane works, 371 retorts were fixed in 1822, 133 being worked on an average of summer and winter. There were 12 gasometers, charged with an average quantity of gas amounting to 197,214 cubic feet. Of coals, 8060 chaldrons were annually consumed; 96,720,000 cubic feet of gas were generated; for the supply of 1978 public lamps, and 7366 private ones, connected with main pipes 40 miles long.At the Curtain-road gas establishment, there were 240 retorts; but the greatest number worked in 1821 was only 80, and the lowest 21. The six gasometers had an average contents of 90,467 cubic feet. Of coals, 3336 chaldrons were annually consumed, yielding 40,040,000 cubic feet of gas, that supplied 3860 private lamps, and 629 public ones, by means of mains 25 miles long. The above three stations belonged to the London Gas Light and Coke Company.The City of London Gas Light Company, Dorset-street, had built up 230 retorts, and 6 gasometers, while two were preparing; having a total capacity of 181,282 cubic feet. Of private lamps 5423 were lighted, and 2413 public ones, from mains extending 50 miles. The quantity of coals carbonized amounted to 8840 chaldrons; producing 106,080,000 cubic feet of gas.The South London Gas Light and Coke Company had mounted at Bankside 143 retorts, with 3 gasometers; the contents of the whole being 41,110 cubic feet, connected with mains from 30 to 40 miles long. At their other station, in Wellington-street, 3 large gasometers were then erecting, with a capacity of 73,565 cubic feet, which were to be supplied with gas from Bankside, till retorts were mounted for them.The Imperial Gas Light and Coke Company had at that time 6 gasometers in progress at their Hackney station.In 1822 there were thus four great companies, having in all 47 gasometers at work, capable of containing 917,940 cubic feet of gas, supplied by 1315 retorts, which generatedper annum upwards of 397,000,000 cubic feet of gas, by which 61,203 private lamps, and 7268 public or street lamps, were lighted in the metropolis. Besides these public companies, there were likewise several private ones.1.Of the generation of illuminating gases.—Pure hydrogen gas burns with too feeble a flame to be employed for illumination. But carburetted hydrogen having the property of precipitating its carbon in the act of burning, its solid particles become incandescent, and diffuse a vivid light. The more carbon it contains, the more brightly does it burn. This gas exists in two distinct states of combination. In the first, two measures of hydrogen gas are combined with one measure of the vapour of carbon, forming together one measure whose specific gravity is of course the sum of the weights of the constituents, or 0·559; atmospherical air being 1·000. This is the gas which is found in mines, and is also evolved in ditches from decomposing vegetable matter. In the second, two measures of hydrogen gas are combined with two of gaseous carbon, forming also one volume or measure whose weight or specific gravity is 0·985. This was at one time called the olefiant gas, because when mixed with chlorine an oily looking compound was produced. It may be called as well oil gas, because it is generated in considerable quantities by the igneous decomposition of oil. Thus the olefiant gas contains in the same volume double the quantity of carbon of common carburetted hydrogen, and it burns with a proportionably brighter flame. The gaseous oxide of carbon, as well as sulphuretted hydrogen gas, burns with a feeble blue light, but the latter produces in combustion sulphurous acid, an offensive and noxious gas.By dry distillation or carbonization in close vessels, all bodies of vegetable and animal origin disengage carburetted hydrogen gas; even charcoal when placed in ignition in contact with steam, by decomposing the water, produces abundance of carbonic acid, carburetted hydrogen, hydrogen, and carbonic oxide. After separating the carbonic acid with lime water, that mixed gas contains in 100 measures, 20 of carburetted hydrogen; the rest being hydrogen and carbonic oxide, so that the gaseous mixture cannot be used for illumination. The best substances for furnishing a gas rich in luminifereous materials are, pitcoal, especially the cannel coal, resin, oil, fats of all kinds, tar, wax, &c. In some cases the gases evolved during the igneous decomposition of bones and other animal matters for the production of ammonia, may be employed for procuring light, but they are apt to emit a fetid odour.When coals are heated in a cast-iron retort to ignition, the progress of decomposition is as follows. First, and before the retort becomes red hot, steam issues along with the atmospheric air. When the retort begins to redden, tar distils in considerable quantity with some combustible gas, of which hydrogen mixed with ammoniacal gas forms a part. The evolution of gas increases as the retort becomes hotter, with a continual production of tar and ammoniacal liquor as well as sulphurous acid from the pyrites of the coal, which unites with the ammonia. When the retort has come to a bright cherry red heat, the disengagement of gas is most active. By and bye the gaseous production diminishes, and eventually ceases entirely, although the heat be increased. In the retort a quantity of carbonized coal or coke remains, while tar is found at the bottom of the receiver, covered with the ammoniacal liquor, and combined with carbonic and sulphurous acids, and sulphuretted hydrogen.If during this distillation, the combustible gas be collected and examined at the several stages of the process, it is found to differ extremely in its luminiferous powers. That which comes off before the retort has acquired its proper temperature, gives a feeble light, and resembles the gas obtained by the ignition of moist charcoal, consisting chiefly of hydrogen. That evolved when the retort has just acquired throughout a vivid red heat, is the best of all, consisting chiefly of bicarburetted hydrogen or olefiant gas. From good coal, it consists, for example in 100 measures, of 13 of olefiant gas, 82·5 of carburetted hydrogen, 3·2 carbonic oxide, 1·3 azote; the mixture having a specific gravity of 0·650. At a later period, as after 5 hours, it contains 7 measures of olefiant gas, 56 of carburetted hydrogen, 11 of carbonic oxide, 21·3 of hydrogen, 4·7 of azote; the specific gravity of the whole being 0·500. Towards the end of the operation, as after 10 hours, it contains twenty measures of carburetted hydrogen, 10 of carbonic oxide, 60 of hydrogen, 10 of azote, with a specific gravity of only 0·345. The hydrogen becomes sulphuretted hydrogen, if there be much pyritous matter in the coal. The larger proportion of the gas is disengaged during the first hour, amounting to about one fifth of the whole; in the three following hours the disengagement is tolerably uniform, constituting in all fifty-four hundredths; in the sixth hour, it is one tenth; in the seventh and eighth hours, sixteen hundredths.From these observations are derived the rules for the production of a good light gas from coals. They show that the distillation should commence with a retort previously heated to a cherry red, since thereby good gas is immediately produced, and a portion of the tar is also converted into gas, instead of being simply distilled over into the condenser pit; that this heat should be steadily continued during the whole operation, from 5 to 8 hours; that it should not be increased, especially towards the end, for fearof generating carbonic oxide and hydrogen gases, as well as of injuring the retort when the cooling agency of gasefication has become feeble; and that the operation should be stopped some time before gas ceases to come over, lest gases with feeble illuminating power should impoverish the contents of the gasometer. Upon the average, a pound of good coal affords four cubic feet of gas, or a chaldron = 26 cwt. London measure, affords from 12,000 to 15,000 cubic feet, according to the form of the retort, and the manner of firing it.When oil, fats, rosin, tar, &c. are employed for the production of a light gas, it is not sufficient to introduce these substances into the retorts, and to heat them, as is done with coals. In this case, the greater part of them would distil over in the state of volatile oils, and very little gas be generated, only as much as corresponded to the quantity of fat, &c. in immediate contact with the retort. It becomes therefore necessary to fill the retorts with pieces of brick or coke; and to keep them in ignition, while the oil, &c. is slowly introduced into their interior. The fats instantly assume the vaporous state, and thus coming into contact upon an extensive surface with the ignited bricks, are decomposed into combustible gases. A small portion of carbonaceous matter remains in the retort, while much olefiant gas is formed, possessing a superior illuminating power to common coal gas, and entirely free from sulphureous impregnation. The best oil gas is generated at a dull red, a heat much below what is requisite for the decomposition of coal. A more intense heat would indeed produce a greater volume of gas, but of a poorer quality, because the olefiant gas thereby deposits one half of its carbon, and is converted into common carburetted hydrogen. Oil affords at a lively red heat, gases which contain in 100 measures, 19 of olefiant gas, 32·4 of carburetted hydrogen, 12·2 of carbonic oxide gas, 32·4 of hydrogen, and 4 of azote; the mean specific gravity being only 0·590. At a more moderate temperature it yields 22·5 of the olefiant, 50·3 carburetted hydrogen, 15·5 carbonic oxide, 7·7 hydrogen, and 4 azote, with a specific gravity of 0·758. It contains when generated by dull ignition, as is usual in works on the manufacturing scale, in 100 parts, from 38 to 40 of olefiant gas, and besides the carburetted hydrogen, a few per cents. of carbonic oxide and azote, with a specific gravity of 0·900, and even upwards. One pound of oil or fluid fat affords 15 cubic feet of gas; of tar affords about 12 cubic feet; of rosin or pitch, 10 cubic feet.When the oil gas is compressed by a force of from 15 to 20 atmospheres, as was the practice of the Portable Gas Company, about one fifth of the volume of the gas becomes liquefied into an oily, very volatile fluid, having the specific gravity 0·821. It is a mixture of three fluids (consisting of carburetted hydrogen), of different degrees of volatility. The most volatile of these boils even under 32° F. Some of the vapour of this gas-oil is mixed with the olefiant gas in the general products of decomposition; in consequence of which they are sometimes richer in carbon than even olefiant gas, and have a higher illuminating power. Oil gas contains about 22 per cent. and coal gas about 31⁄4per cent. of this oily vapour. In the estimations of the composition of the gases given above, this vapour is included under olefiant gas. This vapour combines readily with sulphuric acid, and is thus precipitated from the gaseous mixture. The amount of olefiant gas is shown, by adding to the gas, contained over water, one half of its volume of chlorine, which, in the course of an hour or two, condenses the olefiant gas into an oily looking liquid (chloride of hydrocarbon.) After the mixture, the gases must be screened from the light, otherwise the common carburetted hydrogen would also combine with the chlorine, while water and carbonic acid would make their appearance.The oil employed for affording gas is the crudest and cheapest that can be bought; even the blubber and sediment of whale oil are employed with advantage. After all, however, coal is so much cheaper, and the gas produced from it is now so well purified, that oil and rosin are very little used in gas apparatus.Apparatus for Coal Gas.—Coal gas, as it issues from the retort, cannot be directly employed for illumination; for it contains vapours of tar and coal oil, as also steam impregnated with the carbonate, sulphite, and hydrosulphuret of ammonia. These vapours would readily condense in the pipes through which the gas must be distributed, and would produce obstructions; they must therefore be so far removed by previous cooling, as to be liable to occasion no troublesome condensation at ordinary temperatures. The crude coal gas contains moreover sulphuretted hydrogen, whose combustion for light would exhale an offensive sulphureous odour, that ought to be got rid of as much as possible. Carbonic acid and carbonic oxide gases, generated at first from the decomposition of the steam by the ignited coal, enfeeble the illuminating power of the gas, and should be removed. The disengagement of gas in the retorts is never uniform, but varies with the degree of heat to which they are exposed; for which reason the gas must be received in a gasometer, where it may experience uniform pressure, and be discharged uniformly into the pipes of distribution, in order to ensure a steady discharge of gas, and uniform intensity of light in the burners. A coal gas apparatus ought therefore to be so constructed as not only to generate the gas itself, but to fulfil the above conditions.Infig.482., such an apparatus is represented, where the various parts are shown connected with each other, in section.Coal gas apparatusFig. 482 enlarged(201 kB)Ais the furnace with its set of cylindrical or elliptical retorts, five in number. From each of these retorts, a tubebproceeds perpendicularly upwards, and then by a curve or saddle-tube, it turns downwards, where it enters a long horizontal cylinder underB, shut at each end with a screw cap, and descends to beneath its middle, so as to dip about an inch into the water contained in it. From one end of this cylinder the tubedpasses downwards, to connect itself with a horizontal tube which enters into thetar pitorcisternC, by means of the vertical branchf. This branch reaches to near the bottom of the cylindrical vessel, which sits on the sole of the tar cistern. From the other side of the vertical branchf, the main pipe proceeds to the condenserD, and thence by the pipel, into the purifierE; from which the gas is immediately transmitted by the pipepinto the gasometerF.The operation proceeds in the following way:—As soon as gas begins to be disengaged from the ignited retort, tar and ammoniacal liquor are deposited in the cylindrical receiverB, and fill it up till the superfluity runs over by the piped, the level being constantly preserved at the line shown in the figure. By the same tarry liquid, the orifices of the several pipesb, issuing from the retorts, are closed; whereby the gas in the pipedhas its communication cut off with the gas in the retorts. Hence if one of the retorts be opened and emptied, it remains shut off from the rest of the apparatus. This insulation of the several retorts is the function of the pipe underB, and therefore the recurved tubebmust be dipped as far under the surface of the tarry liquid, as to be in equilibrio with the pressure of the gas upon the water in the purifier. The tubebis closed at top with a screw cap, which can be taken off at pleasure, to permit the interior to be cleansed.Both by the overflow from the receiver-pipeB, and by subsequent condensation in the tubed, tar and ammoniacal liquor collect progressively in the cistern or pit underC, by which mingled liquids the lower orifice of the vertical tubefis closed, so that the gas cannot escape into the empty space of this cistern. These liquids flow over the edges of the inner vessel when it is full, and may, from time to time, be drawn off by the stopcock at the bottom of the cistern.Though the gas has, in its progress hitherto, deposited a good deal of its tarry and ammoniacal vapours, yet, in consequence of its high temperature, it still retains a considerable portion of them, which must be immediately abstracted, otherwise the tarwould pollute the lime in the vesselE, and interfere with its purification. On this account the gas should, at this period of the process, be cooled as much as possible, in order to condense these vapours, and to favour the action of the lime in the purifierE, upon the sulphuretted hydrogen, which is more energetic the lower the temperature of the gas. The coal gas passes, therefore, from the tubefinto the tubehof the condenserD, which is placed in an iron chestgfilled with water, and it deposits more tar and ammoniacal liquor in the under part of the cistern att,t. When these liquids have risen to a certain level, they overflow into the tar-pit, as shown in the figure, to be drawn off by the stopcock as occasion may require.The refrigerated gas is now conducted into the purifierE, which is filled with milk of lime, made by mixing one part of slaked lime with 25 parts of water. The gas, as it enters by the pipel, depresses the water in the wide cylindern, thence passes under the perforated disc in the under part of that cylinder, and rising up through innumerable small holes is distributed throughout the lime liquid in the vesselm. By contact with the lime on this extended surface, the gas is stripped of its sulphuretted hydrogen and carbonic acid, which are condensed into the hydro-sulphuret and carbonate of lime; it now enters the gasometerFin a purified state, through the pipep t, and occupies the spaceq. The gasometer, pressing with a small unbalanced force over the counterweights, expels it through the mainu u, in communication with the pipes of distribution through the buildings or streets to be illuminated.The partsA B C D EandF, of which this apparatus consists, are essential constituents of every good coal-gas work. Their construction rests upon peculiar principles, is susceptible of certain modifications, and therefore deserves to be considered in detail.The Retorts.—These are generally made of cast iron, though they have occasionally been made of baked clay, like common earthenware retorts. The original form was a cylinder, which was changed to an ellipse, with the long axis in a horizontal direction, then into the shape of the letterDwith the straight line undermost, and lastly into a semi-cylinder, with its horizontal diameter 22 inches, and its vertical varying from 9 to 12. The kidney form was at one time preferred, but it has been little used of late.Coal gas retortThe form of retort represented infig.483.has been found to yield the largest quantity of good gas in the shortest time, and with the least quantity of firing. The length is 71⁄2, and the transverse area, from one foot to a foot and a half square. The arrows show the direction of the flame and draught in this excellent bench of retorts, as mounted by Messrs. Barlow.The charge of coals is most conveniently introduced in a tray of sheet iron, made somewhat like a grocer’s scoop, adapted to the size of the retort, which is pushed home to its further end, inverted so as to turn out the contents, and then immediately withdrawn.The duration of the process, or the time of completing a distillation, depends upon the nature of the coal and the form of the retort. With cylindrical retorts it cannot be finished in less than 6 hours, but with elliptical and semi-cylindrical retorts, it may be completed in 4 or 5 hours. If the distillation be continued in the former for 8 hours, and in the latter for 6, gas will continue to be obtained, but during the latter period of the operation, of indifferent quality.ReceiverThe Receiver.—If the furnace contains only 2 or 3 retorts, a simple cylindrical vessel standing on the ground half filled with water, may serve as a receiver; into which the tube from the retort may be plunged. It should be provided with an overflow pipe for the tar and ammoniacal liquor. For a range of several retorts, a long horizontal cylinder is preferable, like that represented atBinfig.484.Its diameter is from 10 to 15 inches. This cylinder may be so constructed as to separate the tar from the ammoniacal liquor, by means of a syphon attached to one of its ends.The Condenser.—The condenser, represented infig.482., consists of a square chest,g, made of wrought iron plates open at top, but having its bottom pierced with a row of holes, to receive a series of tubes. To these holes the upright four-inch tubesh hare secured by flanges and screws, and they are connected in pairs at top by the curved or saddle tubes. The said bottom forms the cover of the chestt,t, which is divided by vertical iron partitions, into half as many compartments as there are tubes.These partition plates are left open at bottom, so as to place the liquids of each compartment in communication. Thereby the gas passes up and down the series of tubes, in proceeding from one compartment to another. The condensed liquids descend into the boxt,t, and flow over into the tar cistern, when they rise above the levelt,t. The tar may be drawn off from time to time by the stopcock. Through the tubek, cold water flows into the condenser chest, and the warm water passes away by a pipe at its upper edge.The extent of surface which the gas requires for its refrigeration before it is admitted into the washing-lime apparatus, depends upon the temperature of the milk of lime, and the quantity of gas generated in a certain time.It may be assumed as a determination sufficiently exact, that 10 square feet of surface of the condenser can cool a cubic foot of gas per minute to the temperature of the cooling water. For example, suppose a furnace or arch with 5 retorts of 150 pounds of coal each, to produce in 5 hours 3000 cubic feet of gas, or 10 cubic feet per minute, there would be required, for the cooling surface of the condenser, 100 square feet = 10 × 10. Suppose 100,000 cubic feet of gas to be produced in 24 hours, for which 8 or 9 such arches must be employed, the condensing surface must contain from 800 to 900 square feet.The Purifier.—The apparatus represented in the preceding figure is composed of a cylindrical iron vessel, with an air-tight cover screwed upon it, through which the cylindernis also fixed air-tight. The bottom of this cylinder spreads out like the brim of a hat, forming a horizontal circular partition, which is pierced with holes. Through a stuffing box, in the cover of this interior cylinder, the vertical axis of the agitator passes, which is turned by wheel and pinion work, in order to stir up the lime from the bottom of the water in the purifier. The vesseloserves for introducing fresh milk of lime, as also for letting it off by a stopcock when it has become too foul for further use.The quantity of lime should be proportioned to the quantity of sulphuretted hydrogen and carbonic acid contained in the gas. Supposing that in good coal gas there is 5 per cent. of these gases, about one pound and a half of lime will be requisite for every hundred cubic feet of coal gas generated, which amounts to nearly one-sixteenth of the weight of coal subjected to decomposition. This quantity of lime mixed with the proper quantity of water will form about a cubic foot of milk of lime. Consequently, the capacity of the purifier, that is, of the interior space filled with liquid, may be taken at four-sevenths of a cubic foot for every hundred cubic feet of gas passing through it in one operation; or for 175 cubic feet of gas, one cubic foot of liquor. After every operation, that is, after every five or six hours, the purifier must be filled afresh. Suppose that in the course of one operation 20,000 cubic feet of gas pass through the machine, this should be able to contain20,000175= 114 cubic feet of milk of lime; whence its diameter should be seven feet, and the height of the liquid three feet. If the capacity of the vessel be less, the lime milk must be more frequently changed.In some of the large gas works of London the purifier has the following construction, whereby an uninterrupted influx and efflux of milk of lime takes place. Three single purifiers are so connected together, that the second vessel stands higher than the first, and the third than the second; so that the discharge tube of the superior vessel, placed somewhat below its cover, enters into the upper part of the next lower vessel; consequently, should the milk of lime in the third and uppermost vessel rise above its ordinary level, it will flow over into the second, and thence in the same way into the first; from which it is let off by the eduction pipe. A tube introduces the gas from the condenser into the first vessel, another tube does the same thing for the second vessel, &c., and the tube of the third vessel conducts the gas into the gasometer. Into the third vessel, milk of lime is constantly made to flow from a cistern upon a higher level. By this arrangement, the gas passing through the several vessels in proportion as it is purified, comes progressively into contact with purer milk of lime, whereby its purification becomes more complete. The agitatorc, provided with two stirring paddles, iskept in continual rotation. The pressure which the gas has here to overcome is naturally three times as great as with a single purifier of like depth.PurifierFig.485.is a simple form of purifier, which has been found to answer well in practice. Through the cover of the vesselA B, the wide cylindere dis inserted, having its lower end pierced with numerous holes. Concentric with that cylinder is the narrower ones z, bound above with the flangea b, but open at top and bottom. The under edgeg hof this cylinder descends a few inches below the endc dof the outer one. About the middle of the vessel the perforated shelfm nis placed. The shaft of the agitatorl, passes through a stuffing box upon the top of the vessel. The gas-pipeg, proceeding from the condenser, enters through the flangea bin the outer cylinder, while the gas-pipehgoes from the cover to the gasometer. A stopcock upon the side, whose orifice of discharge is somewhat higher than the under edge of the outer cylinder, serves to draw off the milk of lime. As the gas enters through the pipeginto the space between the two cylinders, it displaces the liquor till it arrives at the holes in the under edge of the outer cylinder, through which, as well as under the edge, it flows, and then passes up through the apertures of the shelfm ninto the milk of lime chamber; the level of which is shown by the dotted line. The stirrer,l, should be turned by wheel work, though it is here shown as put in motion by a winch handle.In order to judge of the degree of purity of the gas after its transmission through the lime machine, a slender syphon tube provided with a stopcock may have the one end inserted in its cover, and the other dipped into a vessel containing a solution of acetate of lead. Whenever the solution has been rendered turbid by the precipitation of sulphuret of lead, it should be renewed. The saturated and fetid milk of lime is evaporated in oblong cast-iron troughs placed in the ash-pit of the furnaces, and the dried lime is partly employed for luting the apparatus, and partly disposed of for a mortar or manure.By this purifier, and others of similar construction, the gas in the preceding parts of the apparatus, as in the retorts and the condenser, suffers a pressure equal to a column of water about two feet high; and in the last described purifier even a greater pressure. This pressure is not disadvantageous, but is of use in two respects; 1. it shows by a brisk jet of gas when the apparatus is not air-tight, and it prevents common air from entering into the retorts; 2. this compression of the gas favours the condensation of the tar and ammoniacal liquor. The effect of such a degree of pressure in expanding the metal of the ignited retorts is quite inconsiderable, and may be neglected. Two contrivances have, however, been proposed for taking off this pressure in the purifier.PurefierInfig.486.,m mare two similar vessels of a round or rectangular form, furnished at their upper border with a groove filled with water, into which the under edge of the cover fits, so as to make the vessel air-tight. The cover is suspended by a cord or chain, which goes over a pulley, and may be raised or lowered at pleasure. The vessels themselves have perforated bottoms,rr′, covered with wetted moss or hay sprinkled over with slaked and sifted quicklime. The gas passes through the loosely compacted matter of the first vessel, by entering between its two bottoms, rises into the upper spacet, thence it proceeds to the second vessel, and, lastly, through the pipeuinto the gasometer. This method, however, requires twice as much lime as the former, without increasing the purity of the gas.CompressorThe second method consists in compressing the gas by the action of an Archimedes screw, to such a degree, before it is admitted into the purifier, as that it may overcome the pressure of the column of water in that vessel.Fig.487.exhibits this apparatus in section.D Dis the Archimedes worm, the axis of which revolves at bottom upon the gudgeone; it possesses a three-fold spiral, and is turned in the opposite direction to that in which it scoops the water. The cistern which contains it has an air-tight cover. The gas to be purified passes through the pipeCinto the spaceD, over the water leveld; the upper cells of the worm, scoop in the gas at this point, andcarry it downwards, where it enters atginto the cavityEof a second cistern. In order that the gas, after it escapes from the bottom of the worm, may not partially return throughginto the cavityD, an annular plateg his attached to its under edge, so as to turn over it. The compressed gas is conducted from the cavityEthrough the pipeGinto the purifying machine;ais a manometer, to indicate the elastic tension of the gas inD. On the top of the worm a mechanism is fitted for keeping it in constant rotation.A perfect purification of light-gas from sulphuretted hydrogen, either by milk of lime or a solution of the green sulphate of iron, is attended with some difficulty, when carried so far as to cause no precipitation of sulphuret in acetate of lead, because such a degree of washing is required as is apt to diminish its illuminating power, by abstracting the vapour of the rich oily hydrocarburet which it contains. Moreover, the coal gas obtained towards the end of the distillation contains some sulphuret of carbon, which affords sulphurous acid on being burned, and can be removed by no easy method hitherto known. The lime in the purifier disengages from the carbonate and hydrosulphuret of ammonia carried over with the gas, especially when it has been imperfectly cooled in the condenser, a portion of ammoniacal gas, which, however, is not injurious to its illuminating power. The best agent for purifying gas would be the pyrolignite of lead, were it not rather expensive, because it would save the trouble of stirring, and require a smaller and simpler apparatus.The Gasometer.—The gasometer serves not merely as a magazine for receiving the gas when it is purified, and keeping it in store for use, but also for communicating to the gas in the act of burning such an uniform pressure as may secure a steady unflickering flame. It consists of two essential parts; 1. of an under cistern, open at top and filled with water; and 2. of the upper floating cylinder or chest, which is a similar cistern inverted, and of somewhat smaller dimensions, called the gas-holder: seeF,fig.482.The best form of this vessel is the round or cylindrical; both because under equal capacity it requires least surface of metal, and it is least liable to be warped by its own weight or accidents. Since a cylindrical body has the greatest capacity with a given surface when its height is equal to its semi-diameter, its dimensions ought to be such that when elevated to the highest point in the water, the height may be equal to the radius of the base. For example, let the capacity of the gas-holder in cubic feet bek, the semi-diameter of its base bex, the height out of the water beh;his =x=∛k3·14. This height may be increased by one or two feet, according to its magnitude, to prevent the chance of any gas escaping beneath its under edge, when it is raised to its highest elevation in the water.The size of the gasometer should be proportional to the quantity of gas to be consumed in a certain time. If 120,000 cubic feet be required, for instance, in 10 hours for street illumination, and if the gas retorts be charged four times in 24 hours, 30,000 cubic feet of gas will be generated in 6 hours. Hence the gasometer should have a capacity of at least 70,000 cubic feet, supposing the remaining 50,000 cubic feet to be produced during the period of consumption. If the gasometer has a smaller capacity, it must be supplied from a greater number of retorts during the lighting period, which is not advantageous, as the first heating of the supernumerary retorts is wasteful of fuel. Some engineers consider that a capacity of 30,000 cubic feet is the largest which can with propriety be given to a gasometer; in which case, they make its diameter 42 feet, and its height 23. When the dimensions are greater, the sheet iron must be thicker and more expensive; and the hollow cylinder must be fortified by strong internal cross braces.The water cistern is usually constructed in this country with cast-iron plates bolted together, and made tight with rust-cement.GasometerIn cases where the weight of water required to fill such a cistern might be inconvenient to sustain, it may be made in the form represented infig.488.; which, however, will cost nearly twice as much. Parallel with the side of the cistern, a second cylinderC, of the same shape but somewhat smaller, is fixed in an inverted position to the bottom of the first, so as to leave an annular spaceB Bbetween them, which is filled with water, and in which the floating gasometerAplays up and down. The water must stand above the cover of the inverted cylinder.aandbare the pipes for leading the gas in and out. Through an opening in the masonry upon which the gasometer apparatus rests, the spaceCmay be entered, in order to make any requisite repairs.The water cistern may also be sunk in the ground, and the sides made tight with hydraulic mortar, as is shown infig.489., and to make it answer with less water, a concentric cylindrical mass of masonry may be built at a distance of 2 or 3 inches within it.Every large gasometer must be strengthened interiorly with cross iron rods, to stiffen both its top and bottom. The top is supported by rods stretching obliquely down tothe sides, and to the under edge an iron ring is attached, consisting of curved cast-iron bars bolted together; with which the oblique rods are connected by perpendicular ones. Other vertical rods stretch directly from the top to the bottom edge. Upon the periphery of the top, at the end of the rods, several rings are made fast, to which the gas-holder is suspended, by means of a common chain which runs over a pulley at the centre. Upon the other end of the chain there is a counterpoise, which takes off the greater part of the weight of the gas-holder, leaving only so much as is requisite for the expulsion of the gas. The inner and outer surfaces of the gas-holder should be a few times rubbed over with hot tar, at a few days’ interval between each application. The pulley must be made fast to a strong frame.GasometerIf the water cistern be formed with masonry, the suspension of the gas-holder may be made in the following way.A A,fig.489., is a hollow cylinder of cast iron, standing up through the middle of the gasometer, and which is provided at either end with another small hollow cylinderG, open at both ends and passing through the top, with its axis placed in the axis of the gas-holder. In the hollow cylinderG, the counterweight moves up and down, with its chain passing over the three pulleysB,B,B, as shown infig.489.E Fare the gas pipes made fast to a vertical iron rod. Should the gasometer be made to work without a counterweight, as we shall presently see, the central cylinderA A, serves as a vertical guide.In proportion as the gas-holder sinks in the water of the cistern, it loses so much of its weight, as is equal to the weight of the water displaced by the sides of the sinking vessel; so that the gas-holder when entirely immersed, exercises the least pressure upon the gas, and when entirely out of the water, it exercises the greatest pressure. In order to counteract this inequality of pressure, which would occasion an unequal velocity in the efflux of the gas, and of course an unequal intensity of light in its flame, the weight of the chain upon which the gas-holder hangs is so adjusted as to be equal, throughout the length of its motion, to one half of the weight which the gas-holder loses by immersion. In this case, the weight which it loses by sinking into the water, is replaced by the portion of the chain which passing the pulley, and hanging over, balances so much of the chain upon the side of the counterweight; and the weight which it gains by rising out of the water, is counterpoised by the links of the chain which passing over the pulley, add to the amount of the counterweight. The pressure which the gas-holder exercises upon the gas, or that with which it forces it through the first main pipe, is usually so regulated as to sustain a column of from one to two inches of water; so that the water will stand in the cistern from one to two inches higher within, than without the gas-holder. The following computation will place these particulars in a clear light.Let the semi-diameter of the gas-holder, equal to the vertical extent of its motion into and out of the water, =x; let the weight of a foot square of the side of the gas-holder, including that of the strengthening bars and ring, which remain plunged under the water, be =p; then1. the weight of the gas-holder in its highest position = 3pπx2;2. the weight of the sides of the gas-holder which play in the water = 2pπx2;3. the cubic contents of the immersed portion of the gas-holder =2pπx2400;4. its loss of weight in water =112400pπx2;5. the weight of the gas-holder in its lowest position =pπx2(3 -112400)= 2·72pπx2;6. the weight ofninches, height of water =5612nπx2;7. the amount of the counterweight = πx2(3p-56n12);8. the weight of the chain for the lengthx=112800pπx.If we reduce the weight of the gas-holder in its highest and lowest positions to the height of a stratum of water equal to the surface of its top, this height is that of the column of water which would press the gas within the gasometer, were no counterweight employed; it consists as follows;—9. for the highest position =3p56;10. for the lowest =2·72p50;For the case, when the height of the gas-holder is different from its semi-diameter, let this height =m x; then the height of the water level is11. for the highest position =p(1 +2m56);12. for the lowest =p(1 +1·72m6);13. the counterweight = πx2(p(1 + 2m) -56n12);14. the weight of the equalizing chain =112800pπmx2.For example, let the diameter of the gas-holder be 30 feet, the height 15 (the contents in cubic feet will be 10,597),p= 4 pounds; then the counterweight for a height of an inch and a half of water pressure = 3532 pounds; the weight of the chain for a length of 15 feet = 395 pounds. Were no counterweight employed, so that the gas-holder pressed with its whole weight upon the gas, then the height of the equivalent column of water in its highest position = 2·56 inches; and in its lowest, 2·33. The counterweight may hence be lessened at pleasure, if the height of the pressing water-columnnbe increased. The weight of the equalising or compensating portion of the chain remains the same. Whenn= 2 inches, for instance, the counterweight = 1886 pounds.The velocity with which the gas passes along the mains for supplying the various jets of light, may be further regulated by opening the main-cock or slide-valve in a greater or less degree.Gasometers whose height is greater than their semi-diameter, are not only more costly in the construction, but require heavier counterweights and equilibration chains.The above estimate is made on the supposition of the gas in the gas-holder being of the same specific gravity as the atmospherical air, which would be nearly true with regard to oil gas under the ordinary pressure. But coal gas, whose specific gravity may be taken on an average at about 0·5, exercises a buoyancy upon the top of the gas-holder, which of course diminishes its absolute weight. Supposing the cubic foot of gas to be = 0·0364 pounds, the buoyancy will be = 0·0364 πx3pounds; a quantity which deserves to be taken into account for large gasometers. Hence,15. the weight of the gas-holder in its highest position = 3pπx2- 0·1143x3;16. the counterweight = πx2(3p-56n12)- 0·1143x2;17. The weight of the chain for the lengthx, =112800pπx20·1143x32;18. The height of the water pressure for the highest position, without the counterweight =3pπ - 0·1143x56 π;19. the same for the lowest position =2·72p56in feet.The preceding values ofpandx, are,(16) = 3147; (17) = 203; (18) = 2·44 inches; (19) = 2·33 inches.The water columns in the highest and lowest situations of the gas-holder here differ about 0·1 of an inch, and this difference becomes still less whenphas a smaller value, for example, 3 pounds, or when the diameter of the gas-holder is still greater.It would thus appear that for coal-gas gasometers, in which the height of the gas-holder does not exceed its semi-diameter, and especially when it has a considerable size, neither a compensation chain nor a counterweight is necessary. The only thing requisite, is to preserve the vertical motion of the gas-holder by a sufficient number of guide rods or pillars, placed either within the water cistern, or round about it. Should the pressure of the gas in the pipe proceeding from the gasometer, be less than in the gasometer itself, this may be regulated by the main valve, or by water valves of various kinds. Or a small intermediate regulating gasometer may be introduced between the great gas-holder, and the main pipe of distribution. With a diameter of 61 feet in the gas-holder, the pressure in the highest and lowest positions is the same.The gasometers employed in storing up gas until required for use, occupy, upon the old plan, much space, and are attended with considerable expense in erecting. The water tank, whether sunk in the ground, or raised, must be of equal dimensions with the gasometer, both in breadth and depth. The improved construction which we are about to describe, affords a means of reducing the depth of the tank, dispensing with the bridge of suspension, and of increasing at pleasure the capacity of the gasometer, upon a given base; thus rendering a small apparatus capable, if required, of holding a large quantity of gas, the first cost of which will be considerably less than even a small gasometer constructed upon the ordinary plan.
GAS-LIGHT. (Eclairage par gas, Fr.;Gaslicht, Germ.) Dr. Clayton demonstrated, by numerous experiments in 1737 and 1738, that bituminous pit-coal, subjected to a red heat in close vessels, afforded a great deal of an air similar to the fire-damp of mines, but which burned with a brighter flame. It does not appear that this species of factitious air was ever produced from pit-coal for the purpose of artificial illumination till 1792, when Mr. William Murdoch, engineer to Messrs. Bolton and Watt, employed coal gas for lighting his house and offices, at Redruth in Cornwall. The gas was generated in an iron retort, whence it was received in a gasometer, distributed in different situations by pipes, and finally burned at small apertures which could be opened and stopped at pleasure. He moreover made this light movable, by confining the gas in portable tin-plate vessels, and burning it wherever he pleased. Between this period and 1802, Mr. Murdoch continued at intervals to make similar experiments; and upon occasion of the national illumination in the spring of the latter year, at the peace of Amiens, he lighted up part of the Soho manufactory with a public display of gas lights.
The earliest application of this artificial light, on a large systematic scale, was made at Manchester; where an apparatus for lighting the great cotton mills of Messrs. Philips and Lee, was fitted up in 1804 and 1805, under the direction of Mr. Murdoch. A quantity of light, nearly equal to 3000 candles, was produced and distributed in this building. This splendid pattern has been since followed very generally in Great Britain, and more or less in many parts of the continents of Europe and America. By the year 1822, gas-lighting in London had become the business of many public companies. At the Peter-street station, for example, 300 retorts had been erected, supplying 15 gasometers, having each an average capacity of 20·626 cubic feet, but, being never quite filled, their total contents in gas might be estimated at 309,385 cubic feet. The extent of main pipes of distribution belonging to this station was then about 57 miles, with two separate mains in some of the streets. The product of gas was from 10,000 to 12,000 cubic feet from a chaldron of coals. The annual consumption of coals was therefore altogether 9282 chaldrons, affording 11,384,000 cubic feet of gas, allowing 153 retorts to be in constant daily action, upon an average of the year; and illuminating 10,660 private lamps, 2248 street lamps, and 3894 theatre lamps.
At the Brick-lane works, 371 retorts were fixed in 1822, 133 being worked on an average of summer and winter. There were 12 gasometers, charged with an average quantity of gas amounting to 197,214 cubic feet. Of coals, 8060 chaldrons were annually consumed; 96,720,000 cubic feet of gas were generated; for the supply of 1978 public lamps, and 7366 private ones, connected with main pipes 40 miles long.
At the Curtain-road gas establishment, there were 240 retorts; but the greatest number worked in 1821 was only 80, and the lowest 21. The six gasometers had an average contents of 90,467 cubic feet. Of coals, 3336 chaldrons were annually consumed, yielding 40,040,000 cubic feet of gas, that supplied 3860 private lamps, and 629 public ones, by means of mains 25 miles long. The above three stations belonged to the London Gas Light and Coke Company.
The City of London Gas Light Company, Dorset-street, had built up 230 retorts, and 6 gasometers, while two were preparing; having a total capacity of 181,282 cubic feet. Of private lamps 5423 were lighted, and 2413 public ones, from mains extending 50 miles. The quantity of coals carbonized amounted to 8840 chaldrons; producing 106,080,000 cubic feet of gas.
The South London Gas Light and Coke Company had mounted at Bankside 143 retorts, with 3 gasometers; the contents of the whole being 41,110 cubic feet, connected with mains from 30 to 40 miles long. At their other station, in Wellington-street, 3 large gasometers were then erecting, with a capacity of 73,565 cubic feet, which were to be supplied with gas from Bankside, till retorts were mounted for them.
The Imperial Gas Light and Coke Company had at that time 6 gasometers in progress at their Hackney station.
In 1822 there were thus four great companies, having in all 47 gasometers at work, capable of containing 917,940 cubic feet of gas, supplied by 1315 retorts, which generatedper annum upwards of 397,000,000 cubic feet of gas, by which 61,203 private lamps, and 7268 public or street lamps, were lighted in the metropolis. Besides these public companies, there were likewise several private ones.
1.Of the generation of illuminating gases.—Pure hydrogen gas burns with too feeble a flame to be employed for illumination. But carburetted hydrogen having the property of precipitating its carbon in the act of burning, its solid particles become incandescent, and diffuse a vivid light. The more carbon it contains, the more brightly does it burn. This gas exists in two distinct states of combination. In the first, two measures of hydrogen gas are combined with one measure of the vapour of carbon, forming together one measure whose specific gravity is of course the sum of the weights of the constituents, or 0·559; atmospherical air being 1·000. This is the gas which is found in mines, and is also evolved in ditches from decomposing vegetable matter. In the second, two measures of hydrogen gas are combined with two of gaseous carbon, forming also one volume or measure whose weight or specific gravity is 0·985. This was at one time called the olefiant gas, because when mixed with chlorine an oily looking compound was produced. It may be called as well oil gas, because it is generated in considerable quantities by the igneous decomposition of oil. Thus the olefiant gas contains in the same volume double the quantity of carbon of common carburetted hydrogen, and it burns with a proportionably brighter flame. The gaseous oxide of carbon, as well as sulphuretted hydrogen gas, burns with a feeble blue light, but the latter produces in combustion sulphurous acid, an offensive and noxious gas.
By dry distillation or carbonization in close vessels, all bodies of vegetable and animal origin disengage carburetted hydrogen gas; even charcoal when placed in ignition in contact with steam, by decomposing the water, produces abundance of carbonic acid, carburetted hydrogen, hydrogen, and carbonic oxide. After separating the carbonic acid with lime water, that mixed gas contains in 100 measures, 20 of carburetted hydrogen; the rest being hydrogen and carbonic oxide, so that the gaseous mixture cannot be used for illumination. The best substances for furnishing a gas rich in luminifereous materials are, pitcoal, especially the cannel coal, resin, oil, fats of all kinds, tar, wax, &c. In some cases the gases evolved during the igneous decomposition of bones and other animal matters for the production of ammonia, may be employed for procuring light, but they are apt to emit a fetid odour.
When coals are heated in a cast-iron retort to ignition, the progress of decomposition is as follows. First, and before the retort becomes red hot, steam issues along with the atmospheric air. When the retort begins to redden, tar distils in considerable quantity with some combustible gas, of which hydrogen mixed with ammoniacal gas forms a part. The evolution of gas increases as the retort becomes hotter, with a continual production of tar and ammoniacal liquor as well as sulphurous acid from the pyrites of the coal, which unites with the ammonia. When the retort has come to a bright cherry red heat, the disengagement of gas is most active. By and bye the gaseous production diminishes, and eventually ceases entirely, although the heat be increased. In the retort a quantity of carbonized coal or coke remains, while tar is found at the bottom of the receiver, covered with the ammoniacal liquor, and combined with carbonic and sulphurous acids, and sulphuretted hydrogen.
If during this distillation, the combustible gas be collected and examined at the several stages of the process, it is found to differ extremely in its luminiferous powers. That which comes off before the retort has acquired its proper temperature, gives a feeble light, and resembles the gas obtained by the ignition of moist charcoal, consisting chiefly of hydrogen. That evolved when the retort has just acquired throughout a vivid red heat, is the best of all, consisting chiefly of bicarburetted hydrogen or olefiant gas. From good coal, it consists, for example in 100 measures, of 13 of olefiant gas, 82·5 of carburetted hydrogen, 3·2 carbonic oxide, 1·3 azote; the mixture having a specific gravity of 0·650. At a later period, as after 5 hours, it contains 7 measures of olefiant gas, 56 of carburetted hydrogen, 11 of carbonic oxide, 21·3 of hydrogen, 4·7 of azote; the specific gravity of the whole being 0·500. Towards the end of the operation, as after 10 hours, it contains twenty measures of carburetted hydrogen, 10 of carbonic oxide, 60 of hydrogen, 10 of azote, with a specific gravity of only 0·345. The hydrogen becomes sulphuretted hydrogen, if there be much pyritous matter in the coal. The larger proportion of the gas is disengaged during the first hour, amounting to about one fifth of the whole; in the three following hours the disengagement is tolerably uniform, constituting in all fifty-four hundredths; in the sixth hour, it is one tenth; in the seventh and eighth hours, sixteen hundredths.
From these observations are derived the rules for the production of a good light gas from coals. They show that the distillation should commence with a retort previously heated to a cherry red, since thereby good gas is immediately produced, and a portion of the tar is also converted into gas, instead of being simply distilled over into the condenser pit; that this heat should be steadily continued during the whole operation, from 5 to 8 hours; that it should not be increased, especially towards the end, for fearof generating carbonic oxide and hydrogen gases, as well as of injuring the retort when the cooling agency of gasefication has become feeble; and that the operation should be stopped some time before gas ceases to come over, lest gases with feeble illuminating power should impoverish the contents of the gasometer. Upon the average, a pound of good coal affords four cubic feet of gas, or a chaldron = 26 cwt. London measure, affords from 12,000 to 15,000 cubic feet, according to the form of the retort, and the manner of firing it.
When oil, fats, rosin, tar, &c. are employed for the production of a light gas, it is not sufficient to introduce these substances into the retorts, and to heat them, as is done with coals. In this case, the greater part of them would distil over in the state of volatile oils, and very little gas be generated, only as much as corresponded to the quantity of fat, &c. in immediate contact with the retort. It becomes therefore necessary to fill the retorts with pieces of brick or coke; and to keep them in ignition, while the oil, &c. is slowly introduced into their interior. The fats instantly assume the vaporous state, and thus coming into contact upon an extensive surface with the ignited bricks, are decomposed into combustible gases. A small portion of carbonaceous matter remains in the retort, while much olefiant gas is formed, possessing a superior illuminating power to common coal gas, and entirely free from sulphureous impregnation. The best oil gas is generated at a dull red, a heat much below what is requisite for the decomposition of coal. A more intense heat would indeed produce a greater volume of gas, but of a poorer quality, because the olefiant gas thereby deposits one half of its carbon, and is converted into common carburetted hydrogen. Oil affords at a lively red heat, gases which contain in 100 measures, 19 of olefiant gas, 32·4 of carburetted hydrogen, 12·2 of carbonic oxide gas, 32·4 of hydrogen, and 4 of azote; the mean specific gravity being only 0·590. At a more moderate temperature it yields 22·5 of the olefiant, 50·3 carburetted hydrogen, 15·5 carbonic oxide, 7·7 hydrogen, and 4 azote, with a specific gravity of 0·758. It contains when generated by dull ignition, as is usual in works on the manufacturing scale, in 100 parts, from 38 to 40 of olefiant gas, and besides the carburetted hydrogen, a few per cents. of carbonic oxide and azote, with a specific gravity of 0·900, and even upwards. One pound of oil or fluid fat affords 15 cubic feet of gas; of tar affords about 12 cubic feet; of rosin or pitch, 10 cubic feet.
When the oil gas is compressed by a force of from 15 to 20 atmospheres, as was the practice of the Portable Gas Company, about one fifth of the volume of the gas becomes liquefied into an oily, very volatile fluid, having the specific gravity 0·821. It is a mixture of three fluids (consisting of carburetted hydrogen), of different degrees of volatility. The most volatile of these boils even under 32° F. Some of the vapour of this gas-oil is mixed with the olefiant gas in the general products of decomposition; in consequence of which they are sometimes richer in carbon than even olefiant gas, and have a higher illuminating power. Oil gas contains about 22 per cent. and coal gas about 31⁄4per cent. of this oily vapour. In the estimations of the composition of the gases given above, this vapour is included under olefiant gas. This vapour combines readily with sulphuric acid, and is thus precipitated from the gaseous mixture. The amount of olefiant gas is shown, by adding to the gas, contained over water, one half of its volume of chlorine, which, in the course of an hour or two, condenses the olefiant gas into an oily looking liquid (chloride of hydrocarbon.) After the mixture, the gases must be screened from the light, otherwise the common carburetted hydrogen would also combine with the chlorine, while water and carbonic acid would make their appearance.
The oil employed for affording gas is the crudest and cheapest that can be bought; even the blubber and sediment of whale oil are employed with advantage. After all, however, coal is so much cheaper, and the gas produced from it is now so well purified, that oil and rosin are very little used in gas apparatus.
Apparatus for Coal Gas.—Coal gas, as it issues from the retort, cannot be directly employed for illumination; for it contains vapours of tar and coal oil, as also steam impregnated with the carbonate, sulphite, and hydrosulphuret of ammonia. These vapours would readily condense in the pipes through which the gas must be distributed, and would produce obstructions; they must therefore be so far removed by previous cooling, as to be liable to occasion no troublesome condensation at ordinary temperatures. The crude coal gas contains moreover sulphuretted hydrogen, whose combustion for light would exhale an offensive sulphureous odour, that ought to be got rid of as much as possible. Carbonic acid and carbonic oxide gases, generated at first from the decomposition of the steam by the ignited coal, enfeeble the illuminating power of the gas, and should be removed. The disengagement of gas in the retorts is never uniform, but varies with the degree of heat to which they are exposed; for which reason the gas must be received in a gasometer, where it may experience uniform pressure, and be discharged uniformly into the pipes of distribution, in order to ensure a steady discharge of gas, and uniform intensity of light in the burners. A coal gas apparatus ought therefore to be so constructed as not only to generate the gas itself, but to fulfil the above conditions.
Infig.482., such an apparatus is represented, where the various parts are shown connected with each other, in section.
Coal gas apparatusFig. 482 enlarged(201 kB)
Fig. 482 enlarged(201 kB)
Ais the furnace with its set of cylindrical or elliptical retorts, five in number. From each of these retorts, a tubebproceeds perpendicularly upwards, and then by a curve or saddle-tube, it turns downwards, where it enters a long horizontal cylinder underB, shut at each end with a screw cap, and descends to beneath its middle, so as to dip about an inch into the water contained in it. From one end of this cylinder the tubedpasses downwards, to connect itself with a horizontal tube which enters into thetar pitorcisternC, by means of the vertical branchf. This branch reaches to near the bottom of the cylindrical vessel, which sits on the sole of the tar cistern. From the other side of the vertical branchf, the main pipe proceeds to the condenserD, and thence by the pipel, into the purifierE; from which the gas is immediately transmitted by the pipepinto the gasometerF.
The operation proceeds in the following way:—As soon as gas begins to be disengaged from the ignited retort, tar and ammoniacal liquor are deposited in the cylindrical receiverB, and fill it up till the superfluity runs over by the piped, the level being constantly preserved at the line shown in the figure. By the same tarry liquid, the orifices of the several pipesb, issuing from the retorts, are closed; whereby the gas in the pipedhas its communication cut off with the gas in the retorts. Hence if one of the retorts be opened and emptied, it remains shut off from the rest of the apparatus. This insulation of the several retorts is the function of the pipe underB, and therefore the recurved tubebmust be dipped as far under the surface of the tarry liquid, as to be in equilibrio with the pressure of the gas upon the water in the purifier. The tubebis closed at top with a screw cap, which can be taken off at pleasure, to permit the interior to be cleansed.
Both by the overflow from the receiver-pipeB, and by subsequent condensation in the tubed, tar and ammoniacal liquor collect progressively in the cistern or pit underC, by which mingled liquids the lower orifice of the vertical tubefis closed, so that the gas cannot escape into the empty space of this cistern. These liquids flow over the edges of the inner vessel when it is full, and may, from time to time, be drawn off by the stopcock at the bottom of the cistern.
Though the gas has, in its progress hitherto, deposited a good deal of its tarry and ammoniacal vapours, yet, in consequence of its high temperature, it still retains a considerable portion of them, which must be immediately abstracted, otherwise the tarwould pollute the lime in the vesselE, and interfere with its purification. On this account the gas should, at this period of the process, be cooled as much as possible, in order to condense these vapours, and to favour the action of the lime in the purifierE, upon the sulphuretted hydrogen, which is more energetic the lower the temperature of the gas. The coal gas passes, therefore, from the tubefinto the tubehof the condenserD, which is placed in an iron chestgfilled with water, and it deposits more tar and ammoniacal liquor in the under part of the cistern att,t. When these liquids have risen to a certain level, they overflow into the tar-pit, as shown in the figure, to be drawn off by the stopcock as occasion may require.
The refrigerated gas is now conducted into the purifierE, which is filled with milk of lime, made by mixing one part of slaked lime with 25 parts of water. The gas, as it enters by the pipel, depresses the water in the wide cylindern, thence passes under the perforated disc in the under part of that cylinder, and rising up through innumerable small holes is distributed throughout the lime liquid in the vesselm. By contact with the lime on this extended surface, the gas is stripped of its sulphuretted hydrogen and carbonic acid, which are condensed into the hydro-sulphuret and carbonate of lime; it now enters the gasometerFin a purified state, through the pipep t, and occupies the spaceq. The gasometer, pressing with a small unbalanced force over the counterweights, expels it through the mainu u, in communication with the pipes of distribution through the buildings or streets to be illuminated.
The partsA B C D EandF, of which this apparatus consists, are essential constituents of every good coal-gas work. Their construction rests upon peculiar principles, is susceptible of certain modifications, and therefore deserves to be considered in detail.
The Retorts.—These are generally made of cast iron, though they have occasionally been made of baked clay, like common earthenware retorts. The original form was a cylinder, which was changed to an ellipse, with the long axis in a horizontal direction, then into the shape of the letterDwith the straight line undermost, and lastly into a semi-cylinder, with its horizontal diameter 22 inches, and its vertical varying from 9 to 12. The kidney form was at one time preferred, but it has been little used of late.
Coal gas retort
The form of retort represented infig.483.has been found to yield the largest quantity of good gas in the shortest time, and with the least quantity of firing. The length is 71⁄2, and the transverse area, from one foot to a foot and a half square. The arrows show the direction of the flame and draught in this excellent bench of retorts, as mounted by Messrs. Barlow.
The charge of coals is most conveniently introduced in a tray of sheet iron, made somewhat like a grocer’s scoop, adapted to the size of the retort, which is pushed home to its further end, inverted so as to turn out the contents, and then immediately withdrawn.
The duration of the process, or the time of completing a distillation, depends upon the nature of the coal and the form of the retort. With cylindrical retorts it cannot be finished in less than 6 hours, but with elliptical and semi-cylindrical retorts, it may be completed in 4 or 5 hours. If the distillation be continued in the former for 8 hours, and in the latter for 6, gas will continue to be obtained, but during the latter period of the operation, of indifferent quality.
Receiver
The Receiver.—If the furnace contains only 2 or 3 retorts, a simple cylindrical vessel standing on the ground half filled with water, may serve as a receiver; into which the tube from the retort may be plunged. It should be provided with an overflow pipe for the tar and ammoniacal liquor. For a range of several retorts, a long horizontal cylinder is preferable, like that represented atBinfig.484.Its diameter is from 10 to 15 inches. This cylinder may be so constructed as to separate the tar from the ammoniacal liquor, by means of a syphon attached to one of its ends.
The Condenser.—The condenser, represented infig.482., consists of a square chest,g, made of wrought iron plates open at top, but having its bottom pierced with a row of holes, to receive a series of tubes. To these holes the upright four-inch tubesh hare secured by flanges and screws, and they are connected in pairs at top by the curved or saddle tubes. The said bottom forms the cover of the chestt,t, which is divided by vertical iron partitions, into half as many compartments as there are tubes.
These partition plates are left open at bottom, so as to place the liquids of each compartment in communication. Thereby the gas passes up and down the series of tubes, in proceeding from one compartment to another. The condensed liquids descend into the boxt,t, and flow over into the tar cistern, when they rise above the levelt,t. The tar may be drawn off from time to time by the stopcock. Through the tubek, cold water flows into the condenser chest, and the warm water passes away by a pipe at its upper edge.
The extent of surface which the gas requires for its refrigeration before it is admitted into the washing-lime apparatus, depends upon the temperature of the milk of lime, and the quantity of gas generated in a certain time.
It may be assumed as a determination sufficiently exact, that 10 square feet of surface of the condenser can cool a cubic foot of gas per minute to the temperature of the cooling water. For example, suppose a furnace or arch with 5 retorts of 150 pounds of coal each, to produce in 5 hours 3000 cubic feet of gas, or 10 cubic feet per minute, there would be required, for the cooling surface of the condenser, 100 square feet = 10 × 10. Suppose 100,000 cubic feet of gas to be produced in 24 hours, for which 8 or 9 such arches must be employed, the condensing surface must contain from 800 to 900 square feet.
The Purifier.—The apparatus represented in the preceding figure is composed of a cylindrical iron vessel, with an air-tight cover screwed upon it, through which the cylindernis also fixed air-tight. The bottom of this cylinder spreads out like the brim of a hat, forming a horizontal circular partition, which is pierced with holes. Through a stuffing box, in the cover of this interior cylinder, the vertical axis of the agitator passes, which is turned by wheel and pinion work, in order to stir up the lime from the bottom of the water in the purifier. The vesseloserves for introducing fresh milk of lime, as also for letting it off by a stopcock when it has become too foul for further use.
The quantity of lime should be proportioned to the quantity of sulphuretted hydrogen and carbonic acid contained in the gas. Supposing that in good coal gas there is 5 per cent. of these gases, about one pound and a half of lime will be requisite for every hundred cubic feet of coal gas generated, which amounts to nearly one-sixteenth of the weight of coal subjected to decomposition. This quantity of lime mixed with the proper quantity of water will form about a cubic foot of milk of lime. Consequently, the capacity of the purifier, that is, of the interior space filled with liquid, may be taken at four-sevenths of a cubic foot for every hundred cubic feet of gas passing through it in one operation; or for 175 cubic feet of gas, one cubic foot of liquor. After every operation, that is, after every five or six hours, the purifier must be filled afresh. Suppose that in the course of one operation 20,000 cubic feet of gas pass through the machine, this should be able to contain20,000175= 114 cubic feet of milk of lime; whence its diameter should be seven feet, and the height of the liquid three feet. If the capacity of the vessel be less, the lime milk must be more frequently changed.
In some of the large gas works of London the purifier has the following construction, whereby an uninterrupted influx and efflux of milk of lime takes place. Three single purifiers are so connected together, that the second vessel stands higher than the first, and the third than the second; so that the discharge tube of the superior vessel, placed somewhat below its cover, enters into the upper part of the next lower vessel; consequently, should the milk of lime in the third and uppermost vessel rise above its ordinary level, it will flow over into the second, and thence in the same way into the first; from which it is let off by the eduction pipe. A tube introduces the gas from the condenser into the first vessel, another tube does the same thing for the second vessel, &c., and the tube of the third vessel conducts the gas into the gasometer. Into the third vessel, milk of lime is constantly made to flow from a cistern upon a higher level. By this arrangement, the gas passing through the several vessels in proportion as it is purified, comes progressively into contact with purer milk of lime, whereby its purification becomes more complete. The agitatorc, provided with two stirring paddles, iskept in continual rotation. The pressure which the gas has here to overcome is naturally three times as great as with a single purifier of like depth.
Purifier
Fig.485.is a simple form of purifier, which has been found to answer well in practice. Through the cover of the vesselA B, the wide cylindere dis inserted, having its lower end pierced with numerous holes. Concentric with that cylinder is the narrower ones z, bound above with the flangea b, but open at top and bottom. The under edgeg hof this cylinder descends a few inches below the endc dof the outer one. About the middle of the vessel the perforated shelfm nis placed. The shaft of the agitatorl, passes through a stuffing box upon the top of the vessel. The gas-pipeg, proceeding from the condenser, enters through the flangea bin the outer cylinder, while the gas-pipehgoes from the cover to the gasometer. A stopcock upon the side, whose orifice of discharge is somewhat higher than the under edge of the outer cylinder, serves to draw off the milk of lime. As the gas enters through the pipeginto the space between the two cylinders, it displaces the liquor till it arrives at the holes in the under edge of the outer cylinder, through which, as well as under the edge, it flows, and then passes up through the apertures of the shelfm ninto the milk of lime chamber; the level of which is shown by the dotted line. The stirrer,l, should be turned by wheel work, though it is here shown as put in motion by a winch handle.
In order to judge of the degree of purity of the gas after its transmission through the lime machine, a slender syphon tube provided with a stopcock may have the one end inserted in its cover, and the other dipped into a vessel containing a solution of acetate of lead. Whenever the solution has been rendered turbid by the precipitation of sulphuret of lead, it should be renewed. The saturated and fetid milk of lime is evaporated in oblong cast-iron troughs placed in the ash-pit of the furnaces, and the dried lime is partly employed for luting the apparatus, and partly disposed of for a mortar or manure.
By this purifier, and others of similar construction, the gas in the preceding parts of the apparatus, as in the retorts and the condenser, suffers a pressure equal to a column of water about two feet high; and in the last described purifier even a greater pressure. This pressure is not disadvantageous, but is of use in two respects; 1. it shows by a brisk jet of gas when the apparatus is not air-tight, and it prevents common air from entering into the retorts; 2. this compression of the gas favours the condensation of the tar and ammoniacal liquor. The effect of such a degree of pressure in expanding the metal of the ignited retorts is quite inconsiderable, and may be neglected. Two contrivances have, however, been proposed for taking off this pressure in the purifier.
Purefier
Infig.486.,m mare two similar vessels of a round or rectangular form, furnished at their upper border with a groove filled with water, into which the under edge of the cover fits, so as to make the vessel air-tight. The cover is suspended by a cord or chain, which goes over a pulley, and may be raised or lowered at pleasure. The vessels themselves have perforated bottoms,rr′, covered with wetted moss or hay sprinkled over with slaked and sifted quicklime. The gas passes through the loosely compacted matter of the first vessel, by entering between its two bottoms, rises into the upper spacet, thence it proceeds to the second vessel, and, lastly, through the pipeuinto the gasometer. This method, however, requires twice as much lime as the former, without increasing the purity of the gas.
Compressor
The second method consists in compressing the gas by the action of an Archimedes screw, to such a degree, before it is admitted into the purifier, as that it may overcome the pressure of the column of water in that vessel.Fig.487.exhibits this apparatus in section.D Dis the Archimedes worm, the axis of which revolves at bottom upon the gudgeone; it possesses a three-fold spiral, and is turned in the opposite direction to that in which it scoops the water. The cistern which contains it has an air-tight cover. The gas to be purified passes through the pipeCinto the spaceD, over the water leveld; the upper cells of the worm, scoop in the gas at this point, andcarry it downwards, where it enters atginto the cavityEof a second cistern. In order that the gas, after it escapes from the bottom of the worm, may not partially return throughginto the cavityD, an annular plateg his attached to its under edge, so as to turn over it. The compressed gas is conducted from the cavityEthrough the pipeGinto the purifying machine;ais a manometer, to indicate the elastic tension of the gas inD. On the top of the worm a mechanism is fitted for keeping it in constant rotation.
A perfect purification of light-gas from sulphuretted hydrogen, either by milk of lime or a solution of the green sulphate of iron, is attended with some difficulty, when carried so far as to cause no precipitation of sulphuret in acetate of lead, because such a degree of washing is required as is apt to diminish its illuminating power, by abstracting the vapour of the rich oily hydrocarburet which it contains. Moreover, the coal gas obtained towards the end of the distillation contains some sulphuret of carbon, which affords sulphurous acid on being burned, and can be removed by no easy method hitherto known. The lime in the purifier disengages from the carbonate and hydrosulphuret of ammonia carried over with the gas, especially when it has been imperfectly cooled in the condenser, a portion of ammoniacal gas, which, however, is not injurious to its illuminating power. The best agent for purifying gas would be the pyrolignite of lead, were it not rather expensive, because it would save the trouble of stirring, and require a smaller and simpler apparatus.
The Gasometer.—The gasometer serves not merely as a magazine for receiving the gas when it is purified, and keeping it in store for use, but also for communicating to the gas in the act of burning such an uniform pressure as may secure a steady unflickering flame. It consists of two essential parts; 1. of an under cistern, open at top and filled with water; and 2. of the upper floating cylinder or chest, which is a similar cistern inverted, and of somewhat smaller dimensions, called the gas-holder: seeF,fig.482.The best form of this vessel is the round or cylindrical; both because under equal capacity it requires least surface of metal, and it is least liable to be warped by its own weight or accidents. Since a cylindrical body has the greatest capacity with a given surface when its height is equal to its semi-diameter, its dimensions ought to be such that when elevated to the highest point in the water, the height may be equal to the radius of the base. For example, let the capacity of the gas-holder in cubic feet bek, the semi-diameter of its base bex, the height out of the water beh;his =x=∛k3·14. This height may be increased by one or two feet, according to its magnitude, to prevent the chance of any gas escaping beneath its under edge, when it is raised to its highest elevation in the water.
The size of the gasometer should be proportional to the quantity of gas to be consumed in a certain time. If 120,000 cubic feet be required, for instance, in 10 hours for street illumination, and if the gas retorts be charged four times in 24 hours, 30,000 cubic feet of gas will be generated in 6 hours. Hence the gasometer should have a capacity of at least 70,000 cubic feet, supposing the remaining 50,000 cubic feet to be produced during the period of consumption. If the gasometer has a smaller capacity, it must be supplied from a greater number of retorts during the lighting period, which is not advantageous, as the first heating of the supernumerary retorts is wasteful of fuel. Some engineers consider that a capacity of 30,000 cubic feet is the largest which can with propriety be given to a gasometer; in which case, they make its diameter 42 feet, and its height 23. When the dimensions are greater, the sheet iron must be thicker and more expensive; and the hollow cylinder must be fortified by strong internal cross braces.
The water cistern is usually constructed in this country with cast-iron plates bolted together, and made tight with rust-cement.
Gasometer
In cases where the weight of water required to fill such a cistern might be inconvenient to sustain, it may be made in the form represented infig.488.; which, however, will cost nearly twice as much. Parallel with the side of the cistern, a second cylinderC, of the same shape but somewhat smaller, is fixed in an inverted position to the bottom of the first, so as to leave an annular spaceB Bbetween them, which is filled with water, and in which the floating gasometerAplays up and down. The water must stand above the cover of the inverted cylinder.aandbare the pipes for leading the gas in and out. Through an opening in the masonry upon which the gasometer apparatus rests, the spaceCmay be entered, in order to make any requisite repairs.
The water cistern may also be sunk in the ground, and the sides made tight with hydraulic mortar, as is shown infig.489., and to make it answer with less water, a concentric cylindrical mass of masonry may be built at a distance of 2 or 3 inches within it.
Every large gasometer must be strengthened interiorly with cross iron rods, to stiffen both its top and bottom. The top is supported by rods stretching obliquely down tothe sides, and to the under edge an iron ring is attached, consisting of curved cast-iron bars bolted together; with which the oblique rods are connected by perpendicular ones. Other vertical rods stretch directly from the top to the bottom edge. Upon the periphery of the top, at the end of the rods, several rings are made fast, to which the gas-holder is suspended, by means of a common chain which runs over a pulley at the centre. Upon the other end of the chain there is a counterpoise, which takes off the greater part of the weight of the gas-holder, leaving only so much as is requisite for the expulsion of the gas. The inner and outer surfaces of the gas-holder should be a few times rubbed over with hot tar, at a few days’ interval between each application. The pulley must be made fast to a strong frame.
Gasometer
If the water cistern be formed with masonry, the suspension of the gas-holder may be made in the following way.A A,fig.489., is a hollow cylinder of cast iron, standing up through the middle of the gasometer, and which is provided at either end with another small hollow cylinderG, open at both ends and passing through the top, with its axis placed in the axis of the gas-holder. In the hollow cylinderG, the counterweight moves up and down, with its chain passing over the three pulleysB,B,B, as shown infig.489.E Fare the gas pipes made fast to a vertical iron rod. Should the gasometer be made to work without a counterweight, as we shall presently see, the central cylinderA A, serves as a vertical guide.
In proportion as the gas-holder sinks in the water of the cistern, it loses so much of its weight, as is equal to the weight of the water displaced by the sides of the sinking vessel; so that the gas-holder when entirely immersed, exercises the least pressure upon the gas, and when entirely out of the water, it exercises the greatest pressure. In order to counteract this inequality of pressure, which would occasion an unequal velocity in the efflux of the gas, and of course an unequal intensity of light in its flame, the weight of the chain upon which the gas-holder hangs is so adjusted as to be equal, throughout the length of its motion, to one half of the weight which the gas-holder loses by immersion. In this case, the weight which it loses by sinking into the water, is replaced by the portion of the chain which passing the pulley, and hanging over, balances so much of the chain upon the side of the counterweight; and the weight which it gains by rising out of the water, is counterpoised by the links of the chain which passing over the pulley, add to the amount of the counterweight. The pressure which the gas-holder exercises upon the gas, or that with which it forces it through the first main pipe, is usually so regulated as to sustain a column of from one to two inches of water; so that the water will stand in the cistern from one to two inches higher within, than without the gas-holder. The following computation will place these particulars in a clear light.
Let the semi-diameter of the gas-holder, equal to the vertical extent of its motion into and out of the water, =x; let the weight of a foot square of the side of the gas-holder, including that of the strengthening bars and ring, which remain plunged under the water, be =p; then
1. the weight of the gas-holder in its highest position = 3pπx2;
2. the weight of the sides of the gas-holder which play in the water = 2pπx2;
3. the cubic contents of the immersed portion of the gas-holder =2pπx2400;
4. its loss of weight in water =112400pπx2;
5. the weight of the gas-holder in its lowest position =pπx2(3 -112400)= 2·72pπx2;
6. the weight ofninches, height of water =5612nπx2;
7. the amount of the counterweight = πx2(3p-56n12);
8. the weight of the chain for the lengthx=112800pπx.
If we reduce the weight of the gas-holder in its highest and lowest positions to the height of a stratum of water equal to the surface of its top, this height is that of the column of water which would press the gas within the gasometer, were no counterweight employed; it consists as follows;—
9. for the highest position =3p56;
10. for the lowest =2·72p50;
For the case, when the height of the gas-holder is different from its semi-diameter, let this height =m x; then the height of the water level is
11. for the highest position =p(1 +2m56);
12. for the lowest =p(1 +1·72m6);
13. the counterweight = πx2(p(1 + 2m) -56n12);
14. the weight of the equalizing chain =112800pπmx2.
For example, let the diameter of the gas-holder be 30 feet, the height 15 (the contents in cubic feet will be 10,597),p= 4 pounds; then the counterweight for a height of an inch and a half of water pressure = 3532 pounds; the weight of the chain for a length of 15 feet = 395 pounds. Were no counterweight employed, so that the gas-holder pressed with its whole weight upon the gas, then the height of the equivalent column of water in its highest position = 2·56 inches; and in its lowest, 2·33. The counterweight may hence be lessened at pleasure, if the height of the pressing water-columnnbe increased. The weight of the equalising or compensating portion of the chain remains the same. Whenn= 2 inches, for instance, the counterweight = 1886 pounds.
The velocity with which the gas passes along the mains for supplying the various jets of light, may be further regulated by opening the main-cock or slide-valve in a greater or less degree.
Gasometers whose height is greater than their semi-diameter, are not only more costly in the construction, but require heavier counterweights and equilibration chains.
The above estimate is made on the supposition of the gas in the gas-holder being of the same specific gravity as the atmospherical air, which would be nearly true with regard to oil gas under the ordinary pressure. But coal gas, whose specific gravity may be taken on an average at about 0·5, exercises a buoyancy upon the top of the gas-holder, which of course diminishes its absolute weight. Supposing the cubic foot of gas to be = 0·0364 pounds, the buoyancy will be = 0·0364 πx3pounds; a quantity which deserves to be taken into account for large gasometers. Hence,
15. the weight of the gas-holder in its highest position = 3pπx2- 0·1143x3;
16. the counterweight = πx2(3p-56n12)- 0·1143x2;
17. The weight of the chain for the lengthx, =112800pπx20·1143x32;
18. The height of the water pressure for the highest position, without the counterweight =3pπ - 0·1143x56 π;
19. the same for the lowest position =2·72p56in feet.
The preceding values ofpandx, are,
(16) = 3147; (17) = 203; (18) = 2·44 inches; (19) = 2·33 inches.
The water columns in the highest and lowest situations of the gas-holder here differ about 0·1 of an inch, and this difference becomes still less whenphas a smaller value, for example, 3 pounds, or when the diameter of the gas-holder is still greater.
It would thus appear that for coal-gas gasometers, in which the height of the gas-holder does not exceed its semi-diameter, and especially when it has a considerable size, neither a compensation chain nor a counterweight is necessary. The only thing requisite, is to preserve the vertical motion of the gas-holder by a sufficient number of guide rods or pillars, placed either within the water cistern, or round about it. Should the pressure of the gas in the pipe proceeding from the gasometer, be less than in the gasometer itself, this may be regulated by the main valve, or by water valves of various kinds. Or a small intermediate regulating gasometer may be introduced between the great gas-holder, and the main pipe of distribution. With a diameter of 61 feet in the gas-holder, the pressure in the highest and lowest positions is the same.
The gasometers employed in storing up gas until required for use, occupy, upon the old plan, much space, and are attended with considerable expense in erecting. The water tank, whether sunk in the ground, or raised, must be of equal dimensions with the gasometer, both in breadth and depth. The improved construction which we are about to describe, affords a means of reducing the depth of the tank, dispensing with the bridge of suspension, and of increasing at pleasure the capacity of the gasometer, upon a given base; thus rendering a small apparatus capable, if required, of holding a large quantity of gas, the first cost of which will be considerably less than even a small gasometer constructed upon the ordinary plan.