Gas, LIGHTING AND HEATING

Gas, LIGHTING AND HEATING By, depend mainly on the presence of gaseous heavy hydrocarbons in the gas. Pure hydrogen and even pure methane give no light, and, volume for volume, they give little heat, though their flames are flames of high temperature. When illuminating gas is ignited it burns with a flame which is luminous for two reasons: (1) the hydrocarbons form acetylene, which upon becoming highly heated decomposes explosively with a bright flash; and (2) the hydrocarbons are partly decomposed, and leave highly carbonaceous molecular residues which, becoming highly heated in the flame, incandesce and become luminous. I. Coal-gas is produced by the simple distillation of dry coal. Anthracite coal is unsuitable; brown coal and lignite are unsatisfactory: the greatest yield of the best gas is obtained from highly bituminous coals, although these are expensive and leave as residue inferior coke, mainly ash; practically the most useful gas-coal is that which will, either alone or mixed with bituminous coal, yield a fair quantity of good gas and leave good coke in the retorts. The very highly bituminous coals are only used for mixing with ordinary coal: the ordinary bituminous or cannel coals are sometimes used, especially in Scotland, for making richer gas of 25 to 30 candle-power (in standard burners burning 5 cubic feet per hour), but are usually mixed with ordinary coal with the view of improving the coke produced. The ordinary caking coals of the north of England are mainly used in England, mixed with a proportion of cannel or of highly bituminous coal or shale in order to improve the gas, which is generally sup- plied with an illuminating power of from 16 to 20 candles. The gas-coal used on the Continent is intermediate between caking coal and cherry coal, and gives gas of from 12 to 17 candles. By bituminous coal is not meant coal which actually contains bitumen, but coal which contains carbon and hydrogen in a proportion suited to the formation of heavy hydrocarbons when the coal is exposed to heat: no bitumen can be dissolved by alcohol out of a so-called bituminous coal. The proportions of hydrogen and oxygen to the carbon in various materials is shown in the following table:

The hydrocarbons which enable the gas to give a luminous flame depend for their formation upon the presence of hydrogen: oxygen, on the other hand, is detrimental; it takes up hydrogen to form water, and with carbon it forms carbonic acid and carbonic oxide. Anthracite distilled gives no useful result; Newcastle gas-coal gives, per ton, a little over 10,000 cubic feet of gas, of an illuminating power ranging between 14 and 20 candles; Scotch cannel, 10,600 feet, of 30 candles; Scotch Boghead, distilled alone, 13,000 feet, of 40 candle, or 15,000 feet, of 35 candle; and Australian Boghead, 14,000 feet, of 50 candle-gas. These are given merely as typical examples; the results vary greatly according to the temperatures employed and the duration of the exposure to heat. Newcastle cannel coal, for example, if distilled between 750° and 800° F., yields, per ton, 68 gallons of crude oil (whereof may be recovered—paraffin spirit about 2 gallons; lamp-oil, 22½ gallons; heavy oil and paraffin, 24 gallons), 1280 lb. of coke, and only 1400 cubic feet of gas; whereas, when it is distilled for gas in the usual way, it yields, besides the coal-gas, 18½ gallons of coal-tar (wherefrom 3 pints benzol, 3 pints coal-tar naphtha, and 9 gallons of heavy oils, naphthaline, &c.), and 1200 lb. of coke. Protracted distillation at high heats causes the evolution of hydrogen rather than of hydrocarbons; high heats in general cause the production of volatile rather than of condensable hydrocarbons, and this results, if not carried to excess, in a decided advantage—viz. that the gas produced, though of lower quality than the smaller quantity produced at low heats, is greatly less liable to lose its illuminating power by condensation and deposition of hydrocarbons on the way to the consumer. Very roughly, the candle-power is, within a limited range, inversely proportional to the number of feet of gas made (at a given temperature) from a given quantity of coal. Thus, if a ton of coal give 10,000 cubic feet of 15½ candle-gas, then, if the distillation be protracted so that 10,500 feet are produced, the candle-power will sink to 15. Tieftrunk calculates the percentage composition (in volumes) of the gas which comes off in successive hours thus:

Distillation is thus after the fourth hour practically disadvantageous to illuminating power.

The products of distillation of coal, as usually performed in gas-works, are very numerous. The principal of them are marsh-gas, hydrogen, carbonic oxide, carbonic acid, nitrogen, oxygen, sulphuretted hydrogen, ammonia, hydrocyanic acid, bisulphide of carbon, and other organic sulphur compounds; aqueous vapour; ethylene, propylene, butylene, acetylene, ditetrayl, and allylene; caproyl, capryl and rutyl hydrides; caproylene, $\backslash alpha$-anthylene; benzol, toluol, xylo, cymol; paraffin, naphthaline, anthracene, chrysene, pyrene; acetic acid, carbolic acid, cresol, phlorol, rosolic acid; aniline, pyridine, picolin, and several other nitrogenous alkaloid substances; with some hydrochloric and sulphurous acids. These substances have very different volatilities and solubilities; a large number of them may be separated from the gas by mere cooling, and together these form coal-tar, which is a black viscous liquid, sp. gr. 0.98 (from cannel) to 1.15 (from ordinary coal), the yield of which is, from coal, up to 12 gallons, and from cannel up to 17 gallons per ton distilled, the average yield being scarcely 11 gallons. By careful distillation coal-tar yields successively the following products, the percentages of which vary widely in different gas-works: 2-4 per cent. of water, ammonia (which may be extracted from the tar by cold water), and volatile hydrocarbon vapours; 1.5 to 16 per cent. of light oils, including carbolic acid; 20-35 per cent. of heavy oils (creasote oils); 10-20 per cent. of anthracene oils, and a residue of 28-64 per cent. of pitch. The reason of this wide range of variation in the tar lies partly in the nature of the coal used, the temperature of distillation (the higher the heats the thicker the tars), and partly in the mode and temperature of condensation.

After the tar has been mostly deposited the gas is washed with water, which is converted into ammoniacal liquor, containing ammonia, carbonate of ammonium, sulphide of ammonium and some sulphite, chloride, and sulphocyanide of ammonium, and salts of nitrogenous alkaloids. After being cooled and washed the gas still contains carbonic acid, sulphuretted hydrogen, some hydrocyanic acid, and some bisulphide of carbon, and other sulphur compounds. Slaked lime, moistened so as to form a porous mass, will absorb the carbonic acid or sulphuretted hydrogen, but not the hydrocyanic acid and bisulphide of carbon so long as there is free carbonic acid present. Oxide of iron absorbs $H\_2S$, becoming sulphide; and this, when re-exposed to the air, is re-oxidised, the oxide being regenerated, while free sulphur is formed mixed with the oxide; the oxide may be used over and over until the percentage of free sulphur rises to 50 or 56, after which the oxide is 'spent,' and is transferred for the sake of its sulphur to the manufacturing chemist. Spent oxide also contains Prussian blue, or ferrocyanide of iron, $Fe\_4Cy\_\{18\}$; this, together with sulphocyanide of iron, is formed from the hydrocyanic acid. Further, the free sulphur in the oxide arrests bisulphide of carbon and other sulphur compounds. The regeneration of the oxide can be brought about by admitting a percentage, say 2, of air into the gas-stream. The oxygen of the admitted air is taken up in continuous regeneration of the purifying oxide. The disadvantage of this is that the residual nitrogen of the air tells against the illuminating power of the gas; but recently, since pure oxygen has become cheap, oxygen gas alone has been employed with very favourable results. One result of continuous revivication is, that the evil smells associated with the opening of purifiers have become unfamiliar in most works. When continuous regeneration is resorted to, the oxide does not become spent until it contains a considerably higher percentage (as much as 75) of sulphur. Iron oxide, however, does not remove carbonic acid, and Mr R. H. Patterson showed that complete purification might be secured by removing (1) $CO\_2$ by means of lime (the carbonic acid having a stronger affinity for lime than sulphuretted hydrogen has, is retained in the first lime purifier, while $H\_2S$ either passes on directly or is driven off by the succeeding $CO\_2$ from any temporary lodgment it may have gained in the first purifier); (2) $H\_2S$ by a second lime purifier, the resulting sulphide of calcium uniting with the bisulphide of carbon to form thiocarbonate of calcium ($CaS\; +\; CS\_2\; =\; CaCS\_3$, analogous to carbonate of calcium, $CaCO\_3$), or rather a basic compound $CaCS\_3\; \backslash cdot\; CaH\_2O\_2\; \backslash cdot\; 7H\_2O$, and also with other sulphocarbon compounds; and (3) if necessary any remaining $H\_2S$ may be taken up by iron oxide. In 1888-89 Mr Valon found that if 0.6 per cent. of oxygen be added to crude gas, and if lime be used alone as the purifying agent, there is complete and simultaneous removal of the carbonic acid, sulphuretted hydrogen, and sulphide of carbon, the sulphur being separated in the free state and the gas-lime produced being entirely devoid of smell; while, owing to complete separation of the carbonic acid and through not introducing nitrogen, the lighting-power of the gas is at least 1½ candle better than when iron oxide is employed alone.

Purified gas contains, in percentages by volume:

London cannel gas is no longer made; and true Boghead mineral is no longer obtained in Great Britain, though large quantities of an equivalent substance are now shipped from Australia.

When coke is made in a beehive oven, the gas evolved is largely contaminated with nitrogen; but when coke is made from moderately bituminous coal in a by-products oven, the gas produced is practically equivalent to a somewhat poor coal-gas or to a rich fuel-gas. It is understood that the manufacture of this by-products coke-gas is likely to be undertaken on a large scale in Massachusetts and at Pittsburg, where the supply of natural gas shows symptoms of exhaustion.

The illuminating power depends on the 'heavy hydrocarbons;' of these benzol is the most effective (3 parts of it being equal to 25 of ethylene), and in ordinary English gas is present to the amount of from 5 to 10 grains per cubic foot, while ethylene and propylene are together from four to twelve times that quantity. If carbonic acid, sulphuretted hydrogen, and nitrogen be absent, the heavier gas is generally the richer, though a high percentage of carbonic oxide may also make a gas heavy. The specific gravity of coal-gas is from 0.4 to 0.55 (air = 1.00). There are two rough tests for the value of gas: (1) its durability—i.e. the time taken to burn 1 cubic foot of gas in a jet of 5 inches high; this ranges from 50' 40" for English caking-coal gas, to 84' 22" for Boghead gas; (2) the percentage of volume which is condensed by chlorine or bromine, which attack the heavy hydrocarbons. If any carbonic acid remain in the gas, it will diminish the illuminating power about one candle for every 1 per cent. of carbonic acid. If gas be mixed with air the illuminating power rapidly falls off: with 1 per cent. of air, the loss of lighting-power is 6 per cent.; with 2, 11; 3, 18; 4, 26; 5, 33; 10, 67; 20, 93 per cent.; 45, total loss of lighting-power. Ordinary gas mixed with more than 4 and less than 12 times its bulk of air is explosive; most so when mixed with 8 volumes of air or somewhat more (up to 11 volumes) if the gas be richer. Alone, it is not explosive. For ascertaining the illuminating power, the Bunsen photometer (the open 60-inch Bunsen-Letheby photometer, or the enclosed 100-inch Evans photometer) is generally employed. In this, at one end of a rod, there is a candle; at the other end there is a gas-burner, and a meter to measure the supply of gas; the gas-burner and the candle are thus at a fixed distance from one another. Between them there moves, sliding on a graduated bar, a disc of prepared paper; this is slipped up and down until its two sides (or rather the images of its respective sides in two little mirrors which travel with it) appear equally illuminated. This is ascertained by the disappearance of a grease spot or rather, in the newer models, by the vanishing of all difference in appearance between an ungreased centre and the greased rim of the disc. In the Leeson disc there are three thicknesses of paper, of which the middle one is much the thickest, but is perforated at its centre; and this form of disc works better in the comparison of light of somewhat different colours. The Lummel-Brodhun photometer is an idealised Bunsen photometer, in which the place of the paper with its central grease-spot is taken by a purely optical arrangement of totally reflecting or partially reflecting prisms. The bar may be graduated in one of two ways: (1) Equal intervals, so that the respective distances between the disc and the gas-burner and candle may be measured; then the ratio between the intensities is the inverse ratio of the squares of the respective distances; say, for example, that the respective distances of the candle and gas-burner are 20 inches and 80 inches; then the gas-burner's intensity: the candle's :: $(20)^2\; :\; (80)^2$—i.e. :: 16 : 1. (2) The bar may be so graduated as to anticipate and save this calculation, on which principle the mid-point of the bar would be marked 1, and a point one-fifth of the bar's length from either end would be marked 16; the figures so marked show directly the ratios sought for. The pressure of gas must be measured by a gauge and regulated by a governor; the consumpt of the candle must be weighed; the gas used must be exactly 5 cubic feet per hour; the burner is a standard Sugg's London Argand No. 1 for common coal-gas, a standard Steatite Batswing burner for canal gas; the candles are sperm candles, of six to the pound, each burning 120 grains per hour; and the quantity of gas used is to be corrected for temperature and barometric pressure. The candle is a very unsatisfactory unit of light; it varies as much as 6 per cent., and its colour is not the same as that of the gas-flame. Other standards have been proposed; of these the principal are the German standard candle—1.065 English sperm candle; the French Carcel lamp (648 grains colza-oil per hour)=10.441 English sperm candles; Mr Vernon Harcourt's pentane lamp, air + pentane-vapour, $\backslash frac\{1\}\{2\}$ cubic foot per hour, nearly equal to the English standard candle; Mr Methven's and Mr Fiddles's standard, in principal a given area of the bright part of gas-flame, this being, singularly, an almost uniform standard of illumination, not with any kind of illuminating gas, as was at first believed, but quite accurately so with pentane-vapour; Hefner-Alteneck's amyl-acetate lamp, with the flame turned up to a height of 1.6 inch, equal to 0.877 English standard candle; and the Dutch ether-benzol standard (1893)=1.48 English standard candle. Other photometers (Elster's, with movable standard light, &c.) have been proposed. Lowe and Sugg's jet-photometer depends on this, that assuming the height of the flame to be kept constant, the lighting-power of a jet is inversely proportional to the consumpt—or otherwise, that the consumpt being kept constant, the height of the jet-flame is directly proportional to the lighting-power. In Giroud's jet-photometer the height of the flame at constant pressure is taken as the measure of illuminating power; when the flame is about 6 inches high, a variation of about $\backslash frac\{1\}\{2\}$ inch corresponds to a variation of one-candle power, when the whole lighting-power is from 10 to 14 candles per 5 cubic feet. A Committee appointed by the Board of Trade in 1891, reported in 1895 that a flame of some kind must be used as the standard; that the sperm candle is unsatisfactory; that Mr Vernon Harcourt's pentane-vapour and air-flame is constant in brightness and easily reproducible when used as directed, and that it is accurately equal to an average standard candle; and that this should be made the basis of comparison, and called a candle; that for actual work with gas-flames it is better to compare these with more powerful sources of light than a candle, and that for this purpose a Diblin 10-candle standard (an air and pentane-vapour Argand flame with a Methven screen) should be used, with the Methven screen fixed so as to expose 2.15 inches of the flame. They also recommend that instead of burning gas at 5 cubic feet per hour, the gas should be burned at just such a rate as will give the required number of candles, and that the illuminating power be calculated back, and be stated as so many candles per 5 cubic feet. Photometrically the lime-purified gas of the south of England is greatly inferior to the iron-oxide purified gas of the north of England, and yet an impression of greater brightness is often experienced, for the flame is white instead of yellow.

Gas-work apparatus falls under thirteen heads.—The Retort-house contains the benches or sets of retorts in which the coal is distilled. The retorts were formerly small, and of cast-iron only; they are now generally larger and of fireclay; though the use of iron is again becoming familiar in cases where the last retort or two of a set are more easily heated if made of iron than when made of fireclay. Retorts are made round, oval, and D-shaped; the first of these is the strongest and most durable; the oval and the D-shaped are better carbonisers. Clay retorts are usually $2\backslash frac\{1\}\{2\}$ to 3 inches thick, oval, with diameters 15 and 21 inches inside, and 9 feet 4 inches long; but 'through' retorts are often used, corresponding to two ordinary retorts joined together so as to form one tube, some 20 feet long, with a mouthpiece at each end—a form which is more readily manipulated and more readily kept clear of coke-deposit. Even these diameters are somewhat too great, and the result is better with narrower retorts; and in small works smaller and shorter retorts are generally used. Of late years through-retorts, inclined at an angle of some 30°, have come greatly into use, especially in conjunction with mechanical appliances for charging and discharging the retorts: the coal slides down the retort from a hopper and is promptly spread out into a layer of uniform thickness, and the spent coke is easily drawn from the retort in a stream. To an increasing extent the coal is first raised to a height and then lowered in the successive operations to successively lower levels, so that manual labour is economised. The Dinsmore retorts are Z-shaped, and the tarry products are subjected to continued distillation in the upper bends. Mr Isaac Carr's modification of this process has been very successful in his own hands at Widnes; but it seems that the process has not been successful elsewhere. Five or seven retorts, and sometimes ten or more are built into each oven; and all the retorts of one oven are heated from the same source. This may be a coke furnace, in which case some $3\backslash frac\{1\}\{4\}$ cwt. of coke are used in distilling each ton of coal—i.e. about 25 per cent. of the coke made—a proportion which sinks in large works to 20 or 18 per cent.—or tar may be used as fuel, either dropped on hot plates or blown in by air or by steam as spray; or generator furnaces may be employed in which the fuel is first half-burned (CO being formed), and the hot furnace gases thus produced are burned under the retorts; or regenerative furnaces, in which the same thing is done, but the air which meets the furnace gases under the retorts is heated by the waste heat, which would otherwise have been allowed to escape through the flue after the retorts had been heated; the result being a great economy in fuel and in the wear of the retorts. The retorts, once heated up, are kept continuously at an orange-red heat ($2000^\{\backslash circ\}$ F.); they are charged with coal ($2\backslash frac\{1\}\{2\}$ to 3 cwt. each); the charge is raked out after four or six hours, and a fresh charge is put in; the charging and drawing being now often done by machinery. The duration of clay retorts depends on the treatment they receive; fifteen to eighteen months where directly exposed to the fire, or, where protected, three or four years, or even longer. In the Yeadon and Adgie revolving retort, small coal is fed in at one end and coke dust withdrawn at the other as the retort revolves; each granule of coal takes about 15 minutes to traverse the retort. Every retort is provided with a mouthpiece, through which the charge is put in and extracted, and the door of which is pressed home by a screw or lever and may or may not be secured by cement. The gas produced passes from the retort by means of a wide vertical ascending pipe, a very short horizontal bridge-pipe, and a short descending dip-pipe, which dips to a very slight extent below the overflow level of liquid in the hydraulic main. This hydraulic main is a wide tubular closed reservoir of wrought-iron, placed above the retorts; it has a large descending overflow-pipe; it is first filled with tar-water as far as it can be filled; the products of distillation from the retort pass through the hydraulic main; some tar is deposited, some watery liquid condensed; tar accumulates up to the overflow level, so that the gas passing through is washed in hot tar, and the light-giving constituents tend to become dissolved out to a large extent by the tar, unless the tar be kept sufficiently hot or be often enough removed from the hydraulic main. Down the overflow-pipe run the products of distillation, which sink into a tar-well, from which they are pumped out from time to time. This tar-well is also used as a general receptacle for condensation products deposited by the gas in its further course. The gas does not escape by this tar-well, for the overflow-pipe dips to an adequate depth into the liquid in the well; it passes on by a lateral horizontal tube. This device is repeated as often as is necessary.

The gas goes on to undergo a gradual process of cooling (to a temperature not below $55^\{\backslash circ\}$ F.) and farther condensation, partly in pipes led round the retort-house (in which the tar is largely deposited by friction while the gas is still hot), partly in the condenser. There are several types of condenser: (a) a series of vertical iron tubes in which the gas alternately ascends and descends, the cooling being due to the exterior air or to the trickling of water down the surface of the tubes; (b) vertical iron tubes of large size, concentrically arranged in pairs, so that the gas may slowly descend in the annular space between each two tubes, while the cooling air ascends the inner tube—the gas is then led up to the top of another annular space, and so on (Kirkham's); (c) a horizontal spiral; (d) a vertical zig-zag of pipes horizontally-laid; (e) arrangements for retarding the speed and thus enabling the gas, in comparative repose, more readily to deposit any particles; battery condenser; Mohr's condenser, in which the gas is guided through hollow cones, so as to run slowly. The cooled gas is then led to the washer, in which it is passed in fine streams through water, which dissolves ammonia, &c.; but here or farther on, after the scrubber, there is a suction arrangement, either a fan, a pump, or a steam-jet injector, called the exhauster. The coal being thus distilled in a partial vacuum, gas is more readily given off by it; and the gas once formed is rapidly removed from the retort and from the decomposing influence of the hot retort-walls, and its percentage in hydrocarbons is thus kept as high as may be; but there is at the same time a contrary tendency towards deterioration of quality, along with increase of yield, when the exhaust is at work. After the washer comes the scrubber, in which the gas is made to ascend a lofty column filled with coke or deal boards, down which water trickles, or is made to ascend a space filled with descending spray. Sometimes the gas is made, as in Pelouze and Audouin's so-called condenser, to deposit the last traces of tar by impact against solid surfaces; or may be made to run with or against a stream of hot tar, and thus to pick up hydrocarbons from the tar. Sometimes the functions of washer and scrubber are combined in one apparatus; sometimes a scrubber is used alone. The gas next passes through the purifiers, in which it has to pass slowly up, or better down, through an ample extent of thick layers of porous lime, or of iron oxide somewhat moist and rendered porous by sawdust, chaff, or other vehicle, or aided by porous magnesia, or through both, or else through washed Weldon slime. The gas ought, before this stage, to be free from all impurities, except carbonic acid, sulphuretted hydrogen, and bisulphide of carbon, and these are removed in the purifiers. There are various devices for absorbing these by means of ammonia and hydrocarbons separated in the earlier stages (Young, Claus, Hills). The British parliamentary standard of purity is that 10 cubic feet of gas shall not stain lead paper (absence of sulphuretted hydrogen); that the ammonia in the gas shall not exceed four grains per 100 cubic feet; and that the whole sulphur in the gas shall not exceed twenty-two grains per 100 cubic feet. The purifiers are so arranged that while a sufficient large area of purifying material shall always be encountered by the gas, one part of the purifiers after another is thrown out of action, and renewal of the material is thus possible, when required, without interruption to the purification. The valves and connecting pipes are so arranged as to permit this alternation to be readily effected: and throughout a gas-work, the pipes are so arranged as to permit any single piece of apparatus to be cut out of the gas-stream when required.

The gas goes on from the purifiers to the station-meter-house, in which there are (a) the station-meter, a large 'wet' meter for measuring the whole make of purified gas; (b) the exhaust, previously referred to; (c) pressure gauges, and (d) pressure-recording instruments; (e) the station-governor, by adjustment of which the pressure of gas as supplied from the gasholder to the mains is to be regulated. From the station-meter the gas goes on to the gasholder, or holders, to be stored and issued as required. The gasholder is an inverted cylindrical vessel of sheet-iron, placed in a tank of stone, brick, concrete, cast or wrought iron, steel, or a combination of these, but generally of brick or stone, lined with Portland cement, or backed with clay puddle, and, where possible, sunk into the ground. The tank contains water, in which the cylindrical vessel floats and rises or sinks. As the floating holder rises and sinks, it is kept vertical by tall columns which surround it, and guide its motion. On the tops of these columns are pulleys, over which run chains which at one end are connected to the crown of the gasholder, while at the other they bear suspended balance-weights. These balance-weights are not quite heavy enough to balance the weight of the floating vessel, which thus tends to descend and press the gas (contained between the water and the crown of the holder) out into the mains, and also back through the station-meter; but they so nearly poise the floating holder that the small pressure at which the gas is delivered through the station-meter is sufficient to lift the holder, and thus to enable gas to accumulate in it when there is no outflow through the main; and when there is such an outflow, the gas-holder oscillates up and down according to the proportion between the gas taken off from the mains and that supplied from the retorts. When the diameter of a gasholder is proportionately great, it does not need counterbalancing. It is comparatively not a heavy structure, and it contains a gas which is lighter than air, so that the pressure upon the base, so far as due to the sheet-iron holder and its contents, readily comes to be but little more than that which would have been due to an equivalent quantity of air. Mechanical ingenuity has been spent upon framing the holder by means of ribs, and internal bars, so as to give the maximum strength (freedom from buckling) with the least weight; and upon the construction of telescopic holders, in which the holder is constructed in two, three, or four lifts or cylinders, of which only the inner one has a crown. In each pair of cylinders the inner one has its lower free edge turned up, so that when it rises it hooks into the down-turned upper free edge of the outer cylinder, and, as the gasholder goes on filling, lifts the outer cylinder from the tank, and so, if there be more than two lifts, for each succeeding cylinder; the gas being prevented from escaping between any two of these mobile cylinders by the water which the inner one lifts from the tank in its upturned edge. Recently the construction of the gasometer has been managed in such a way as to dispense with the columnar guides. Necessarily the space within the gasholder above the tank water is, by means of pipes, placed in communication both with the station-meter and the mains. The function of the gasholders is a most important one; they act as a reservoir, and usually are of a capacity sufficient to contain a twenty-four hours maximum supply (the quantity used on a midwinter day); and they also equalise the pressure. The gasholder of the South Metropolitan Co. at East Greenwich has six lifts, a diameter of 300 feet, a height when inflated of 180 feet, and a capacity of 12,000,000 cubic feet. The gasholder ensures a regular supply at all hours both of day and night; and by its means a comparatively small plant, kept continuously working, is enabled to meet demands for which, if the gas were supplied direct from the retorts, it would be quite inadequate.

Before reaching the mains the pressure of the gas is regulated by the station-governor; an excessive pressure in the mains would result in excessive leakage. There are various devices for securing the automatic adjustment of resistance, whose amount is made to increase or diminish with the pressure; either by the gas lifting to a greater or less degree the floating bell of a small gasholder, and thereby altering the position of a conical or parabolic plug suspended within the entrance to the main, or (Hunt's) by working a throttle-valve.

The gas is conveyed from the works by main-pipes or mains, generally of cast-iron, carefully jointed; the jointing is effected either by turning and boring so as to make the pipes fit easily with a little white and red lead, or by using pipes which do not exactly fit, and making them do so by means of caulking, melted lead, india-rubber, or rust cement; in some cases the pipes are connected by ball-and-socket joints; in others, special provision is made for expansion.

At each lowest point provision is made for taking off water, as by a trapped drip-well, the liquid in which can be pumped out into a cart and taken to the gas-works. When mains supply a district the altitudes in which vary considerably, the tendency is for the local pressures to vary correspondingly; a difference of 100 feet in level makes a difference of 1.5 inch of water in a pressure-gauge; and therefore it is necessary to use district-governors which control the pressure in particular districts. To the mains are connected branch or service pipes, usually of wrought-iron or lead, in which the deposition of moisture is provided for, either by making the whole service-pipe drain into the main, or by fitting up a drip-well at each lowest point.

The gas supplied is measured by meters, of which there are two main varieties, the wet and the dry. The wet meter is a device for measuring out successive units of volume of gas; the reading will be the same whether the gas be delivered at low or at high pressures; and therefore the lower the pressure the less the absolute quantity of material in gas measured through a wet meter, and vice versa. In a wet meter there is a cylinder mounted on an axis; this cylinder is hollow, the hollow being divided into four parts or chambers by partitions, the longitudinal boundaries of which present the form of an Archimedean screw or the rifling of a gun; the gas enters one of these spiral chambers at one end; as the gas is pressed in, it displaces water and makes the hollow space lighter than water; it thus makes the hollow tend to rise, and in that way works the cylinder partly round. No gas can pass through the chamber until it is completely full. When one chamber has been completely filled, two things happen: the entering stream of gas now finds an inlet into the succeeding chamber; and, secondly, the gas in the first chamber finds a possible outlet at its opposite end, through a slit which now begins to emerge above water-level. As the cylinder goes on rotating, the first chamber comes to sink under water; water enters the chamber and gas leaves it; and so for each of the four chambers in succession. The axle, thus made to rotate in proportion to the amount of gas delivered, works a train of wheelwork which by means of pointers shows the number of 10,000's, the number of 1000's, and the number of 100's of cubic feet of gas which have passed through the cylinder. The water must be kept at a constant level; it may freeze, for which reason the meter should be kept in a sufficiently warm place (not too warm, else the gas will expand and the meter give too high a reading), or else a non-freezing liquid should be used; and the water damps the gas. There are contrivances for maintaining the water-level constant; the meter sometimes shuts off the gas when the water is too low. Thus there may be an automatic addition of water from a subsidiary reservoir, or an automatic maintenance of level by a hinged float which sinks into the water when liquid fails to support it in its uppermost position (as in the constant-level inkstands); or, there may be (Warner and Cowan) a contrivance for transferring the excess of gas delivered at each revolution, when the water is too low, back again for measurement. When the meter is driven too fast the record is too low; but backwash in the meter then causes flickering at the jet; and the general use of meters too small for the work which they have to do is conducive to leakage in the district within which they abound, on account of the high pressure necessary to force gas through them.

Dry meters are, in principle, a variety of piston-meter; the fluid is measured by displacing a piston or diaphragm, and thereby filling a measured cavity. They consist of two or three separate chambers; each chamber is divided into two by a diaphragm, which may be displaced to one side or the other. The gas is admitted to the one side of this diaphragm until it is displaced to the full extent of its range; when this occurs the gas is admitted to its other side, and the gas previously admitted is allowed to go on to the burner, and so on alternately. The chambers act alternately, thus passing the dead-points. The diaphragms are connected with wheelwork which record their successive oscillations, and represent on the dials the corresponding number of cubic feet passed through the apparatus. By an act of parliament (1859) all gas-meters must register not more than 2 per cent. in favour of the seller and not more than 3 per cent. in favour of the purchaser of gas; and meters must bear the seal of an inspector appointed under the act. Meters have recently been introduced which enable the poorer consumer to purchase gas by pennyworths on the familiar 'penny in a slot' principle ('coin' meters), or to pay into the meter a definite sum which will allow the mechanism to transmit the prearranged quantity of gas ('stop' meters). In Brussels the gas burned by day and that used at night were for some years registered on different dials of the same meter.

The lighting power of a gas is measured in terms of the number of candles to which a 5-feet standard flat-flame is equivalent. The lighting value of a gas is measured by the number of candle-hours it will yield per 1000 cubic feet when burned in standard burners; thus 1000 cubic feet of 20-candle gas will keep up a light of 20 candles for 200 hours (using 5 cubic feet per hour), and its lighting value is 4000 candle-hours, or, as it is generally abbreviated, 4000 'candles.' Since a standard candle shines for one hour at the expense of 120 grains of sperm consumed, the lighting value of a gas is frequently stated as so many grains of sperm; thus the 'sperm value' of 20-candle gas is $20\; \backslash times\; 200\; \backslash times\; 120\; =\; 48,000$ grains per 1000 cubic feet. During recent years cannel coal has become too expensive to make gas from, and the use of cannel gas has been given up in the limited region of the west end of London to which it was formerly supplied. Gas-makers have, therefore, had to reduce their standard, as in Edinburgh, where the 28-candle gas has been replaced by 24-candle gas, or else to turn their attention to the enrichment of a poorer gas made from ordinary coal. This enrichment is effected by the addition of hydrocarbon vapours in various forms to the poorer coal-gas. If gas of higher quality be made by a more costly process, so that it costs say $d^1$ pence per 1000 cubic feet to make gas of a lighting power $C^1$, instead of $d$ pence to make gas of a lighting power $C$, the cost per additional candle of lighting power is $\backslash \{(d^1\; -\; d)\; \backslash div\; (C^1\; -\; C)\backslash \}$ pence per 1000 cubic feet of gas made. If the enriching gas be added in the proportion of $f$ cubic feet to 1000 of coal-gas, of a lighting value of $C$ candle-hours per 5 cubic feet, then if the resulting $(1000\; +\; f)$ cubic feet of enriched gas have a lighting value of $C^1$ candle-hours per 5 cubic feet, and if the original gas and the added enriching gas respectively cost $d$ and $d^1$ pence per 1000 cubic feet, the additional cost per 1000 cubic feet of gas made is $\backslash \{f(d^1\; -\; d)\; \backslash div\; [(1000\; +\; f)(C^1\; -\; C)]\backslash \}$ pence per additional candle of lighting power. If we add a richer gas to a poorer, the lighting power of the mixture is generally not equal to the arithmetical mean as deduced by calculation; there is generally deterioration due to dilution; but it often happens that if we add a little poor gas to an exceedingly rich one the lighting power is higher than we would have expected. But if we apply to the actual results of enrichment the same methods which we would use if there had been no deteriora- tion, we obtain a useful nominal value for the lighting power of the richer gas, which is called its 'enrichment value.' Thus if we mix $13\backslash frac\{1\}\{2\}$ cubic feet of oil-gas, of an unknown enrichment value $C^1$, with 1000 cubic feet of 14-candle coal-gas, and obtain $1013\backslash frac\{1\}\{2\}$ cubic feet of 15-candle gas, we find, from the equation $1013\backslash frac\{1\}\{2\}\; \backslash times\; 15\; =\; (1000\; \backslash times\; 14)\; +\; 13\backslash frac\{1\}\{2\}C^1$, that $C^1\; =\; 90$ candles, the nominal lighting power of the enriching gas, or its enrichment value. As means of enrichment by mere admixture, we have benzol-vapour, which is much used on the Continent, and which for small enrichment adds about 4700 candle-hours per gallon of benzol evaporated into the gas; carburine or light petroleum oil (practically hexane, $C\_6H\_\{14\}$), used to some extent in London under the Maxim patents, and adding about 1600 candle-hours per gallon evaporated; and oil-gas. Oil has also been employed as spray injected into the coal-retorts themselves; and coal-gas is largely carburetted by being exposed, along with the vapours obtained by the distillation of oil, to a high temperature, so that these vapours may be rendered more 'permanent,' or less liable to condense in transit through the pipes.

It is of great importance that in the first place gasfittings should be adequate to supply the maximum demand for gas; and in the second, that the gas should emerge from each burner under a low pressure. If the gasfittings—pipes, &c.—be inadequate, as they mostly are, full flames cannot be produced, and the light is unsatisfactory; if, on the other hand, the full pressure of the mains is communicated too directly to the gas-burners themselves, there is a tendency to flare. This can be mitigated by partially turning off at the meter; but even then the variable demand may result in variable pressures at the burners. There should be a governor for each gas-burner, or for each small group of gas-burners; these are now readily procurable, and when they are used a full flame is obtained which is constantly and steadily kept up by a comparatively slow supply of gas; the incandescent particles or heavy heated hydrocarbon vapours upon which luminosity depends are allowed to remain as long as possible in the flame, and the gas is thoroughly burned; and air is not swirled into the interior of the flame by the swift current of gas, thus spoiling the luminosity. An ordinary burner gives greatly superior results when governed; since the electric light has caused more attention to be paid to the efficient burning of gas, the burners themselves have been greatly improved; but burners should always be selected with reference to the quality of gas to be used in them.

The ordinary ratstail burner has long given place to the batswing and fishtail burners, the former of which are made with a clean slit across the head of the burner; the latter have two passages converging towards one another, the result being that the two streams of gas meet one another and spread out into a flat sheet of flame. The former use much gas at ordinary pressures, and a very small pressure ($\backslash frac\{1\}\{2\}$-inch of water just below the burner) is sufficient to bring out the full lighting-power. In hollow-top burners the pressure is relieved by the gas swirling in a cavity below the outlet-slit. Burners of these classes should always be selected with steatite tops; metal burners soon rust and spoil the flame. In Argand burners the gas issues through a ring of holes; the flame is tubular, and is surrounded by a chimney; air ascends both inside and outside the tubular flame. In Dumas burners the circle of holes is replaced by a circular slit, and a regulator controls the admission of air. These various burners have also been collected in groups to form the so-called sunlights, and so forth; but the recent remarkable progress in gas-lighting has been due to the study of the mutual actions of flames, and to the use of hot air and sometimes hot gas. For example, we have concentric Argand flames (Sugg); porcelain cylinders in the axis of an Argand flame to keep the flame from flickering, to keep up the heat of the flame, and also themselves to radiate light when incandescent; burners in which gas from a circular slit plays on the under surface of a porcelain globe; and especially regenerative burners of various models, generally with inverted flames, in which the heated products of combustion are made to heat the incoming air. Globes and shades cut off a good deal of light; a clear glass globe cuts off from 9 to 12 per cent.; ground glass about 40; opal globes about 60. Globes should never have a lower aperture narrower than 4 or 5 inches; the ordinary narrow aperture makes a strong draught of air, which materially weakens the brightness of the flame, and insteades it. For use with incandescent mantles globes are now made with surfaces mathematically faceted ('holophanes') or channelled ('diffusers') which distribute the incident light and spread out the light so as to make it apparently fill the globe.

Sometimes gas is burned with air in a small Bunsen burner, and over the flame is fitted a basket of platinum wire (Lewis), or a small mantle consisting of thoria along with a little ceria (Auer von Welsbach), which emits a brilliant white light on incandescence; or the ordinary flame of gas may be rendered more luminous by passing the gas over melted naphthaline, which it takes up (Albo-carbon). In Denayrouze's modification of the Bunsen burner, the gas and air are effectively mixed by means of a little fan-wheel driven by a minute electromotor; the flame is altered in character and becomes intensely hot; if a Welsbach mantle be used with such a burner, the lighting effect goes up as high as 270 candles with a consump. of 9 cubic feet of London gas per hour. In Bandsept's Bunsen burner, the gas and air are similarly mixed by means of a baffler immediately under the flame; the result is about $\backslash frac\{3\}\{4\}$ the light given by a Denayrouze.

For heating purposes, coal-gas mixed with air produces a smokeless flame and a higher temperature than it does when burned in luminous flames; and so for direct heating the Bunsen Burner (q.v.) principle is suitable. In one modification of the Bandsept Bunsen burner the air is driven through an inverted injector under high pressure, dragging gas with it, and being mixed therewith; and the flame is produced under the surface of any liquid which it may be desired to heat up. Thus about 90 per cent. of the heat evolved is utilised directly. Gas produces the same quantity of heat, provided that it is completely burned, in whatever way it is burned. Convenience, cleanliness, may often determine the use of Bunsen flames; but where radiation is expected to come into play the luminous flame is more effective—as for cooking (see WARMING). Coal-gas for cooking is economical, as it can be turned off when not wanted, and turned on at once; and it is smokeless if properly burned. Of course it ought not to be left unprovided with a chimney. For ventilation, a well-arranged system of lamps, especially of the regenerative type, will provide motive power for carrying away their own products of combustion and for renewing the air of the room. Gas is largely used for gas-engines (q.v.), which in 1896 were being made up to 1000 horse-power.

The price of light obtained from coal-gas may be ascertained by finding the cost of a candle-hour—the light of one standard sperm candle for one hour—in each case. The table combines the data of Stevenson Macadam, Letheby, Thompson, Poris, and others, and gives the price per candle-hour, in thousandths of a penny:

The price of gas, like the quality, will vary from place to place, owing to differences in the price of coal, the cost of the works, and so forth. In the London Gas-light and Coke Company's accounts we find the gross cost of manufacture of each 1000 cubic feet of gas sold is 23.418 pence; the residuals—coke, breeze, tar, and ammoniacal liquor—return 9.036d.: so the net current cost at the works is 14.382d. for each 1000 cubic feet sold; the cost of distribution is 3.571d.; public lighting involves an outlay of 0.437d.; rates and taxes come to 2.696d.; management to 0.894d.; various charges (bad debts, annuities, legal expenses, &c.), come to 0.546d.—altogether 22.526d.: which meter and stove rents, &c., bring down to 22.144d. The average price of the gas sold is 33.705d.; the difference, 11.561d. per thousand on a sale of 9,453,889,000 cubic feet in six months, corresponds to a gross profit of just over 8$\backslash frac\{1\}\{2\}$ per cent. per annum on the paid-up capital of £11,198,000. The capital value of the works of this company in January 1896 was £11,792,851, 9s. 11d.; that of the South Metropolitan Company was £3,405,715, 4s.; and that of the Commercial Company, £877,951, 10s. 9d.

The risks of gas-lighting are twofold—explosion and poisoning. Explosion cannot occur until there is about 6.6 per cent. of gas in the air, but it is dangerous to 'look for a leak with a light.' As to poisoning, the gas must escape into a room without being noticed until there is about one-half per cent. of carbonic oxide—i.e. from 4 to 12 per cent. of coal-gas—in the air of the room, before danger to life becomes imminent; and this percentage is rarely attained by ordinary escapes into rooms of fair size. Fatal accidents have generally happened from escapes into small rooms, and also from the travelling of gas from broken mains through earth into an earth-floored house, which may draw the earth-gases through it in a deodorised condition. A gas-escape is most likely to be serious in its consequences when it takes place into the upper part of a room; the percentage near the ceiling may then come to be much greater than it is at first lower down (see POISONS).

From 1639 onwards the attention of scientific men had repeatedly been turned to 'burning springs' or streams of 'inflammable air' issuing from wells and mines in the coal districts of England, and communications on the subject were addressed to the Royal Society of London. Some time before 1691 the Rev. Dr John Clayton, Dean of Kildare, addressed a letter to the Hon. Robert Boyle, in which he described experiments on the production and storage of inflammable gas distilled from coal; and this letter was published in the Royal Society's Transactions for 1789. In 1787 Lord Dundonald made some domestic experiments on lighting by coal-gas. In 1792 William Murdoch lit up his house and office at Redruth in Cornwall; in 1798 he lit up a part of Boulton & Watt's manufactory at Soho, Birmingham; and in 1805, with 1000 burners, the mills of Messrs Philips and Lee at Salford. In 1801 Le Bon lit his house with coal-gas, and in 1802 he proposed to light a part of the city of Paris. In 1803 Wintzer or Winsor lectured in London upon the new light; he was a sanguine projector, holding forth fantastic hopes, but was instrumental in founding the Chartered Gas Company which obtained its Act of Parliament in 1810. In 1813 he was replaced by Mr Samuel Clegg, who had been managing Boulton and Watt's gas-lighting since 1805 in succession to Mr Murdoch, and who was the inventor of the hydraulic main, the wet meter, and the wet-lime purifier. In 1813 Westminster Bridge was lighted by gas, and immediately thereafter the new method of lighting made very rapid progress in Great Britain and other countries; and in the contest for supremacy between coal-gas and oil, wood, and peat-gas, which were at one time somewhat extensively tried, coal-gas took the lead.

II. Oil-gas is prepared from heavy mineral oils (sq. gr. = 0.9) or paraffins, from their residues, and sometimes from spent grease, suint, waste mutton fat (in Australia), &c. One hundred lb. of oil yields from 722 to 1092 cubic feet of gas, of which one cubic foot per hour yields a light of 10 to 12 candles. The oil is made to flow in a thin steady stream into cast-iron retorts, heated to between 900° and 1000° C.: these retorts are horizontal or vertical, or are in some cases so arranged that gas formed in one retort or section of a retort is further heated in another retort or in another section of the same retort. The condensation requires special attention; oil-gas has a tendency to carry non-permanent vapours with it, and these must be removed. The purification necessitates the use of scrubbers, purifiers, and so on as in coal-gas. Even in refined paraffin and petroleum oils there is sulphur present often far in excess of that contained in an equivalent quantity of coal-gas. Oil-gas must be burned at a low pressure and in small burners; the standard burner is No. 1 (1 cubic foot per hour). Oil-gas is used for lighting railway carriages; the gas, carefully purified, is compressed at 10 atmospheres' pressure; it is then transferred to the reservoirs borne by the railway carriage, each of which carries, at 6 atmospheres' pressure, enough gas for 33 to 40 hours' lighting; a regulator governs the pressure at the burners, and each burner, consuming 0.777 cubic feet per hour, gives 7 candle-light. Compressed oil-gas has also been applied to the lighting of buoys, and to some extent to steamship lighting. In the Young & Bell process, oil is made to trickle from cooler to hotter regions, but at no point is the temperature relatively very high; as the oil descends, any given constituent of it meets a temperature competent partly to decompose it into lighter and heavier hydrocarbon gases and vapours: the gaseous and vaporous mixture produced travels upwards and meets the down-flowing stream; this stream dissolves everything except the lightest gases and vapours, which pass off as oil-gas, without being subjected to any excessive temperature, while the materials dissolved find their way back towards the retort, and are again subjected to heat and further decomposition. The only by-product is a very pure form of coke. This gas has an enrichment value of about 90, and may be applied to the enrichment either of ordinary coal-gas or to that of poor gas or water gas. Mr Tatham mixes oil-gas with about 15 per cent. of oxygen, and thereby enables the gas to be burned directly in greater volume with ordinary small burners, so that a lighting power is attained equivalent to 100 candles per 5 cubic feet. The light is brilliantly white, and the flames are not so small that they are chilled by the burner itself.

III. Peat-gas and IV. Wood-gas are occasionally used. Wood-gas is a by-product in the preparation of pyroligneous (crude acetic) acid; its lighting-power is about 20 candles; the yield is 546 to 642 cubic feet per 1000 lb. of wood; of the crude gas 20 to 25 per cent. consist of carbonic acid. Peat yields 320 to 500 cubic feet of gas per 100 lb.; lighting-power about 18 candles; the carbonic acid in the crude gas is about 30 per cent.

V. Producer Gas.—When a limited stream of air is driven through glowing coke, the coke is first burned to carbonic acid; the carbonic acid, as it travels through the remainder of the brightly glowing coke, takes up carbon and, for the most part, becomes carbonic oxide; the resultant gaseous mixture consists of carbonic oxide (about 26 per cent.), the nitrogen of the air employed (about 70 per cent.), and some undecomposed carbonic acid (about 4 per cent.). This mixture is combustible with a clean flame, and this kind of fuel is now largely employed (generally with utilisation of the waste heat to warm the incoming current of air, as in the so-called regenerative furnaces) for heating the retorts in coal-gas-making, in metallurgical operations, in glass and pottery making, and in boiler firing. The furnace hearth becomes a clear, clean, deoxidising region of intense heat without visible flame. The gas from the producer is very hot; if it be passed at once into the furnace, a large proportion of the heat of the coke may be utilised; if it be allowed to cool, a considerable percentage is lost. The usual yield of producer gas is from coal (Siemens) about 160,000, from coke about 175,000 cubic feet per ton; the heating values are, for cooled gas, respectively 29,700 and 26,900 calories per thousand cubic feet, or altogether 60 and 68 per cent. of those of the respective materials employed.

VI. Producer Water-gas.—When mixed air and steam are driven through glowing coke (or anthracite, Dowson), the air keeps the coke glowing, and, as in the previous case, produces carbonic oxide, carbonic acid, and nitrogen; the steam acts on the glowing coke and produces hydrogen and carbonic oxide; the result is a mixture whose composition varies according to the relative quantities of air and steam, and according to the temperature in the producer; as an average it may be said to consist of 9 per cent. of carbonic acid, 24 of carbonic oxide, 13 of hydrogen, and 54 of nitrogen. If an excess of steam be used, there is more hydrogen, more carbonic acid, and less carbonic oxide. The usual yield is about 168,000 cubic feet per ton of material; the heating value is about 33,500 calories per 1000 cubic feet; altogether about 80 per cent. of that of the coke and anthracite employed.

VII. Water-gas.—In 1793 Lavoisier discovered that when steam, unmixed with air, is passed through glowing coke, the coke is oxidised; carbonic oxide and hydrogen gas are produced, theoretically, pure and in equal volumes; practically, the product contains 3 to 8 per cent. of carbonic acid, and 4 to 9 of nitrogen. The yield is from coke (7,000,000 calories per ton) about 35,000 cubic feet, with a heating value of about 75,000 calories per 1000 cubic feet, or on the whole about 40 per cent. of the heat-value of the coke; from coal (7,800,000 calories per ton) about 42,000 cubic feet, at 95,000 calories, or about 49 per cent. In the process the steam cools down the glowing coke; consequently air must be sent through the coke at intervals (about 4 minutes steam and 10 minutes air) in order to restore its glow; and a series of producers must be so conjoined as to act alternately with one another, before the process can result in a continuous supply of water-gas. The by-product, producer gas, which may be produced in large quantities (110,000 cubic feet, at 26,900 calories per 1000) by regulating the supply of air while the coke-glow is being worked up, may be used for boilers or for gas-engines. When it is so utilised, the net cost of making simple water-gas is between 5d. and 6d. per 1000 cubic feet, about 8d. per 1000 less than coal-gas. Water-gas gives on combustion an extremely high temperature, which saves time in furnace work; gold, silver, and copper, and even an alloy of 70 parts of gold and 30 of platinum are readily melted in quantity by it; hence for bringing objects such as Fahnehjelm's combs (a series of rods of magnesia) into brilliant luminous incandescence, for welding, or for metallurgical operations involving high temperatures, it is very suitable; and in gas-engines it works cleanly. When water-gas is used with Fahnehjelm combs, the quantity of gas used is (Dr F. Fischer) 180 litres, or $6\backslash frac\{1\}\{2\}$ cubic feet per hour, the light being, when the burner is new, 22 to 24 candles, and after 60 hours, reduced to 16. The combs (15s. per hundred) require renewal after 100 hours' use. As a carrier of heat, coal-gas is twice as effective in respect of quantity of heat; its heating-power is about 150,000 calories per 1000 cubic feet, which represents about 20 per cent. of the whole heat of the coal distilled, or about 50 per cent. after allowing for the heating-power retained in the coke, breeze, and tar; and this concentration of heating-power in smaller bulk may in some cases transfer the advantage of cheapness, through smaller cost of distribution, to coal-gas. Water-gas is much used in the United States. It is supplied to houses, either pure or mixed with the coal-gas produced in the manufacture of the coke from which the water-gas is made, and it is then known as 'fuel-gas'; but more generally it is carburetted by being exposed to a high temperature along with naphtha or petroleum vapours, and the resultant mixture is employed as illuminating gas. Unfortunately the high percentage of carbonic oxide, which is odourless, has caused a high death-roll.

VIII. Acetylene.—This gas, $C\_2H\_2$, long a chemical curiosity merely, is now prepared on the large scale by the action of water upon calcium carbide, which is made by exposing a mixture of lime and carbon to the temperature of the electric arc in the electric furnace. The carbon unites with the hydrogen of the water, forming acetylene; the calcium with the oxygen, forming lime, which, as slaked lime, remains in the water. This gas gives, with a half cubic-foot burner, an intensely white solid-looking flame of 24 candle-power. For enrichment its enrichment value is about 100 candles for about the first five candles of additional illuminating power; after which the effect of dilution wears off, and the enrichment value may go up to about 150. Sufficiently dilute pure acetylene is not appreciably poisonous; but it has a characteristic disagreeable odour, partly due, when it is made from carbide, to traces of phosphuretted hydrogen. A ton of carbide produces about 11,000 cubic feet of acetylene; and though estimates have been published which show a cost of £4 per ton, the manufacturers have not been able, in Europe, to put it on the market at less than 405s. a ton (1896).

IX. Natural Gas issues from the earth in many places—the eternal fires at Baku (q.v.), for example; from other gas-wells in the Caucasus, natural or opened in boring for oil; in China; but principally in North America. At Fredonia, New York state, gas escaping from the earth was used in 1821. In 1859 boring for oil in Pennsylvania and elsewhere became general; the gas associated with this was conveyed to a distance and burned as a nuisance. The general utilisation of the gas began in 1872 at Fairview, Butler County, Pennsylvania. Many of the gas-wells lasted only four or five years. In 1874 the gas was used in iron-smelting, and by 1884 one Pittsburg company used gas equivalent to the produce of 400 tons of coal a day. Pittsburg, formerly lying under a continuous black pall of smoke, became bright and clear. But now the supply has fallen off, and Pittsburg has been supplied with gas from West Virginia, at a distance of 102 miles (and see above at p. 98). Chicago is supplied with natural gas from Greentown, Ind., at a distance of 116 miles. Nearly all the gas obtained is now distributed by pipes and pumping engines, and in the United States of America about 400 million cubic feet per day are thus distributed. Natural gas is also found by boring elsewhere in Pennsylvania, in Ohio, Indiana, Kentucky, Illinois, Kansas, the Dakotas, and at Los Angeles in California. The North American gas consists mainly of marsh-gas; sometimes it contains nothing else than marsh-gas and a little carbonic acid; sometimes there are various percentages of hydrogen, ethylene, traces of carbonic oxide, nitrogen, oxygen, or heavy hydrocarbons. The Baku gas contains 3 per cent. of heavy hydrocarbons, and is more regularly deficient in hydrogen. The American gas is used for all metallurgical processes except the blast-furnace, and is very convenient for glass-making. In some places the gas is carburetted or used with Fahnehjelm's combs. Natural gas may possibly underlie the English salt-beds.

See King's work on coal-gas edited by Newbigging, whose Gas Managers' Handbook is also valuable; Wanklyn's Gas Engineers' Chemical Manual, and Butterfield, Gas Manufacture (1896) for chemistry; Field's Analysis, and the Gas World's yearly analyses.