Tait's gasometerMr. Tait, of Mile-End Road, the inventor, has, we believe, been for some years connected with gas establishments, and is therefore fully aware of the practical defects or advantages of the different constructions of gasometers now in use.Fig.490.is a section of Mr. Tait’s improved contrivance;a ais the tank, occupied with water,b btwo iron columns, with pulley-wheels on the top,c c, chains attached to a ring of iron,d d, extending round the gasometer, which chains pass over the pulley-wheels, and are loaded at their extremities, for the purpose of balancing the weight of the materials of which the gasometer is composed.The gasometer is formed by 2 or 3 cylinders, sliding one within the other, like the tubes of a telescope;e,e,e, is the first or outer cylinder, closed at the top, and having the ring of irond, passing round it, by which the whole is suspended;f f, is the second cylinder, sliding freely within the first, and there may be a third and fourth within these if necessary.When there is no gas in the apparatus, all the cylinders are slidden down, and remain one within the other immersed in the tank of water; but when the gas rises through the water pressing against the top of the gasometer, its buoyancy causes the cylindereto ascend. Round the lower edge of this cylinder a groove is formed by the turning in of the plate of iron, and as it rises, the edge takes hold of the top rim of the cylinderf, which is overlapped for that purpose. The groove at the bottom of the cylinder fills itself with water as it ascends, and by the rim of the second cylinder falling into it, an air-tight hydraulic joint is produced.Thus, several cylinders may be adapted to act in a small tank of water, by sliding one within the other, with lapped edges forming hydraulic joints, and by supporting the apparatus in the way shown, the centre of gravity will always be below the points of suspension. A gasometer may be made upon this plan of any diameter, as there will be no need of frame work, or a bridge to support it; and the increasing weight of the apparatus, as the cylinders are raised one after the other, may be counterpoised by loading the ends of the chainsc c.The water in the gasometer need not be renewed; but merely so much of it as evaporates or leaks out, is to be replaced. Indeed the surface of the water in the cistern gets covered with a stratum of coal oil, a few inches deep, which prevents its evaporation, and allows the gas to be saturated with this volatile substance, so as to increase its illuminating powers.Intermediate vesselThe gasometer may be separated from the purifier by an intermediate vessel, such as is representedfig.491., with which the two gas pipes are connected.Ais thecylindrical vessel of cast iron,a, the end of the gas pipe which comes from the purifier, immersed a few inches deep into the liquid with which the vessel is about two-thirds filled;bis the gas-pipe which leads into the gasometer,cis a perpendicular tube, placed over the bottom of the vessel, and reaching to within one-third of the top, through which the liquid is introduced into the vessel, and through which it escapes when it overflows the leveld. In this tube the liquid stands towards the inner level higher, in proportion to the pressure of the gas in the gasometer. The fluid which is condensed in the gas pipe,b, and in its prolongation from the gasometer, runs off into the vesselA; and therefore the latter must be laid so low that the said tube may have the requisite declivity. A straight stop-cock may also be attached to the side over the bottom, to draw off any sediment.II.Application of Light-Gas.1.Distribution of the pipes.—The pressure by which the motion of the gas is maintained in the pipes, corresponds to a certain height of water in the cistern of the gasometer. From the magnitude of this pressure, and the quantity of gas which in a given time, as an hour, must be transmitted through a certain length of pipes, depends the width or the diameter that they should have, in order that the motion may not be retarded by the friction which the gas, like all other fluids, experiences in tubes, and thereby the gas might be prevented from issuing with the velocity required for the jets of flame. The velocity of the gas in the main pipe increases in the ratio of the square root of the pressing column of water upon the gasometer, and therefore by increasing this pressure, the gas may be forced more rapidly along the remoter and smaller ramifications of the pipes. Thus it happens, however, that the gas will be discharged from the orifices near the gasometer, with superfluous velocity. It is therefore advisable to lay the pipes in such a manner, that in every point of their length, the velocity of discharge may be nearly equal. This may be nearly effected as follows;—From experiment it appears that the magnitude of the friction, or the resistance which the air suffers in moving along the pipes, under a like primary pressure, that is for equal initial velocity, varies with the square root of the length. The volume of gas discharged from the end of a pipe, is directly proportional to the square of its diameter, and inversely as the square root of its length; or, calling the lengthL, the diameterD, the cubic feet of gas discharged in an hourk; thenk=D2√L. Experience likewise shows, that for a pipe 250 feet long, which transmits in an hour 200 cubic feet of gas, one inch is a sufficient diameter.Consequently, 200 :k∷1144 √250:D2√L; and D =√k√L455,000From this formula the following table of proportions is calculated.Number ofcubic feetper hour.Length ofpipe,in feet.Diameter,in inches.501000·402502001·005006001·9770010002·65100010003·16150010003·87200010004·47200020005·32200040006·33200060007·00600010007·75600020009·21800010008·958000200016·65These dimensions are applicable to the case where the body of gas is transmitted throughpipes without being let off in its way by burners, that is, to the mains which conduct the gas to the places where it is to be used. If the main sends off branches for burners, then for the same length the diameter may be reduced, or for like diameter the length may be greater. For example, if a pipe of 5·32 inches, which transmits 2000 cubic feet through a length of 2000 feet, gives off, in this space, 1000 cubic feet of gas; then the remainder of the pipe, having the same diameter, can continue to transmit the gas through a length of 2450 feet =(450,000k)2, with undiminished pressure for the purposes of lighting. Inversely, the diameter should be progressively reduced in proportion to the number of jets sent off in the length of the pipe.Suppose for instance, the gasometer to discharge 2000 cubic feet per hour, and the last point of the jets to be at a distance of 4000 feet. Suppose also that from the gasometer to the first point of lighting, the gas proceeds through 1000 feet of close pipe, the diameter of the pipe will be here 4·47 inches; in the second 1000 feet of length, suppose the pipe to give off, at equal distances, 1000 cubic feet of gas, the diameter in this length (calculated at 1500 cubic feet for 1000 feet long) = 3·87 inches; in the third extent of 1000 feet, 600 cubic feet of gas will be given off, and the diameter (reckoning 700 cubic feet for 1000 feet long) will be 2·65 inches; in the fourth and last space (for 200 cubic feet in 1000 feet long) the pipe has a diameter of only an inch and a half, for which, in practice, a two-inch cast iron pipe is substituted; this being the smallest used in mains, into which branch pipes can be conveniently inserted.The same relations hold with regard to branch pipes through which the gas is transmitted into buildings and other places to be illuminated. If such pipes make frequent angular turnings, whereby they retard the motion of the gas, they must be a third or a half larger in diameter. The smallest tubes of distribution are never less than one fourth of an inch in the bore.Where, from one central gas work, a very great quantity of light is required in particular localities, there ought to be placed near these spots gasometers of distribution, which, being filled during the slack hours of the day, are ready to supply the burners at night, without making any considerable demand upon the original main pipe. Suppose the first main be required to supply 8000 cubic feet in the hour, for an illumination of 8 hours, at the distance of 2000 feet, a pipe 102⁄3inches in diameter would be necessary; but if two or three gasometers of distribution, or station gasometers be had recourse to, into which the gas during the course of 24 hours would flow through the same distance continuously from the central gas works, the quantity required per hour from them would be only one third of 8000, = 2666·6 cubic feet; consequently the diameter for such a pipe is only 6·15 inches.Gas pipesAll the principal as well as branch pipes, whose interior diameter exceeds an inch and a half, are made of cast iron from 6 to 8 feet long, with elbow pipes cast in them where it is necessary. These pipe lengths are shown infig.492., having at one end a wide socketa, and at the other a nozzleb, which fits the former. After inserting the one in the other in their proper horizontal position, a coil of hemp soaked with tar is driven home at the junction; then a luting of clay is applied at the mouth, within which a ring of lead is cast into the socket, which is driven tight home with a mallet and blunt chisel.The pipes should be proved by a force pump before being received into the gas works; two or three lengths of them should be joined before laying them down, and they should be placed at least two feet below the surface, to prevent their being affected by changes of temperature, which would loosen the joints. The tubes for internal distribution, when of small size are made of lead, copper, wrought iron, or tin.Water trapInstead of a stopcock for letting off the gas in regulated quantities from the gasometer, a peculiarly formed water or mercurial valve is usually employed.Fig.493.shows the mode of construction for a water trap or lute, and is, in fact, merely a gasometer in miniature.C D E Fis a square cast iron vessel, in the one side of which a pipeAis placed in communication with the gasometer, and in the other, one with the mainB. The movable cover or lidH G I Khas a partition,L M, in its middle. If this cover be raised by its counterweight, the gas can pass without impediment fromAtoB; but if the counterweight be diminished so as to let the partition plateL Msink into the water, the communication of the two pipes is thereby interrupted. In this case the water-level stands in the compartmentAso much lower than outside of it, and in the compartmentB, as is equivalent to the pressure in the gasometer; therefore the pipesAandBmust project thus far above the water. In order to keep the water always at the same height, and to prevent it from flowing into the mouths of these pipes, the rimC Dof the outer vessel stands somewhat lower than the orificesA B; and thence the vessel may be kept always full of water.Quicksilver valveIf a quicksilver valve be preferred, it may be constructed as shown infig.494.A Bare the terminations of the two gas pipes, which are made fast in the rectangular iron vesselM.Eis an iron vessel of the same form, which is filled with quicksilver up to the levela, and which, by means of the screwG, which presses against its bottom, and works in the fixed female screwC C, may be moved up or down, so that the vesselMmay be immersed more or less into the quicksilver. The vesselMis furnished with a vertical partitionm; the passage of the gas fromAtoBis therefore obstructed when this partition dips into the quicksilver, and from the gradual depression of the vesselEby its screw, the interval between the quicksilver and the lower edge of the partition, through which the gas must enter, may be enlarged at pleasure, whereby the pressure of the gas inBmay be regulated to any degree. The transverse section of that interval is equal to the area of the pipe or rather greater; the breadth of the vesselMfromAtoBamounts to the double of that space, and its length to the mere diameter ofAorB. The greatest height to which the partitionmcan rise out of the quicksilver, is also equal to the above diameter, and in this case the lineacomes to the place ofb. The vertical movement of the outer vesselE, is secured by a rectangular rim or hoop which surrounds it, and is made fast to the upper part of the vesselM, within which guide it moves up and down. Instead of the leverD D, an index with a graduated plate may be employed to turn the screw, and to indicate exactly the magnitude in the opening of the valve.Gas-meterIn order to measure the quantity of gas which passes through a pipe for lighting a factory, theatre, &c., the gas-meter is employed, of whose construction a sufficiently precise idea may be formed from the consideration offig.495., which shows the instrument in a section perpendicular to its axis.Within the cylindrical casea, there is a shorter cylinderb b, shut at both ends, and movable round an axis, which is divided into four compartments, that communicate by the openingd, with the interval between this cylinder and the outer case. The mode in which thiscylinder turns round its axis is as follows:—The end of the tubec, which is made fast to the side of the case, and by which the gas enters, carries a pivot or gudgeon, upon which the centre of its prop turns; the other end of the axis runs in the cover, which here forms the side of a superior open vessel, in which, upon the same axis, there is a toothed wheel. The vessel is so far filled with water, that the tubecjust rises above it, which position is secured by the level of the side vessel. When the gas enters through the tubec, by its pressure upon the partitione, (fig.495.) it turns the cylinder from right to left upon its axis, till the exterior openingdrises above the water, and the gas expands itself in the exterior space, whence it passes off through a tube at top. At every revolution a certain volume of gas thus goes through the cylinder, proportional to its known capacity. The wheel on the axis works in other toothed wheels, whence, by means of an index upon a graduated disc or dial, placed at top or in front of the gas-meter, the number of cubic feet of gas, which pass through this apparatus in a given time, is registered.B.Employment of the gas for lighting.—The illuminating power of different gases burned in the same circumstances, is proportional, generally speaking, to their specific gravity, as this is to the quantity of carbon they hold in combination. The following table exhibits the different qualities of gases in respect to illumination.Density or specific gravity.Proportion of lightafforded bycoal gas to oil gas.Coal gas.Oil gas.0·6590·818100:1400·5780·910100:2250·6051·110100:2500·4070·940100:3540·4290·965100:3560·5081·175100:310Mean 0·5290·96100:272In the last three proportions, the coal gas was produced from coals of middle quality; in the first three proportions from coals of good quality; and therefore the middle proportion of 100 to 270 may be taken to represent the fair average upon the great scale. On comparing the gas from bad coals, with good oil gas, the proportion may become 100 to 300. Nay, coal gas of specific gravity 0·4, compared to oil gas of 1·1, gives the proportion of 1 to 4. A mould tallow candle, of 6 in the pound, burning for an hour, is equivalent to half a cubic foot of ordinary coal gas, and to four tenths of a foot of good gas. The flame of the best argand lamp of Carcel, in which a steady supply of oil is maintained by pump-work, consuming 42 grammes = 649 grains English in an hour, and equal in light to 9·38 such candles, is equivalent to 3·75 cubic feet of coal gas per hour. The sinumbra lamp, which consumes 50 grammes = 772 grains English, of oil per hour, and gives the light of 8 of the above candles, is equivalent to the light emitted by 3·2 cubic feet of coal gas burning for an hour. A common argand lamp, equal to 4 candles, which consumes 30 grammes = 463 grains English per hour, is represented by 1·6 cubic feet of gas burning during the same time. A common lamp, with a flat wick and glass chimney, whose light is equal to 1·13 tallow candles, and which consumes 11 grammes = 169·8 grains English per hour, is represented by 0·452 of a cubic foot of gas burning for the same time.Construction of the Burners.—The mode of burning the gas as it issues from the jets has a great influence upon the quantity and quality of its light. When carburetted hydrogen gas is transmitted through ignited porcelain tubes, it is partially decomposed with a precipitation of some of its carbon, while the resulting gas burns with a feebler flame. Coal gas, when kindled at a small orifice in a tube, undergoes a like decomposition and precipitation. Its hydrogen, with a little of its carbon, burns whenever it comes into contact with the atmospherical air, with a bluish coloured flame; but the carbonaceous part not being so accendible, takes fire only when mixed with more air; therefore at a greater distance from the beak, and with a white light from the vivid ignition of its solid particles. Upon this principle pure hydrogen gas may be made to burn with a white instead of its usual blue flame, by dusting into it particles of lamp black; or by kindling it at the extremity of a tube containing finely pulverized zinc. The metallic particles become ignited, and impart their bright light to the pale blue flame. Even platinum wire and asbestos, when placed in the flame of hydrogen gas, serve to whiten it. Hence it has been concluded, that the intensity of light which a gas is capable of affording is proportional to the quantity of solid particles which itcontains, and can precipitate in the act of burning. Carbonic oxide gas burns with the feeblest light next to hydrogen, because it deposits no carbon in the act of burning. Phosphuretted hydrogen gives a brilliant light, because the phosphoric acid, into which its base is converted during the combustion, is a solid substance, capable of being ignited in the flame. Olefiant gas, as also the vapour of hydro-carbon oil, emits a more vivid light than common coal gas; for the first is composed of two measures of hydrogen and two measures of the vapour of carbon condensed into one volume; while the last contains only one measure of the vapour of carbon in the same bulk, and combined with the same proportion of hydrogen. Olefiant gas may therefore be expected to evolve a double quantity of carbon in its flame, which should emit a double light.The illuminating power of the flame of coal gas is, on the contrary, impaired, when, by admixture with other species of gas which precipitate no carbon, its own ignited particles are diffused over a greater surface. This happens when it is mixed with hydrogen, carbonic oxide, carbonic acid, and nitrogen gases, and the diminution of the light is proportional to the dilution of the coal gas.Gas burnerIn like manner the illuminating power of coal gas is impaired, when it is consumed too rapidly to allow time for the separation and ignition of its carbonaceous matter; it burns, in this case, without decomposition, and with a feeble blue flame. 1. This occurs when the light-gas is previously mixed with atmospherical air, because the combustion is thereby accelerated throughout the interior of the flame, so as to prevent the due separation of carbon. A large admixture of atmospherical air makes the flame entirely blue. 2. When it issues, with considerable velocity, from a minute orifice, whereby the gas, by expansion, gets intimately mixed with a large proportion of atmospherical air. If the jet be vertical, the bottom part of the flame is blue, and the more so the less carbon is contained in the gas. The same thing may be observed in the flame of tallow, wax, or oil lights. The burning wick acts the part of a retort, in decomposing the fatty matter. From the lower part of the wick the gases and vapours of the fat issue with the greatest velocity, and are most freely mixed with the air; while the gases disengaged from the upper part of the wick compose the interior of the flame, and being momentarily protected from the action of the atmosphere, acquire the proper high temperature for the deposition of carbon, which is then diffused on the outer surface in an ignited state, and causes its characteristic white light. Hence with coal gas, the light increases in a certain ratio with the size of the flame as it issues from a larger orifice, because the intermixture of air becomes proportionately less. 3. If by any means too great a draught be given to the flame, its light becomes feebler by the rapidity and completeness with which the gas is burned, as when too tall a chimney is placed over an argand burner, seefig.496.Fig.497.c, is a view of the upper plate, upon which the glass chimneybrests. The gas issues through the smaller openings of the inner ring, and forms a hollow cylindrical flame, upon the outside as well as the inside of which the atmospherical air acts. The illuminating power of this flame may be diminished at pleasure, according as more or less air is allowed to enter through the orifices beneath. With a very full draught the light almost vanishes, leaving only a dull blue flame of great heating power, like that of the blowpipe, corresponding to the perfect combustion of the gas without precipitation of its carbon. 4. On the other hand, too small a draught of air is equally prejudicial; not merely because a portion of the carbon thus escapes unconsumed in smoke, but also because the highest illuminating power of the flame is obtained only when the precipitated charcoal is heated to whiteness, a circumstance which requires a considerable draught of air. Hence the flame of dense oil gas, or of oil in a wick, burns with a yellow light without a chimney; but when it is increased in intensity by a chimney draught, it burns with a brilliant white flame.From the consideration of the preceding facts, it is possible to give to coal gas its highest illuminating power. The burners are either simple beaks perforated with a small round hole, or circles with a series of holes to form an argand flame, as shown infig.497, or two holes drilled obliquely, to make the flame cross, like a swallow’s tail, or with a slit constituting the sheet of flame called a bat’s wing, like most of the lamps in the streets of London. These burners are mounted with a stop-cock for regulating the quantity of gas.The height of the flame, which with like pressure depends upon the size of the orifice, and with like orifice upon the amount of pressure, the latter being modified by the stop-cock, is for simple jets in the open air, as follows:—Length of the flame23456 inchesIntensity of the light55·6100150197·8247·4Volume of gas consumed60·5101·4126·3143·7182·2Light with equal consumption100109131150150When the length exceeds five inches, nothing is gained in respect to light. For oilgas the same statements will serve, only on account of its superior richness in carbon, it does not bear so long a flame without smoke. Thus:—Length of the flame12345 inchesIntensity of the light2263·796·5141178Gas consumed33·178·590118153Light with equal consumption100122159181174The diameter of the orifice for single jets, or for several jets from the same beak, is one twenty-eighth of an inch for coal gas, and one forty-fifth for oil gas.Gas burnerWhen several jets issue from the same burner, the light is improved by making all the flames unite into one. In this case the heat becomes greater, for the combined flame presents a smaller surface to be cooled, than the sum of the smaller flames. The advantage gained in this way, may be in the ratio of 3 to 2, or 50 per cent. In an argand burner, the distances of the orifices for coal gas should be from16⁄100to18⁄100of an inch, and for oil gas12⁄100. If the argand ring has ten orifices, the diameter of the central opening should be =4⁄10of an inch; if 25 orifices, it should be one inch for coal gas; but for oil gas with 10 orifices, the central opening should have a diameter of half an inch, and for 20 orifices, one inch. The pin holes should be of equal size, otherwise the larger ones will cause smoke, as in an argand flame with an uneven wick. The glass chimney is not necessary to promote the combustion of an argand coal gas flame, but only to prevent it from flickering with the wind, and therefore it should be made so wide as to exercise little or no influence upon the draught. A narrow chimney is necessary merely to prevent smoke, when a very strong light, with a profusion of gas is desired. Oil gas burned in an argand beak requires a draught chimney, like a common argand lamp, on account of the large quantity of carbon to be consumed. The most suitable mode of regulating the degree of draught can be determined only by experiment, and the best construction hitherto ascertained is that represented infig.498.Fig.499.exhibits the view from above, of the rim or ringc, upon which the chimneybstands, and which surrounds the perforated beak. The ring is made of open fretwork, to permit the free passage of air upwards to strike the outside of the flame. The thin annular discd, which is laid over its fellow discc, in the bottom of the chimney-holder, being turned a little one way or other, will allow more or less air to pass through for promoting more or less, the draught or ventilation. The draught in the central tube of the burner may be regulated by the small disce, whose diameter is somewhat smaller than that of the ring of the burner, and which by turning the milled headf, of the screw, may be adjusted with the greatest nicety, so as to admit a greater or smaller body of air into the centre of the cylindrical flame.In mounting gas-lights, and in estimating beforehand their illuminating effects, we must keep in mind the optical proposition, that the quantity of light is inversely as the square of the distance from the luminous body, and we must distribute the burners accordingly. When for example a gas-light placed at a distance of ten feet, is required for reading or writing to afford the same light as a candle placed at a distance of two feet; squaring each distance, we have 100 and 4; therefore1004= 25, shows us that 25 such lights will be necessary at the distance of 10 feet.Concerning portable gas-light, with the means of condensing it, and carrying it from the gas works to the places where it is to be consumed, we need say nothing, as by the improvements lately made in the purification and distribution of coal-gas, the former system has been superseded.It is well known that light gas deteriorates very considerably by keeping, especially when exposed to water over an extensive surface; but even to a certain degree over oil, or in close vessels. An oil-gas which when newly prepared has the specific gravity of 1·054, will give the light of a candle for an hour, by consuming 200 cubic inches; will, after two days, give the same light by consuming 215 cubic inches per hour; and after four days, by consuming 240 cubic inches in the like time. With coal-gas the deterioration appears to be more rapid. When newly prepared, if it affords the light of a candle with a consumption of 400 cubic inches per hour; it will not give the same light after being kept two days, except with a consumption of 430 inches; and after four days, of 460. Oil-gas three weeks old has become so much impaired in quality that 600 inches of it were required per hour to furnish the light of a candle. All light gas should be used therefore as soon as possible after it is properly purified.Economical considerations.—The cost of gas-light depends upon so many local circumstances, that no estimate of it can be made of general application; only a fewleading points may be stated. The coals required for heating the retorts used to constitute one half of the quantity required for charging the retorts themselves. When five retorts are heated by one fire, the expenditure for fuel is only one third of that when each retort has a fire. The coak which remains in the retorts constitutes about 60 per cent. of the weight of the original coal; but the volume is increased by the coaking in the proportion of 100 to 75. When the coak is used for heating the retorts, about one half of the whole is required. If we estimate the coak by its comparative heating power, it represents 65 per cent. of the coals consumed. One hundred pounds of good coal yield in distillation 10 pounds of ammoniacal liquor, from which sulphate or muriate of ammonia may be made, by saturation with sulphuric or muriatic acid, and evaporation. The liquor contains likewise some cyanide of ammonia, which may be converted into prussian blue by the addition of sulphate of iron, after saturation with muriatic acid.Two hundred pounds of coal afford about 17 pounds of tar. This contains in 100 pounds 26 pounds of coal oil, and 48 pounds of pitch. The tar is sometimes employed as a paint to preserve wood and walls from the influence of moisture, but its disagreeable smell limits its use. The coal oil when rectified by distillation, is extensively employed for dissolving caoutchouc in making the varnish of waterproof cloth, and also for burning in a peculiar kind of lamps under the name of naphtha. Oil of turpentine however is often sold and used for this purpose, by the same name. If the coal oil be mixed with its volume of water, and the mixture be made to boil in a kettle, the mingled vapours when passed through a perforated nozzle may be kindled, and employed as a powerful means of artificial heat. The water is not decomposed, but it serves by its vapour to expand the bulk of the volatile oil, and to make it thereby come into contact with a larger volume of atmospherical air, so as to burn without smoke, under a boiler or any other vessel. The pitch may be decomposed into a light-gas.The relative cost of light from coal gas and oil gas may be estimated as one to six, at least. Rosin gas is cheaper than oil gas. SeeRosin.I shall conclude this article with a summary of the comparative expense of different modes of illumination, and some statistical tables.One pound of tallow will last 40 hours in six mould candles burned in succession, and costs 8d.; a gallon of oil, capable of affording the light of 15 candles, for 40 hours costs 5s., being therefore1⁄2of the price of mould candles, and6⁄15of the price of dips. The cost of wax is about 31⁄2times that of tallow; and coal gas, as sold at the rate of 9s.for 1000 cubic feet, will be one sixth the price of mould candles; for 500 cubic inches of coal gas give a light equal to the above candle for an hour; therefore 40 × 500 = 20,000 cubic inches = 11·57 cubic feet, worth 11⁄4d., which multiplied by 6 gives 71⁄2d.the average price of mould candles per pound.The author of the articleGas-lightin the Encyclopædia Britannica, observes, in reference to the economy of this mode of illumination, that while the price of coal, in consequence of the abundant and regular supply of that article, is liable to little fluctuation, the cost of wax, tallow, and oil, on account of the more precarious nature of the sources from which they are obtained, varies exceedingly in different seasons. “Assuming that a pound of tallow candles, which last when burned in succession forty hours, costs nine-pence,” (seven-pence halfpenny is the average price), “that a gallon of oil, yielding the light of 600 candles for an hour, costs two shillings,” (five shillings is the lowest price of a gallon of such oil as a gentleman would choose to burn in his lamp), “that the expense of the light from wax is three times as great as from tallow, and that a thousand cubic feet of coal gas cost nine shillings;” he concludes the relative cost to be for the same quantity of light,—from wax, 100; tallow, 25; oil, 5; and coal-gas, 3. I conceive the estimate given above to be much nearer the truth; when referred to wax called 100, it becomes, for tallow, 28·6; oil, 14·3; coal gas, 4·76.Gas-lighting has received a marvellous development in London. In the year 1834, the number of gas lamps in this city was 168,000, which consumed daily about 4,200,000 cubic feet of gas. For the purpose of generating this gas, more than 200,000 chaldrons, or 10,800,000 cubic feet of coals were required.For the following valuable statistical details upon gas-light, my readers are indebted to Joseph Hedley, Esq., engineer, of the Alliance Gas Works, Dublin; a gentleman who to a sound knowledge of chemistry, joins such mechanical talent and indefatigable diligence, as qualify him to conduct with success, any great undertaking committed to his care. He has long endeavoured to induce the directors of the London gas-works to employ a better coal, and generate a more richly carburetted gas, which in much smaller quantity would give as brilliant a light, without heating the apartments unpleasantly, as their highly hydrogenated gas now does. Were his judicious views adopted, coal gas would soon supersede oil, and even wax candles, for illuminating private mansions.Copy of a paper laid before a Committee of the House of Commons, showing not only the relative values of the Gases produced at the undermentioned places, but showing in like manner the relative economy of Gas as produced at the different places, over candles. By Joseph Hedley, Esq.Names of the Placeswhere Experimentswere made.Illuminatingpower of asingle Jet ofGas-flamefour incheshigh, takenby acomparisonof Shadows.The Jet ofGas burnt,four incheshigh,consumedper hourand wasequal to theCandlesin the lastcolumn.Gas requiredto be equalto 100 lbs.of mouldCandles,6 to the lb.,9 incheslong each.[A]Sellingpriceof Gasper meterper 1000cubic feet.Cost ofGas equalin illumi-natingpower to100 lbs. ofcandles.[B]Averagediscountallowedoff thechargefor Gas.Net cost ofGas equalto 100 lbs.of Candles.Specificgravityof theGas.Equal toCandles.CubicFeet.CubicFeet.s.d.L.s.d.PerCent.L.s.d.Birmingham;-2·5721·2227041001709147·541Birmingham andStaffordshire;two CompaniesStockport3·254·85148910001411121⁄20130·539Manchester3·060·8251536800123111⁄401010·534Liverpool OldCompany[C]2·3691·1264610016561⁄4149·462Liverpool NewGas Company4·408·91164100011861⁄40910·580Bradford2·1901·2312390181121⁄2146·420Leeds2·970·855164480013261⁄40124·530Sheffield2·4341·04244080019661⁄40183·466Leicester2·4351·12575760193150165·528Nottingham1·6451·34200901179151113·424Derby1·9371·235211001154151100·448Preston2·1361·153069100110815162·419London2·0831·13309210011011noneallowed.11011·412[A]100 lbs. of candles are estimated to burn 5700 hours.[B]The candles cost 3l.2s.6d.[C]The Liverpool Old Company have since resorted to the use of Cannel coal, and consequently very nearly assimilate to the Liverpool New Company in illuminating power.Names of the Placeswhere Experimentswere made.Illuminatingpower of asingle Jet ofGas-flamefour incheshigh, takenby acomparisonof Shadows.The Jet ofGas burnt,four incheshigh,consumedper hourand wasequal to theCandlesin the lastcolumn.Gas requiredto be equalto 100 lbs.of mouldCandles,6 to the lb.,9 incheslong each.[A]Sellingpriceof Gasper meterper 1000cubic feet.Equal toCandles.CubicFeet.CubicFeet.s.d.Birmingham;-2·5721·222704100Birmingham andStaffordshire;two CompaniesStockport3·254·851489100Manchester3·060·825153680Liverpool OldCompany[C]2·3691·12646100Liverpool NewGas Company4·408·91164100Bradford2·1901·2312390Leeds2·970·855164480Sheffield2·4341·04244080Leicester2·4351·1257576Nottingham1·6451·3420090Derby1·9371·23521100Preston2·1361·153069100London2·0831·133092100[A]100 lbs. of candles are estimated to burn 5700 hours.[C]The Liverpool Old Company have since resorted to the use of Cannel coal, and consequently very nearly assimilate to the Liverpool New Company in illuminating power.Names of the Placeswhere Experimentswere made.Cost ofGas equalin illumi-natingpower to100 lbs. ofcandles.[B]Averagediscountallowedoff thechargefor Gas.Net cost ofGas equalto 100 lbs.of Candles.Specificgravityof theGas.L.s.d.PerCent.L.s.d.Birmingham;-1709147·541Birmingham andStaffordshire;two CompaniesStockport01411121⁄20130·539Manchester0123111⁄401010·534Liverpool OldCompany[C]16561⁄4149·462Liverpool NewGas Company011861⁄40910·580Bradford181121⁄2146·420Leeds013261⁄40124·530Sheffield019661⁄40183·466Leicester0193150165·528Nottingham1179151113·424Derby1154151100·448Preston110815162·419London11011noneallowed.11011·412[B]The candles cost 3l.2s.6d.[C]The Liverpool Old Company have since resorted to the use of Cannel coal, and consequently very nearly assimilate to the Liverpool New Company in illuminating power.Memorandum.—It will not fail to be observed that in deducing the comparative value between candles and gas by these experiments, the single jet (and in every instance, of course, it was the same), has been the medium. This however, though decidedly the most correct way of making the comparative estimate of the illuminating power of the several gases, is highly disadvantageous in the economical comparison, inasmuch as gas burnt in a properly regulated argand burner, with its proper sized glass, air aperture, and sufficient number of holes, gives an advantage in favour of gas consumed in an argand, over a jet burner, of from 30 to 40 per cent. At the same time it must not be overlooked that in many situations where great light is not required, it will be found far more economical to adopt the use of single jets, which by means of swing brackets and light elegant shades, becomes splendid substitutes for candles, in banking establishments, offices, libraries, &c. &c.Note.—In Glasgow, Edinburgh, Dundee, Perth, and the Scotch towns generally the Parrot or Scotch Cannel coal is used; in illuminating power and specific gravity the gas produced is equal to that from the best description of Cannel coal in England. The price per 1000 cubic feet ranges about 9s., with from 5 to 30 per cent. off for discounts, leaving the net price about 9s.to be equal in the above table to 100 lbs. of candles.Epitome of Experiments made in Gas produced from different qualities of Coal, and consumed in different kinds of Burners:Tried at the Sheffield Gas Light Company’s Works, and laid before a Committee of the House of Commons. By Joseph Hedley, Esq.Date1835.Descriptionof Burner.Species ofCoal.SpecificGravityof Gas.Distanceof CandlefromShadow.Gasconsumedper Hour.Heightof GasFlame.Equal toMould TallowCandles, 6 tothe pound,9 incheslong each.Gas equalto 100 lbs.of MouldCandles.Cost of Gasat 8s.per1000 cubic feet.Cost of 100lbs. of MouldCandles at 7s.6d.per dozenlbs.May.Inches.CubicFeet.Inches.Candles.CubicFeet.L.s.d.L.s.d.8Single JetDeep Pit·410751·42·36241501931⁄2-3269DittoMortormley·45074·9542·434222401791⁄29DittoCannel·660611⁄4·743·5411270908-Argand14 holes-Deep Pit·410343·331⁄211·53163101301⁄29DittoMortormley·450333·131⁄212·24144301161⁄29DittoCannel·660292·631⁄315·859350753⁄4Date1835.Descriptionof Burner.Species ofCoal.SpecificGravityof Gas.Distanceof CandlefromShadow.Gasconsumedper Hour.Heightof GasFlame.May.Inches.CubicFeet.Inches.8Single JetDeep Pit·410751·49DittoMortormley·45074·9549DittoCannel·660611⁄4·748-Argand14 holes-Deep Pit·410343·331⁄29DittoMortormley·450333·131⁄29DittoCannel·660292·631⁄3Date1835.Descriptionof Burner.Species ofCoal.Equal toMould TallowCandles, 6 tothe pound,9 incheslong each.Gas equalto 100 lbs.of MouldCandles.Cost of Gasat 8s.per1000 cubic feet.Cost of 100lbs. of MouldCandles at 7s.6d.per dozenlbs.May.Candles.CubicFeet.L.s.d.L.s.d.8Single JetDeep Pit2·36241501931⁄2-3269DittoMortormley2·434222401791⁄29DittoCannel3·5411270908-Argand14 holes-Deep Pit11·53163101301⁄29DittoMortormley12·24144301161⁄29DittoCannel15·859350753⁄4
Tait's gasometer
Mr. Tait, of Mile-End Road, the inventor, has, we believe, been for some years connected with gas establishments, and is therefore fully aware of the practical defects or advantages of the different constructions of gasometers now in use.Fig.490.is a section of Mr. Tait’s improved contrivance;a ais the tank, occupied with water,b btwo iron columns, with pulley-wheels on the top,c c, chains attached to a ring of iron,d d, extending round the gasometer, which chains pass over the pulley-wheels, and are loaded at their extremities, for the purpose of balancing the weight of the materials of which the gasometer is composed.
The gasometer is formed by 2 or 3 cylinders, sliding one within the other, like the tubes of a telescope;e,e,e, is the first or outer cylinder, closed at the top, and having the ring of irond, passing round it, by which the whole is suspended;f f, is the second cylinder, sliding freely within the first, and there may be a third and fourth within these if necessary.
When there is no gas in the apparatus, all the cylinders are slidden down, and remain one within the other immersed in the tank of water; but when the gas rises through the water pressing against the top of the gasometer, its buoyancy causes the cylindereto ascend. Round the lower edge of this cylinder a groove is formed by the turning in of the plate of iron, and as it rises, the edge takes hold of the top rim of the cylinderf, which is overlapped for that purpose. The groove at the bottom of the cylinder fills itself with water as it ascends, and by the rim of the second cylinder falling into it, an air-tight hydraulic joint is produced.
Thus, several cylinders may be adapted to act in a small tank of water, by sliding one within the other, with lapped edges forming hydraulic joints, and by supporting the apparatus in the way shown, the centre of gravity will always be below the points of suspension. A gasometer may be made upon this plan of any diameter, as there will be no need of frame work, or a bridge to support it; and the increasing weight of the apparatus, as the cylinders are raised one after the other, may be counterpoised by loading the ends of the chainsc c.
The water in the gasometer need not be renewed; but merely so much of it as evaporates or leaks out, is to be replaced. Indeed the surface of the water in the cistern gets covered with a stratum of coal oil, a few inches deep, which prevents its evaporation, and allows the gas to be saturated with this volatile substance, so as to increase its illuminating powers.
Intermediate vessel
The gasometer may be separated from the purifier by an intermediate vessel, such as is representedfig.491., with which the two gas pipes are connected.Ais thecylindrical vessel of cast iron,a, the end of the gas pipe which comes from the purifier, immersed a few inches deep into the liquid with which the vessel is about two-thirds filled;bis the gas-pipe which leads into the gasometer,cis a perpendicular tube, placed over the bottom of the vessel, and reaching to within one-third of the top, through which the liquid is introduced into the vessel, and through which it escapes when it overflows the leveld. In this tube the liquid stands towards the inner level higher, in proportion to the pressure of the gas in the gasometer. The fluid which is condensed in the gas pipe,b, and in its prolongation from the gasometer, runs off into the vesselA; and therefore the latter must be laid so low that the said tube may have the requisite declivity. A straight stop-cock may also be attached to the side over the bottom, to draw off any sediment.
II.Application of Light-Gas.
1.Distribution of the pipes.—The pressure by which the motion of the gas is maintained in the pipes, corresponds to a certain height of water in the cistern of the gasometer. From the magnitude of this pressure, and the quantity of gas which in a given time, as an hour, must be transmitted through a certain length of pipes, depends the width or the diameter that they should have, in order that the motion may not be retarded by the friction which the gas, like all other fluids, experiences in tubes, and thereby the gas might be prevented from issuing with the velocity required for the jets of flame. The velocity of the gas in the main pipe increases in the ratio of the square root of the pressing column of water upon the gasometer, and therefore by increasing this pressure, the gas may be forced more rapidly along the remoter and smaller ramifications of the pipes. Thus it happens, however, that the gas will be discharged from the orifices near the gasometer, with superfluous velocity. It is therefore advisable to lay the pipes in such a manner, that in every point of their length, the velocity of discharge may be nearly equal. This may be nearly effected as follows;—
From experiment it appears that the magnitude of the friction, or the resistance which the air suffers in moving along the pipes, under a like primary pressure, that is for equal initial velocity, varies with the square root of the length. The volume of gas discharged from the end of a pipe, is directly proportional to the square of its diameter, and inversely as the square root of its length; or, calling the lengthL, the diameterD, the cubic feet of gas discharged in an hourk; thenk=D2√L. Experience likewise shows, that for a pipe 250 feet long, which transmits in an hour 200 cubic feet of gas, one inch is a sufficient diameter.
Consequently, 200 :k∷1144 √250:D2√L; and D =√k√L455,000
From this formula the following table of proportions is calculated.
These dimensions are applicable to the case where the body of gas is transmitted throughpipes without being let off in its way by burners, that is, to the mains which conduct the gas to the places where it is to be used. If the main sends off branches for burners, then for the same length the diameter may be reduced, or for like diameter the length may be greater. For example, if a pipe of 5·32 inches, which transmits 2000 cubic feet through a length of 2000 feet, gives off, in this space, 1000 cubic feet of gas; then the remainder of the pipe, having the same diameter, can continue to transmit the gas through a length of 2450 feet =(450,000k)2, with undiminished pressure for the purposes of lighting. Inversely, the diameter should be progressively reduced in proportion to the number of jets sent off in the length of the pipe.
Suppose for instance, the gasometer to discharge 2000 cubic feet per hour, and the last point of the jets to be at a distance of 4000 feet. Suppose also that from the gasometer to the first point of lighting, the gas proceeds through 1000 feet of close pipe, the diameter of the pipe will be here 4·47 inches; in the second 1000 feet of length, suppose the pipe to give off, at equal distances, 1000 cubic feet of gas, the diameter in this length (calculated at 1500 cubic feet for 1000 feet long) = 3·87 inches; in the third extent of 1000 feet, 600 cubic feet of gas will be given off, and the diameter (reckoning 700 cubic feet for 1000 feet long) will be 2·65 inches; in the fourth and last space (for 200 cubic feet in 1000 feet long) the pipe has a diameter of only an inch and a half, for which, in practice, a two-inch cast iron pipe is substituted; this being the smallest used in mains, into which branch pipes can be conveniently inserted.
The same relations hold with regard to branch pipes through which the gas is transmitted into buildings and other places to be illuminated. If such pipes make frequent angular turnings, whereby they retard the motion of the gas, they must be a third or a half larger in diameter. The smallest tubes of distribution are never less than one fourth of an inch in the bore.
Where, from one central gas work, a very great quantity of light is required in particular localities, there ought to be placed near these spots gasometers of distribution, which, being filled during the slack hours of the day, are ready to supply the burners at night, without making any considerable demand upon the original main pipe. Suppose the first main be required to supply 8000 cubic feet in the hour, for an illumination of 8 hours, at the distance of 2000 feet, a pipe 102⁄3inches in diameter would be necessary; but if two or three gasometers of distribution, or station gasometers be had recourse to, into which the gas during the course of 24 hours would flow through the same distance continuously from the central gas works, the quantity required per hour from them would be only one third of 8000, = 2666·6 cubic feet; consequently the diameter for such a pipe is only 6·15 inches.
Gas pipes
All the principal as well as branch pipes, whose interior diameter exceeds an inch and a half, are made of cast iron from 6 to 8 feet long, with elbow pipes cast in them where it is necessary. These pipe lengths are shown infig.492., having at one end a wide socketa, and at the other a nozzleb, which fits the former. After inserting the one in the other in their proper horizontal position, a coil of hemp soaked with tar is driven home at the junction; then a luting of clay is applied at the mouth, within which a ring of lead is cast into the socket, which is driven tight home with a mallet and blunt chisel.
The pipes should be proved by a force pump before being received into the gas works; two or three lengths of them should be joined before laying them down, and they should be placed at least two feet below the surface, to prevent their being affected by changes of temperature, which would loosen the joints. The tubes for internal distribution, when of small size are made of lead, copper, wrought iron, or tin.
Water trap
Instead of a stopcock for letting off the gas in regulated quantities from the gasometer, a peculiarly formed water or mercurial valve is usually employed.Fig.493.shows the mode of construction for a water trap or lute, and is, in fact, merely a gasometer in miniature.C D E Fis a square cast iron vessel, in the one side of which a pipeAis placed in communication with the gasometer, and in the other, one with the mainB. The movable cover or lidH G I Khas a partition,L M, in its middle. If this cover be raised by its counterweight, the gas can pass without impediment fromAtoB; but if the counterweight be diminished so as to let the partition plateL Msink into the water, the communication of the two pipes is thereby interrupted. In this case the water-level stands in the compartmentAso much lower than outside of it, and in the compartmentB, as is equivalent to the pressure in the gasometer; therefore the pipesAandBmust project thus far above the water. In order to keep the water always at the same height, and to prevent it from flowing into the mouths of these pipes, the rimC Dof the outer vessel stands somewhat lower than the orificesA B; and thence the vessel may be kept always full of water.
Quicksilver valve
If a quicksilver valve be preferred, it may be constructed as shown infig.494.A Bare the terminations of the two gas pipes, which are made fast in the rectangular iron vesselM.Eis an iron vessel of the same form, which is filled with quicksilver up to the levela, and which, by means of the screwG, which presses against its bottom, and works in the fixed female screwC C, may be moved up or down, so that the vesselMmay be immersed more or less into the quicksilver. The vesselMis furnished with a vertical partitionm; the passage of the gas fromAtoBis therefore obstructed when this partition dips into the quicksilver, and from the gradual depression of the vesselEby its screw, the interval between the quicksilver and the lower edge of the partition, through which the gas must enter, may be enlarged at pleasure, whereby the pressure of the gas inBmay be regulated to any degree. The transverse section of that interval is equal to the area of the pipe or rather greater; the breadth of the vesselMfromAtoBamounts to the double of that space, and its length to the mere diameter ofAorB. The greatest height to which the partitionmcan rise out of the quicksilver, is also equal to the above diameter, and in this case the lineacomes to the place ofb. The vertical movement of the outer vesselE, is secured by a rectangular rim or hoop which surrounds it, and is made fast to the upper part of the vesselM, within which guide it moves up and down. Instead of the leverD D, an index with a graduated plate may be employed to turn the screw, and to indicate exactly the magnitude in the opening of the valve.
Gas-meter
In order to measure the quantity of gas which passes through a pipe for lighting a factory, theatre, &c., the gas-meter is employed, of whose construction a sufficiently precise idea may be formed from the consideration offig.495., which shows the instrument in a section perpendicular to its axis.
Within the cylindrical casea, there is a shorter cylinderb b, shut at both ends, and movable round an axis, which is divided into four compartments, that communicate by the openingd, with the interval between this cylinder and the outer case. The mode in which thiscylinder turns round its axis is as follows:—The end of the tubec, which is made fast to the side of the case, and by which the gas enters, carries a pivot or gudgeon, upon which the centre of its prop turns; the other end of the axis runs in the cover, which here forms the side of a superior open vessel, in which, upon the same axis, there is a toothed wheel. The vessel is so far filled with water, that the tubecjust rises above it, which position is secured by the level of the side vessel. When the gas enters through the tubec, by its pressure upon the partitione, (fig.495.) it turns the cylinder from right to left upon its axis, till the exterior openingdrises above the water, and the gas expands itself in the exterior space, whence it passes off through a tube at top. At every revolution a certain volume of gas thus goes through the cylinder, proportional to its known capacity. The wheel on the axis works in other toothed wheels, whence, by means of an index upon a graduated disc or dial, placed at top or in front of the gas-meter, the number of cubic feet of gas, which pass through this apparatus in a given time, is registered.
B.Employment of the gas for lighting.—The illuminating power of different gases burned in the same circumstances, is proportional, generally speaking, to their specific gravity, as this is to the quantity of carbon they hold in combination. The following table exhibits the different qualities of gases in respect to illumination.
In the last three proportions, the coal gas was produced from coals of middle quality; in the first three proportions from coals of good quality; and therefore the middle proportion of 100 to 270 may be taken to represent the fair average upon the great scale. On comparing the gas from bad coals, with good oil gas, the proportion may become 100 to 300. Nay, coal gas of specific gravity 0·4, compared to oil gas of 1·1, gives the proportion of 1 to 4. A mould tallow candle, of 6 in the pound, burning for an hour, is equivalent to half a cubic foot of ordinary coal gas, and to four tenths of a foot of good gas. The flame of the best argand lamp of Carcel, in which a steady supply of oil is maintained by pump-work, consuming 42 grammes = 649 grains English in an hour, and equal in light to 9·38 such candles, is equivalent to 3·75 cubic feet of coal gas per hour. The sinumbra lamp, which consumes 50 grammes = 772 grains English, of oil per hour, and gives the light of 8 of the above candles, is equivalent to the light emitted by 3·2 cubic feet of coal gas burning for an hour. A common argand lamp, equal to 4 candles, which consumes 30 grammes = 463 grains English per hour, is represented by 1·6 cubic feet of gas burning during the same time. A common lamp, with a flat wick and glass chimney, whose light is equal to 1·13 tallow candles, and which consumes 11 grammes = 169·8 grains English per hour, is represented by 0·452 of a cubic foot of gas burning for the same time.
Construction of the Burners.—The mode of burning the gas as it issues from the jets has a great influence upon the quantity and quality of its light. When carburetted hydrogen gas is transmitted through ignited porcelain tubes, it is partially decomposed with a precipitation of some of its carbon, while the resulting gas burns with a feebler flame. Coal gas, when kindled at a small orifice in a tube, undergoes a like decomposition and precipitation. Its hydrogen, with a little of its carbon, burns whenever it comes into contact with the atmospherical air, with a bluish coloured flame; but the carbonaceous part not being so accendible, takes fire only when mixed with more air; therefore at a greater distance from the beak, and with a white light from the vivid ignition of its solid particles. Upon this principle pure hydrogen gas may be made to burn with a white instead of its usual blue flame, by dusting into it particles of lamp black; or by kindling it at the extremity of a tube containing finely pulverized zinc. The metallic particles become ignited, and impart their bright light to the pale blue flame. Even platinum wire and asbestos, when placed in the flame of hydrogen gas, serve to whiten it. Hence it has been concluded, that the intensity of light which a gas is capable of affording is proportional to the quantity of solid particles which itcontains, and can precipitate in the act of burning. Carbonic oxide gas burns with the feeblest light next to hydrogen, because it deposits no carbon in the act of burning. Phosphuretted hydrogen gives a brilliant light, because the phosphoric acid, into which its base is converted during the combustion, is a solid substance, capable of being ignited in the flame. Olefiant gas, as also the vapour of hydro-carbon oil, emits a more vivid light than common coal gas; for the first is composed of two measures of hydrogen and two measures of the vapour of carbon condensed into one volume; while the last contains only one measure of the vapour of carbon in the same bulk, and combined with the same proportion of hydrogen. Olefiant gas may therefore be expected to evolve a double quantity of carbon in its flame, which should emit a double light.
The illuminating power of the flame of coal gas is, on the contrary, impaired, when, by admixture with other species of gas which precipitate no carbon, its own ignited particles are diffused over a greater surface. This happens when it is mixed with hydrogen, carbonic oxide, carbonic acid, and nitrogen gases, and the diminution of the light is proportional to the dilution of the coal gas.
Gas burner
In like manner the illuminating power of coal gas is impaired, when it is consumed too rapidly to allow time for the separation and ignition of its carbonaceous matter; it burns, in this case, without decomposition, and with a feeble blue flame. 1. This occurs when the light-gas is previously mixed with atmospherical air, because the combustion is thereby accelerated throughout the interior of the flame, so as to prevent the due separation of carbon. A large admixture of atmospherical air makes the flame entirely blue. 2. When it issues, with considerable velocity, from a minute orifice, whereby the gas, by expansion, gets intimately mixed with a large proportion of atmospherical air. If the jet be vertical, the bottom part of the flame is blue, and the more so the less carbon is contained in the gas. The same thing may be observed in the flame of tallow, wax, or oil lights. The burning wick acts the part of a retort, in decomposing the fatty matter. From the lower part of the wick the gases and vapours of the fat issue with the greatest velocity, and are most freely mixed with the air; while the gases disengaged from the upper part of the wick compose the interior of the flame, and being momentarily protected from the action of the atmosphere, acquire the proper high temperature for the deposition of carbon, which is then diffused on the outer surface in an ignited state, and causes its characteristic white light. Hence with coal gas, the light increases in a certain ratio with the size of the flame as it issues from a larger orifice, because the intermixture of air becomes proportionately less. 3. If by any means too great a draught be given to the flame, its light becomes feebler by the rapidity and completeness with which the gas is burned, as when too tall a chimney is placed over an argand burner, seefig.496.Fig.497.c, is a view of the upper plate, upon which the glass chimneybrests. The gas issues through the smaller openings of the inner ring, and forms a hollow cylindrical flame, upon the outside as well as the inside of which the atmospherical air acts. The illuminating power of this flame may be diminished at pleasure, according as more or less air is allowed to enter through the orifices beneath. With a very full draught the light almost vanishes, leaving only a dull blue flame of great heating power, like that of the blowpipe, corresponding to the perfect combustion of the gas without precipitation of its carbon. 4. On the other hand, too small a draught of air is equally prejudicial; not merely because a portion of the carbon thus escapes unconsumed in smoke, but also because the highest illuminating power of the flame is obtained only when the precipitated charcoal is heated to whiteness, a circumstance which requires a considerable draught of air. Hence the flame of dense oil gas, or of oil in a wick, burns with a yellow light without a chimney; but when it is increased in intensity by a chimney draught, it burns with a brilliant white flame.
From the consideration of the preceding facts, it is possible to give to coal gas its highest illuminating power. The burners are either simple beaks perforated with a small round hole, or circles with a series of holes to form an argand flame, as shown infig.497, or two holes drilled obliquely, to make the flame cross, like a swallow’s tail, or with a slit constituting the sheet of flame called a bat’s wing, like most of the lamps in the streets of London. These burners are mounted with a stop-cock for regulating the quantity of gas.
The height of the flame, which with like pressure depends upon the size of the orifice, and with like orifice upon the amount of pressure, the latter being modified by the stop-cock, is for simple jets in the open air, as follows:—
When the length exceeds five inches, nothing is gained in respect to light. For oilgas the same statements will serve, only on account of its superior richness in carbon, it does not bear so long a flame without smoke. Thus:—
The diameter of the orifice for single jets, or for several jets from the same beak, is one twenty-eighth of an inch for coal gas, and one forty-fifth for oil gas.
Gas burner
When several jets issue from the same burner, the light is improved by making all the flames unite into one. In this case the heat becomes greater, for the combined flame presents a smaller surface to be cooled, than the sum of the smaller flames. The advantage gained in this way, may be in the ratio of 3 to 2, or 50 per cent. In an argand burner, the distances of the orifices for coal gas should be from16⁄100to18⁄100of an inch, and for oil gas12⁄100. If the argand ring has ten orifices, the diameter of the central opening should be =4⁄10of an inch; if 25 orifices, it should be one inch for coal gas; but for oil gas with 10 orifices, the central opening should have a diameter of half an inch, and for 20 orifices, one inch. The pin holes should be of equal size, otherwise the larger ones will cause smoke, as in an argand flame with an uneven wick. The glass chimney is not necessary to promote the combustion of an argand coal gas flame, but only to prevent it from flickering with the wind, and therefore it should be made so wide as to exercise little or no influence upon the draught. A narrow chimney is necessary merely to prevent smoke, when a very strong light, with a profusion of gas is desired. Oil gas burned in an argand beak requires a draught chimney, like a common argand lamp, on account of the large quantity of carbon to be consumed. The most suitable mode of regulating the degree of draught can be determined only by experiment, and the best construction hitherto ascertained is that represented infig.498.Fig.499.exhibits the view from above, of the rim or ringc, upon which the chimneybstands, and which surrounds the perforated beak. The ring is made of open fretwork, to permit the free passage of air upwards to strike the outside of the flame. The thin annular discd, which is laid over its fellow discc, in the bottom of the chimney-holder, being turned a little one way or other, will allow more or less air to pass through for promoting more or less, the draught or ventilation. The draught in the central tube of the burner may be regulated by the small disce, whose diameter is somewhat smaller than that of the ring of the burner, and which by turning the milled headf, of the screw, may be adjusted with the greatest nicety, so as to admit a greater or smaller body of air into the centre of the cylindrical flame.
In mounting gas-lights, and in estimating beforehand their illuminating effects, we must keep in mind the optical proposition, that the quantity of light is inversely as the square of the distance from the luminous body, and we must distribute the burners accordingly. When for example a gas-light placed at a distance of ten feet, is required for reading or writing to afford the same light as a candle placed at a distance of two feet; squaring each distance, we have 100 and 4; therefore1004= 25, shows us that 25 such lights will be necessary at the distance of 10 feet.
Concerning portable gas-light, with the means of condensing it, and carrying it from the gas works to the places where it is to be consumed, we need say nothing, as by the improvements lately made in the purification and distribution of coal-gas, the former system has been superseded.
It is well known that light gas deteriorates very considerably by keeping, especially when exposed to water over an extensive surface; but even to a certain degree over oil, or in close vessels. An oil-gas which when newly prepared has the specific gravity of 1·054, will give the light of a candle for an hour, by consuming 200 cubic inches; will, after two days, give the same light by consuming 215 cubic inches per hour; and after four days, by consuming 240 cubic inches in the like time. With coal-gas the deterioration appears to be more rapid. When newly prepared, if it affords the light of a candle with a consumption of 400 cubic inches per hour; it will not give the same light after being kept two days, except with a consumption of 430 inches; and after four days, of 460. Oil-gas three weeks old has become so much impaired in quality that 600 inches of it were required per hour to furnish the light of a candle. All light gas should be used therefore as soon as possible after it is properly purified.
Economical considerations.—The cost of gas-light depends upon so many local circumstances, that no estimate of it can be made of general application; only a fewleading points may be stated. The coals required for heating the retorts used to constitute one half of the quantity required for charging the retorts themselves. When five retorts are heated by one fire, the expenditure for fuel is only one third of that when each retort has a fire. The coak which remains in the retorts constitutes about 60 per cent. of the weight of the original coal; but the volume is increased by the coaking in the proportion of 100 to 75. When the coak is used for heating the retorts, about one half of the whole is required. If we estimate the coak by its comparative heating power, it represents 65 per cent. of the coals consumed. One hundred pounds of good coal yield in distillation 10 pounds of ammoniacal liquor, from which sulphate or muriate of ammonia may be made, by saturation with sulphuric or muriatic acid, and evaporation. The liquor contains likewise some cyanide of ammonia, which may be converted into prussian blue by the addition of sulphate of iron, after saturation with muriatic acid.
Two hundred pounds of coal afford about 17 pounds of tar. This contains in 100 pounds 26 pounds of coal oil, and 48 pounds of pitch. The tar is sometimes employed as a paint to preserve wood and walls from the influence of moisture, but its disagreeable smell limits its use. The coal oil when rectified by distillation, is extensively employed for dissolving caoutchouc in making the varnish of waterproof cloth, and also for burning in a peculiar kind of lamps under the name of naphtha. Oil of turpentine however is often sold and used for this purpose, by the same name. If the coal oil be mixed with its volume of water, and the mixture be made to boil in a kettle, the mingled vapours when passed through a perforated nozzle may be kindled, and employed as a powerful means of artificial heat. The water is not decomposed, but it serves by its vapour to expand the bulk of the volatile oil, and to make it thereby come into contact with a larger volume of atmospherical air, so as to burn without smoke, under a boiler or any other vessel. The pitch may be decomposed into a light-gas.
The relative cost of light from coal gas and oil gas may be estimated as one to six, at least. Rosin gas is cheaper than oil gas. SeeRosin.
I shall conclude this article with a summary of the comparative expense of different modes of illumination, and some statistical tables.
One pound of tallow will last 40 hours in six mould candles burned in succession, and costs 8d.; a gallon of oil, capable of affording the light of 15 candles, for 40 hours costs 5s., being therefore1⁄2of the price of mould candles, and6⁄15of the price of dips. The cost of wax is about 31⁄2times that of tallow; and coal gas, as sold at the rate of 9s.for 1000 cubic feet, will be one sixth the price of mould candles; for 500 cubic inches of coal gas give a light equal to the above candle for an hour; therefore 40 × 500 = 20,000 cubic inches = 11·57 cubic feet, worth 11⁄4d., which multiplied by 6 gives 71⁄2d.the average price of mould candles per pound.
The author of the articleGas-lightin the Encyclopædia Britannica, observes, in reference to the economy of this mode of illumination, that while the price of coal, in consequence of the abundant and regular supply of that article, is liable to little fluctuation, the cost of wax, tallow, and oil, on account of the more precarious nature of the sources from which they are obtained, varies exceedingly in different seasons. “Assuming that a pound of tallow candles, which last when burned in succession forty hours, costs nine-pence,” (seven-pence halfpenny is the average price), “that a gallon of oil, yielding the light of 600 candles for an hour, costs two shillings,” (five shillings is the lowest price of a gallon of such oil as a gentleman would choose to burn in his lamp), “that the expense of the light from wax is three times as great as from tallow, and that a thousand cubic feet of coal gas cost nine shillings;” he concludes the relative cost to be for the same quantity of light,—from wax, 100; tallow, 25; oil, 5; and coal-gas, 3. I conceive the estimate given above to be much nearer the truth; when referred to wax called 100, it becomes, for tallow, 28·6; oil, 14·3; coal gas, 4·76.
Gas-lighting has received a marvellous development in London. In the year 1834, the number of gas lamps in this city was 168,000, which consumed daily about 4,200,000 cubic feet of gas. For the purpose of generating this gas, more than 200,000 chaldrons, or 10,800,000 cubic feet of coals were required.
For the following valuable statistical details upon gas-light, my readers are indebted to Joseph Hedley, Esq., engineer, of the Alliance Gas Works, Dublin; a gentleman who to a sound knowledge of chemistry, joins such mechanical talent and indefatigable diligence, as qualify him to conduct with success, any great undertaking committed to his care. He has long endeavoured to induce the directors of the London gas-works to employ a better coal, and generate a more richly carburetted gas, which in much smaller quantity would give as brilliant a light, without heating the apartments unpleasantly, as their highly hydrogenated gas now does. Were his judicious views adopted, coal gas would soon supersede oil, and even wax candles, for illuminating private mansions.
Copy of a paper laid before a Committee of the House of Commons, showing not only the relative values of the Gases produced at the undermentioned places, but showing in like manner the relative economy of Gas as produced at the different places, over candles. By Joseph Hedley, Esq.
Memorandum.—It will not fail to be observed that in deducing the comparative value between candles and gas by these experiments, the single jet (and in every instance, of course, it was the same), has been the medium. This however, though decidedly the most correct way of making the comparative estimate of the illuminating power of the several gases, is highly disadvantageous in the economical comparison, inasmuch as gas burnt in a properly regulated argand burner, with its proper sized glass, air aperture, and sufficient number of holes, gives an advantage in favour of gas consumed in an argand, over a jet burner, of from 30 to 40 per cent. At the same time it must not be overlooked that in many situations where great light is not required, it will be found far more economical to adopt the use of single jets, which by means of swing brackets and light elegant shades, becomes splendid substitutes for candles, in banking establishments, offices, libraries, &c. &c.
Memorandum.—It will not fail to be observed that in deducing the comparative value between candles and gas by these experiments, the single jet (and in every instance, of course, it was the same), has been the medium. This however, though decidedly the most correct way of making the comparative estimate of the illuminating power of the several gases, is highly disadvantageous in the economical comparison, inasmuch as gas burnt in a properly regulated argand burner, with its proper sized glass, air aperture, and sufficient number of holes, gives an advantage in favour of gas consumed in an argand, over a jet burner, of from 30 to 40 per cent. At the same time it must not be overlooked that in many situations where great light is not required, it will be found far more economical to adopt the use of single jets, which by means of swing brackets and light elegant shades, becomes splendid substitutes for candles, in banking establishments, offices, libraries, &c. &c.
Note.—In Glasgow, Edinburgh, Dundee, Perth, and the Scotch towns generally the Parrot or Scotch Cannel coal is used; in illuminating power and specific gravity the gas produced is equal to that from the best description of Cannel coal in England. The price per 1000 cubic feet ranges about 9s., with from 5 to 30 per cent. off for discounts, leaving the net price about 9s.to be equal in the above table to 100 lbs. of candles.
Note.—In Glasgow, Edinburgh, Dundee, Perth, and the Scotch towns generally the Parrot or Scotch Cannel coal is used; in illuminating power and specific gravity the gas produced is equal to that from the best description of Cannel coal in England. The price per 1000 cubic feet ranges about 9s., with from 5 to 30 per cent. off for discounts, leaving the net price about 9s.to be equal in the above table to 100 lbs. of candles.
Epitome of Experiments made in Gas produced from different qualities of Coal, and consumed in different kinds of Burners:
Tried at the Sheffield Gas Light Company’s Works, and laid before a Committee of the House of Commons. By Joseph Hedley, Esq.