When coal is burning in a common fire, we may see jets of smoky gas issuing from the pieces of coal before they become red hot. This vapour, coming in contact with flame in another part of the fire, may often be observed to ignite, thus supplying an instance of gas-lighting in its most elementary form. In the ordinary fire the air has free access, and the inflammable gases and vapours continue to burn with flames more or less bright, and when these have ceased the carbonaceous portion continues afterward to glow until nearly the whole has been consumed, except the solid residue which we call the ashes. These ashes in general contain a portion of unconsumed carbon, mixed with what is chemically the ash, namely, certain incombustible salts, constituting the white part of the ashes. If, however, we heat the coal in a vessel which prevents access of air, and allows the gases to escape, the coal is decomposed much in the same way as when it is burnt in the open fire; but the products formed are no longer burnt, the supply of oxygen being cut off. Every one knows the familiar experiment of filling the bowl of a common clay tobacco-pipe with powdered coal, then covering it with a dab of clay, and placing it in a fire. The gas which soon comes from the stem of the pipe does not take fire unless a light be applied, when it may be seen to burn with a bright flame, and after the flow of gas has ceased, nearly the whole of the carbon of the coal will be found unconsumed in the bowl of the pipe. This simple experiment illustrates perfectly the first step in the manufacture of coal-gas, namely, the process of heating coal to redness in closed vessels, by which operation the substances originally contained in coal are destroyed, and their elements enter into new combinations.

These elements are few in number; for, except the very small portion which remains as incombustible white ash, coal is constituted of carbon, hydrogen, oxygen, nitrogen, and a little sulphur. All the varied and interesting products obtained by the destructive distillation of coal are combinations of two or more of these four or five elements. Illuminating gas is far from being the only product when coal is heated without access of air; for of the numerous substances volatized at the red heat of the gas-retort a great number are not only incapable of affording light, but liable to generate noxious compounds when burnt. Besides this there are numerous bodies which, though leaving the retort in the gaseous form, immediately assume the liquid or solid state at ordinary temperatures. All such substances must be separated before permanent gases are obtained fit for illuminating purposes and capable of being carried through pipes to distant places. Thus an important part of the apparatus for gas manufacture consists in arrangements for separating the condensable bodies, and for removing useless or injurious gases from the remainder.

The products resulting from the destructive distillation of the coal may, therefore, be classified as—a, solids left behind in the retort; b, solids and liquids condensed by cooling the vapours which issue from the retort; c, coal-gas—a mixture of gases from which certain useless and noxious constituents must be removed. Fig. 347 is intended to give a diagrammatic view of the apparatus employed in the generation, purification, and storage of gas, the various parts being shown in section. A is the furnace containing several retorts, of which B is one. From each retort a tube, d, rises vertically, and curving downward like an inverted U, it enters a long horizontal cylinder, f; half filled with water, beneath the surface of which the open end of the recurved tube dips. The cylinder containing water passes horizontally along the whole range of furnaces in the gas-works, and is known as the hydraulic main. It is here, then, the tar and the moisture first condense, and the pipe is always kept half full of these liquids, so that the ends of the pipes, d, from the retorts, dipping beneath its surface, form traps or water-valves, which allow any retort to be opened without permitting the gas to escape. As the tar accumulates in the hydraulic main, it flows over through a pipe, g, leading downwards into the tar-well, H. The gases take the same course; but while the tar flows down the vertical tube, R, the gases pass on through j into the condensers or refrigerators. Gas cannot escape from the open end of the tube, for it is always closed by the liquids—tar and ammoniacal liquor—which accumulate and flow over the top of the open inner vessel into the cistern, S, from which they are drawn off from time to time by the stop-cock, I. Although when the gas has arrived at this cistern much of its tar and ammoniacal vapours have been condensed, a portion is still retained by reason of the high temperature of the gas; and this has to be removed before it is permitted to enter the purifier. This is the object of passing the gas through the series of pipes, j j, forming the condenser. These are kept cool by the large surface they expose to the air, and, when necessary, cold water from the cistern, K, may be made to flow over them. The tar and other liquids condense in the iron chest, T, which is so divided by partitions as to compel the gas to pass through the whole series of tubes; and as the liquid accumulates, it also overflows into the tar-well. The cooled gas then enters the purifier, L L, in which are layers of slaked lime placed on a number of shelves. By contact with the extensive surface of slaked lime the gas has its sulphuretted hydrogen, carbonic acid, and some other impurities, removed; and it then, through the tube n, enters the gasholder, in which it is stored up for use.

Hydrated oxide of iron is now much used for purifying coal-gas. The oxide is mixed with sawdust, and placed in layers 10 in. thick. Sulphide of iron and water is formed; and when the mixture has ceased to absorb any more, it is removed and exposed to a current of air; the hydrated oxide is thus reproduced and sulphur set free. The process may be repeated many times in succession, until the absorbent power is impaired by the accumulation of sulphur.

The gasholder—or “gasometer,” as it is often improperly named—is an immense cylindrical bell, made of wrought iron plates, and inverted in a tank of water, in which it rises or falls. It is counterpoised by weights attached to chains passing over pulleys, so as to press the gas with a small force in order to drive it along the main, which communicates with the pipes supplying it to the various consumers. The pressure impelling the gas through the mains does not in general exceed that of a column of water two or three inches high.

It will be necessary, after this slight outline describing the essential parts of the apparatus, to enter more fully into the details of the several parts.

The retorts are constructed of wrought iron, cast iron, or earthenware, and in shape are cylindrical, with a diameter of 12 in. to 18 in., or more, and a length of 6 ft. to 10 ft. Though sometimes circular in section, other forms are commonly used—such as the elliptical, and especially the ?-shaped. The retorts are closed except at the mouth-end, Fig. 348, from the top of which rises the stand-pipe, A, which has usually a diameter of about 5 in. When the charge has been introduced, the mouth is closed by a plate of iron, B, closely and securely applied by means of a screw, C, as shown in the figure—a perfectly tight joint being obtained by a luting of lime mortar spread on the part of the lid which comes into contact with the mouth of the retort. The retorts are always set horizontally in the furnace—each furnace usually including a set of five retorts. The charge of coals is introduced on a tray of sheet iron adapted to the size of the retort, which, when properly pushed in, is inverted so as turn out the contents, and then withdrawn.

The time required to completely expel the volatile constituents from the charge in a gas retort varies very much, because there are great diversities in the composition of the different kinds of coal employed. Some varieties of coal, such as cannel, are easily decomposed, and the operation may be complete in about three hours; while other kinds may require double that time. The quantity of gas procurable from a given weight of coal also varies according to the kind of coal made use of. Thus, while a hundredweight of cannel may give 430 cubic feet of gas, the same weight of Newcastle coals will yield but 370 cubic feet. The nature of the gases given off from a retort will be different at the different stages of the operation.

The scene presented by the retort-house of a large gas manufactory, when viewed at night, is a singular spectacle. The strange lurid gleams which shoot out amid the general darkness as the retorts are opened to withdraw the coke, and the black forms of the workmen partially illuminated by the glare, or flitting like dark shadows across it, form a picture which might engage the pencil of a Rembrandt. In Fig. 348a is depicted the retort-house at the Imperial Gas Works, King’s Cross. Here the retorts are arranged in several tiers—the coal being brought, and the coke withdrawn, by the aid of an iron carriage running on rails parallel to the line of furnaces.

In the process of heating, a proper regulation of the temperature is of the highest importance. It is found that when the retorts are heated to bright cherry-red, the best results are obtained. At a lower temperature a larger quantity of condensable vapours are given off, which collect in the gasholders and distributing pipes as solid or liquid, and occasion much inconvenience, while the quantity of gas obtained is decreased. On the other hand, if the temperature be too high, some of the gases are decomposed, and the quantity of carbon contained in the product is so much diminished as seriously to impair the illuminating power. Again, every second the gases after their production remain in the red-hot retort diminishes their light-giving value; for those hydro-carbons on which the luminiferous power of the gas depends, are then liable to partial decomposition; a portion of their carbon is deposited on the walls of the retort in a dense layer, gradually choking it up, while the liberated hydrogen does not add to the illuminating but to the heating constituents of the gas. A plan has been patented by Mr. White, of Manchester, for rapidly removing the illuminating gases from the retort by sweeping them out by means of a current of what has been termed “water gas.” This water gas is produced by causing steam to pass over heated coke, and is a mixture of carbonic acid, carbonic oxide, and hydrogen. Though only two of these are combustible gases—and even they do not yield light by their combustion, and, by adding to the bulk of the gas, serve rather to dilute it—yet it has been found that in some cases twice the amount of light is obtainable by White’s process than the same weight of coal supplies when treated in the ordinary manner.

The hydraulic main, as already mentioned, being kept half full of tar into which the lower ends of the dip-pipes descend, prevents the gas from escaping through the stand-pipes when the lid of a retort is removed for the introduction of a fresh charge. The hydraulic main is from 12 to 18 in. diameter, and the dip pipes pass into it by gas-tight joints. Various forms of purifiers are in use besides the simple one already mentioned. Some of these have arrangements for agitating the gas with a purifying liquid by mechanical means, the motion being supplied by a steam engine.

The gasholder, as it sinks in the water of the cistern, presses with less force on the contained gas, and unless this inequality of pressure were counteracted there would be very unequal velocities in the flow of gas from the burner. The equality of pressure is obtained by making the weight of the chains by which the gasholder is suspended equal to half the weight the gasholder loses in the same length of its motion. Gasholders are also constructed without chains or counterpoises, as these are found to be unnecessary where the height of the gasholder does not exceed half its width. In such cases, especially when the vessel is very large, the difference of pressure at the highest and lowest position is quite inconsiderable, and nothing more is necessary than that upright guides or pillars be placed to preserve the vertical motion of the vessel. Another improvement, which enables a lofty gasholder to be used without increasing the depth of the tank, consists in forming the gasholder of several cylinders, which slide in and out of one another like the draw-tubes of a telescope. Each cylinder has a groove formed by turning up the iron inside the rim, and at the top of the next cylinder the edge is turned outwards so as to drop in the groove or channel, which thus forms a gas-tight joint, for it is of course filled with water as it rises. The pressure is, however, more accurately regulated by an apparatus called the governor, through which the gas passes in before it enters the mains. The construction and action of the regulator will be understood from Fig. 349, where A represents a kind of miniature gasholder, inverted in the cistern, B. From the centre of the interior of the bell hangs a cone, C, within the contracted orifice of the inlet-pipe. If this cone be drawn up, the size of the orifice, D, is reduced, and, on the other hand, by its descent it enlarges the opening through which the gas passes outward. By properly adjusting the weights of the counterpoise, E, such a position of the cone may be found that the gas passes into the mains at an assigned pressure. Suppose, now, that from any cause the pressure of gas in F increases, that pressure acting upon the inverted bell, A, causes it to rise and carry with it the cone, which, by narrowing the orifice of the outlet, checks the flow of gas. Similarly, a decrease of pressure in the mains would be followed by the descent of the cone, and consequently freer egress of gas. In hilly towns it is necessary to fix regulators of this kind at certain heights in order to equalize the pressure. It is found that a difference of 30 ft. in level affects the pressure of gas in the same main to about the same amount as would a column of water one-fifth of an inch high, the pressure being least at the lowest point.

Coal-gas is a mixture of several gases, and these may be classified as, first, the light-giving gases, or those which burn with a luminous flame; secondly, gases which burn with a non-luminous flame, and which therefore contribute to the heat, and not to the light, of a gas-flame, and have the effect of diluting the gas; third, gases and vapours which are properly termed impurities, as they are either incombustible or by their combustion give rise to injurious products. Of the first kind the principal is olefiant gas, a gas which burns with a brilliant white flame without smoke. It is a compound of hydrogen and carbon, six parts by weight of carbon being combined with one part by weight of hydrogen. Besides olefiant gas other gaseous hydro-carbons are found in smaller quantities. These contain a larger proportion of carbon than olefiant gas. The second class contains hydrogen, light carburetted hydrogen, and carbonic oxide. Hydrogen is one element of water, of which it forms one-ninth of the weight. It burns with a flame giving singularly little light, but having intensely heating power; in fact, one of the brightest lights we can produce is obtained by allowing the flame of burning hydrogen to heat a piece of lime. Light carburetted hydrogen, like olefiant gas, is a compound of hydrogen and carbon, but the proportion of carbon to hydrogen is only half what it is in olefiant gas, namely, three parts to one. This gas enters largely into the composition of coal-gas, and occurs naturally in the coal seams, being, in fact, the dreaded fire-damp of the miner. It is much lighter than olefiant gas, for while that gas is of nearly the same specific gravity as atmospheric air, light carburetted hydrogen is only a little more than half that specific gravity. It is this ingredient of coal-gas which renders it so light as to be available for inflating balloons. It burns with either a bluish or a slightly yellow flame, yielding hardly any light. Olefiant gas and the other luminiferous hydro-carbons, when exposed to a bright red heat, split up for the most part into this gas and carbon. This explains the importance of rapidly removing the gas from the retort in which it is generated, a point which has been referred to above. Carbonic oxide is a gas which one may often see burning with a pale blue flame above the glowing embers of a common fire, the flame giving, however, little light. It is a compound of carbon and oxygen, containing only one-half the quantity of oxygen which its carbon is capable of uniting with, and therefore ready to unite with another proportion, which it does in burning, carbonic acid being the product.

The third class of constituents of coal-gas—the impurities—are those which the manufacturer strives to remove by passing the gas over lime, milk of lime, oxide of iron, &c. Sulphuretted hydrogen, a compound of sulphur and hydrogen, has an extremely nauseous odour resembling that of rotten eggs. It is always formed in the distillation of coal, and if not removed from the gas in the process of purification, it has a very objectionable effect; for one product of its combustion is sulphurous acid, and in a room where such gas is burnt much damage may be done by the acid vapours; for example, the bindings of books, &c., soon become deteriorated from this cause. The detection of sulphuretted hydrogen in coal-gas is quite easy, for it is only necessary to hold in a current of the gas a piece of paper dipped in a solution of the acetate of lead. If in a few minutes the paper becomes discoloured the presence of sulphuretted hydrogen is indicated.

But the bÊte noire of the gas-maker is a substance called “sulphide of carbon,” which is formed whenever sulphur and carbonaceous matters are brought together at an elevated temperature. Sulphide of carbon is, in the pure state, a colourless liquid, of an intensely offensive odour, resembling the disagreeable effluvia of putrefying cabbages. The liquid is extremely volatile, and coal-gas usually contains some of its vapour. When too high a temperature is used in the generation of the gas, it contains a large quantity of this deleterious ingredient, especially if the amount of sulphur contained in the coal is at all considerable. This sulphide of carbon vapour is very inflammable, and one product of its combustion is a large quantity of sulphurous acid. This substance cannot be removed from coal-gas by any process sufficiently cheap to admit of its application on the large scale. It is said, however, that by passing the gas over a solution of potash in methylated spirit, the sulphide of carbon vapour can be completely got rid of. The price of these materials renders the process available in special cases only, where the damage done by the sulphurous acid would be serious, as in libraries, &c. Besides the impurities we have already enumerated, many others are present in greater or less quantity. Carbonic acid—the gas resulting from the complete combustion of carbon—should be entirely removed by the lime purifiers, as the presence of even a small percentage detracts materially from the illuminating power. This gas is not inflammable and cannot support combustion. It has decided acid properties, and readily unites with alkaline bases forming carbonates: it is upon this behaviour that its removal by lime depends. The illuminating power of coal-gas containing only 1 per cent. of carbonic acid is reduced thereby by about one-fifteenth of its whole amount.

The proper mode of burning the gas so as to obtain the maximum amount of light it is capable of yielding requires a compliance with certain physical and chemical conditions. The artificial production of light depends upon the fact that by sufficiently heating any substance, it becomes luminous, and the higher the temperature the greater the luminosity. The light emitted by solid bodies moderately heated is at first red in colour; as the temperature rises it becomes yellow, which gradually changes to white when the heat becomes very intense. The widest difference exists, however, in the temperature required to render solids or liquids luminous, and that needed to cause gases to give off light. In all luminous flames the light is emitted by solid particles highly heated. Every luminous gas-flame contains solid particles of carbon, as may be easily shown by the soot deposited on any cold body—such as a piece of metal—introduced into the flame. On the other hand, the flame of burning hydrogen, which produces only aqueous vapour, furnishes no light, but a heat so intense, that a piece of lime introduced into the jet becomes luminous to a degree hardly supportable by the eye. The conditions requisite, therefore, for burning illuminating gas are, first, just such a supply of air as will prevent particles of carbon from escaping unconsumed in the form of smoke, and yet not enough to burn up the carbon before it has separated from the hydrogen, and passed through the flame in the solid state; second, the attainment of the highest possible temperature in the flame, compatible with the former condition. When the supply of oxygen is not in excess, the hydrogen of the gaseous hydro-carbon appears to burn first; the carbon is set free, and its solid particles immersed in the flame of the burning hydrogen are there intensely heated; but ultimately reaching the outer part of the flame, they enter into combination with the oxygen of the air, producing carbonic acid; or if present in excessive quantity, they are thrown off as smoke. If the purpose of burning the gas is to obtain heating effects only, this is accomplished by supplying air in such quantities, that the carbon enters into combination with oxygen in the body of the flame, without a previous separation from the hydrogen with which it is combined. In this case a higher temperature is attained, and the flame is wholly free from smoke; so that vessels of any kind placed over it remain perfectly clean and free from the least deposit of soot. The last result is of great advantage in chemical processes, especially where glass vessels require to be heated, for the chemist retains an uninterrupted view of the actions taking place in his flasks and retorts.

No better illustration of the nature of the combustion in a gas-flame can be found than is furnished by Bunsen’s burner, Fig. 350, now universally employed as a source of heat in chemical laboratories. In this burner the gas issues from a small orifice at the level of a, near the bottom of the tube, b, which is open at the top, and is in free communication at the bottom with openings through which air enters and mixes with the gas, as they rise together in the tube and are ignited at the top. If the pressure of the gas be properly regulated, the flame does not descend in the tube, but the mixture burns at the top of the tube, producing a pale blue flame incapable of emitting light, but much hotter than an ordinary flame, for the combustion is much quicker. If the openings at a be stopped, the supply of air to the interior of the tube is cut off, and then the gas burns at the top of the tube, b, in the ordinary manner, giving a luminous flame. Ordinary gas-jets burning in the streets, at open stalls or shops, may be seen on a windy night to have their light almost extinguished by the increased supply of oxygen, carried mechanically into the body of the flame, the white light instantly changing to pale blue. The disappearance of the light in such cases is due, as in Bunsen’s burner, to the supply of oxygen in sufficient quantity to combine at once with the carbon as well as the hydrogen of the hydro-carbons.

The burners now chiefly used for the consumption of coal-gas for illuminating purposes are the bat’s-wing, the fish-tail, and various forms of Argand. The bat’s-wing burner is simply a fine slit cut in an iron nipple, and it produces a flat fan-like flame. The fish-tail is formed by boring two holes so that two jets of gas inclined at an angle of about 60° infringe on each other and produce a flat sheet of flame. The Argand, in its simplest form, consists of a tubular ring perforated with a number of small holes from which the gas issues. Many modifications of this kind of burner have been devised, in all of which a glass chimney is requisite to obtain a current of air sufficient to consume the gas without smoke, and it is important that the height of the chimney should be adapted to the amount of light required if the gas is to be used economically. Argand burners are specially advantageous where a concentrated light is required. Fig. 351 represents a ventilating gas-burner, contrived by Faraday, the object being to remove from the apartment the whole of the products of the combustion of the gas. A is the pipe conveying the gas to the Argand burner, B, the flame of which is enclosed in the usual cylindrical glass chimney, C C, open at the top. This is enclosed in a wider glass cylinder closed at the top by a double disc of talc, D D, and opening at its base into the ventilating tube, E E. The direction of the currents produced by the heat of the flame is shown by the arrows. The whole is entirely enclosed by a globe of ground glass. Means are provided for regulating the draught in the pipe, E E, which, when heated, creates of itself a strong current of air through the apparatus.

The illuminating power of coal-gas may be measured directly by comparing the intensity of the light emitted by a gas-flame consuming a known quantity of gas per hour with the light yielded by some standard source. The standard usually employed is a spermaceti candle burning at the rate of 120 grains of sperm per hour. It is not necessary that the candle actually used should consume exactly this amount, but the consumption of sperm by the candle during the course of each experiment is ascertained by the loss of weight, and the results obtained are easily reduced to the standard of 120 grains per hour. An instrument is used for determining the relative intensities of the illumination, called Bunsen’s photometer. It consists of a graduated rule, or bar of wood or metal, about 10 ft. long. At one end of this bar is placed the standard candle, at the other is the gas-flame. A stand slides along the rule supporting a circular paper screen at the level of the two flames, and at right angles to the line joining them. This paper screen is made of thin writing-paper, which has been brushed over with a solution of spermaceti, except a spot in the centre, or, more simply, a grease-spot is made in the middle of a piece of paper. In consequence the paper surrounding the spot is much more transparent; yet when it is placed so that both sides are equally illuminated, a spectator will not perceive the spot in the centre when viewing the screen on either side. When the screen has been placed by trial in such a position between the two sources of light, it is only necessary to measure its distance from each flame in order to compute the number of times the illuminating power of the gas-flame exceeds that of the candle. This computation is based on the fact that the intensity of the light from any source diminishes as the square of the distance from the source. Thus, if a sheet of paper be illuminated by a candle at 2 ft. distance, it will receive only one-fourth of the light that would fall upon it were its distance but 1 ft., and if removed to 3 ft. distance it has only one-ninth of the light. In the instrument used for measuring the illuminating power of gas the rule is graduated in accordance with this law, so that the relative intensities may be read off at once. The gas passes through a meter for measuring accurately the quantity per minute which is consumed by the burner, and there is also a gauge for ascertaining the pressure. Another mode of estimating the illuminating power of coal-gas is by determining the quantity of carbon contained in a given volume. For, in general, the richness of the gas in carbon is a fair index of the quantity of its luminiferous constituents. This may be readily effected by exploding the gas with oxygen, and measuring the amount of carbonic acid produced. Still more accurate determinations of the illuminating value of gas may be obtained by a detailed chemical analysis.

The illuminating power of any gas is so calculated that it represents the number of times that the light emitted by a jet of the gas, burning at the rate of 5 cubic feet per hour, exceeds the light given off by the standard sperm candle burning 120 grains of sperm per hour. For example, when it is said that the illuminating power of London gas is 13, it is meant that when the gas is burnt in an ordinary burner at the rate of 5 cubic feet per hour, the light is equal to that given by thirteen sperm candles burning together 13 × 120 grains per hour. The quality of gas varies very much, as it depends upon the kind of coal employed, and upon the mode in which the manufacture is conducted. The following are the results of experiments made to determine the illuminating power of the gas supplied to several large towns:

	Candles.
London	12·1
Paris	12·3
Birmingham	15·0
Berlin	15·5
Carlisle	16·0
Liverpool	22·0
Manchester	22·0
Glasgow	28·0

The relative quantities of tar, ammonia water, and coke yielded in various gas manufactories also vary very considerably for the same reasons.

In the early days of gas illumination the consumers were charged according to the number of burners; but this arrangement proved so unsatisfactory that the gas-meter became a necessity, and already in 1817 meters had been devised, which were not essentially different from those now in use. Although gas is used in so many houses, there are few persons who have any notion of the mechanism of the gas-meter. Our space will not allow full details of the construction, but the following particulars may be mentioned. In the ordinary “wet” meter there is a drum divided into four compartments by radiating partitions; this drum revolves on a horizontal axis, and the lower half of the drum, or rather more, is beneath the surface of water contained in the case, the water being at the same level inside and outside the drum. The gas enters one of the closed chambers formed between the surface of the water and a partition of the drum. Its pressure tends to increase the size of the chamber, hence the drum revolves. The preceding division of the drum being filled with gas, this is driven into the exit pipe by the motion of the drum, as it is included in a space comprised between the water and a partition. Each division in turn comes into communication with the gas-main, and as it is filled passes on towards the position in which a passage is opened for it to the exit-pipe. Each turn of the drum, therefore, carries forward a definite quantity of gas, and the only thing necessary is a train of wheels, to register the number of revolutions made by the drum. The “wet” meter is much inferior in almost every respect to the “dry” meter, in which no water is used. The principle of the “dry” meter is very simple. The gas pours into an expanding chamber, partly constructed of a flexible material, and which may be compared to the bellows of a circular accordion. The expansion is made to compress another similar chamber, already filled with gas, which is thus forced through the exit-pipe. When the first chamber has expanded to a definite volume, it moves a lever, and this reverses the communications. The expanded chamber is now opened to the exit-pipe, and the other to the entrance-pipe, and so on alternately. A train of wheels registers the number of movements on a set of dials.

Recent years have brought no essential changes in the methods of gas making, although of course improvements in many minor details of the processes and of the apparatus have been effected. These demand no description at our hands, as they are of interest only to those concerned with the actual technology of gas-making, nor need some of the later forms of burners for using the gas be noticed, as these are sufficiently familiar. They really do effect a considerable economy in the consumption of gas, especially in cases where a more powerful light is required. But the reader will have already learnt from a foregoing section on Electric Lighting that the importance of gas as an illuminant is already on the wane. Indeed, it will not be too much to say that, before the close of the present century, every town will have its streets, and still more certainly, all its places of public assembly, such as theatres, concert halls, churches, libraries, &c., fitted with installations for electric illumination, and even in shops and private houses, it is probable that before long, gas will be superseded by the electric light. Some of the disadvantages of burning gas have already been referred to, and the danger attending its accidental escape into apartments is illustrated by the yearly tale of victims to suffocation and violent explosions. The inherent disadvantage of gas used as an illuminant, is the enormous quantity of heat produced by its combustion, compared with the amount of light evolved. The absolute quantity of heat required to render a body highly luminous is really very small, for masses of matter almost inappreciable become very luminous, provided only that their temperature be sufficiently raised. Thus, for example, the few residual particles of gas in a Geissler’s tube (p. 431) become incandescent by electrical discharges, while the number of them is too small to sensibly heat the glass vessel, and the very attenuated carbon filament in an electric glow lamp suffices by the mere contraction and concentration of the current within it raising its temperature high enough, to diffuse a brighter light than a large gas-flame. This explains the fact alluded to elsewhere, that if instead of burning the gas we use it in a gas engine, driving a dynamo connected with an electric light installation, we shall obtain a much greater luminous effect. As there is no combustion, the surrounding air is neither heated nor deteriorated with gaseous products and smoke.

Without any rivalry from the electric-light, gas, as a domestic luminant, has now met with a competitor on the ground of cheapness in the mineral oils mentioned in the preceding article. If these could be deprived of their unpleasant odour, and a perfectly safe lamp contrived for burning them, it would be only under very favourable conditions that gas could compete with them on the score of economy. But of late years two applications of gas to other purposes than to illumination will have been observed. First to heating, for warming, cooking, and other domestic purposes, and also in various processes in the arts. In all the appliances so used, the principle of Bunsen’s burner (p. 722) is adopted, and stoves, fireplaces, and kitchen-ranges, heated by gas have obvious advantages in their greater cleanliness and readiness. The other new application of gas is as a motive power in the gas engine, by which a very convenient supply of mechanical energy is afforded. There can be little doubt that in the future, gas will be greatly used for these purposes, and perhaps be for them consumed as largely as at present. A singular thing in the history of gas-manufacture is the great value that the bye-products have attained, that is to say, the ammoniacal liquor, the coke, and especially the tar. So many valuable substances are now derived from this last, that even if coal should cease to be destructively distilled for gas, the operation would still be largely carried on if only for the tar.

A jet of hydrogen gas burning in a dark room is all but invisible, yet no gas can give so intense a heat. The lime-light, which no doubt is perfectly familiar to everyone as an illuminant in magic lantern projections, is simply a jet of mixed hydrogen and oxygen gases directed on a piece of lime, which is rendered incandescent by the heat. The flame of the Bunsen burner (p. 772) is distinguishable only by a very pale blue colour, and it is impossible to discern objects, or to read by its light in an otherwise dark room. But if a piece of thin platinum wire formed into a coil, as by twisting round a pencil, be introduced into the flame, the wire will glow with great brilliancy, and its thickness will seem much increased. It will, in fact, emit so much light that reading by its glow becomes easy. This shows that, as already stated, a solid will give off light at a temperature which scarcely suffices to make a gas visible. Thus a Bunsen burner flame can be made to give light simply by putting into it some incombustible solid, which itself incapable or suffering any chemical change under the conditions, nevertheless becomes luminous by merely acquiring the temperature of the almost invisible heated gas. The cause of the luminosity of the ordinary gas burner, as compared with the almost invisible Bunsen burner flame, has, indeed, been already explained on a previous page, but the phenomenon is again, by the experiment just referred to, brought clearly before our attention; and it becomes obvious that substances other than the carbon of the hydro-carbon constituents of the coal gas will emit rays of light. Chemical analysis shows that by far the larger proportion of the constituents of ordinary coal-gas consist of gases which do not themselves produce luminous flames, and that, taking 16 candle-gas, about 10 candles of the illuminating power is due to compounds of which the gas does not contain more than 4 per cent. Nearly half the bulk of purified coal-gas is hydrogen, which itself gives no light whatever when burnt; marsh-gas, which burns with only a slight luminosity, forms 35 per cent. of ordinary coal-gas; and there is usually present about 7 per cent. of carbonic oxide, which in burning gives only a pale blue flame. This shows that by far the greatest product of the combustion of coal-gas is not light but heat. The flame of hydrogen is much the hottest known, and as that gas enters so largely into the composition of coal-gas, and the complete combustion of all the other constituents takes place when the gas is previously mixed with air, as in the Bunsen burner, we are provided with an economical means of obtaining high temperatures. But coal-gas was in the first instance intended to provide us with a cheap illuminant, and although for some time the gas itself was very impure, and it was long before the crude appliances for burning it were superseded by contrivances giving steadier and more brilliant lights, such as the Argand and the regenerative burners. It is only quite recently that the full illuminating possibilities of coal-gas have been developed by the happy notion of converting the heating power of its flame into light-giving power, by the simple plan of suspending a suitable solid over the hot but non-luminous Bunsen burner.

The manner in which an effective method of doing this was discovered is not a little curious. The construction of the ordinary incandescent electric lamp, Fig. 280h, involves the necessity of enclosing the carbon filament in an exhausted glass bulb; and it was when Auer von Welsbach was engaged in attempting to find some substance that could be brought into incandescence by the electric current, and yet be incombustible even in the open air, that his investigations led to the invention we have now to describe—an invention apparently destined to give a new lease of life to coal-gas illumination.

It is singular also that Welsbach, in seeking for the most suitable materials for heating to incandescence in the Bunsen burner flame, should find them in certain very rare minerals, containing a group of elements formerly of interest only to the scientific chemist, and up to that time devoid of any practical applications. The names of these elements, the oxides of which are called “earths,” will, of course, be strange to non-chemical readers, but we give their names, with the remark that the nearest familiar substance they at all resemble is aluminium, of which the oxide, or “earth,” is alumina. These rare metals, the oxides of which are the materials of the Welsbach “mantle,” are all discoveries of the present century, or nearly so, and they are called lanthanum, zirconium, thorium, cerium, didymium, yttrium, erbium, &c. They occur as silicates or phosphates very sparingly, and in a few localities in Norway; but some of them have now been found more abundantly in America. The minerals, from which for the most part the oxides are obtained, are called monazite, orthite, and thorite. It was found after many trials that a blend of these earths in certain proportions gives a mantle that yields a pure white light, while any preponderance of one or another would impart some tint to the light. This proper blending of the constituents forms a great improvement on the first mantles, which generally shed a greenish light.

The mantles are made by an ingenious process, in which a network of cotton thread is knitted into the form of a tube; this is cut up into suitable lengths, and a piece attached to form the top. The network is then saturated with a solution of the nitrates of the rare earths above-mentioned, and dried on glass rods. After this a loop of asbestos thread is passed through the top, by which the mantle may be attached to its support. The mantle is now shaped to the required form, and the cotton thread burnt off, when a thin skeleton of the oxides is left reproducing the form of the original network. The mantle is again strongly heated, and after cooling is dipped into a solution of collodion, dried, and carefully laid in a box. The collodion serves to strengthen the mantle sufficiently for transit, for it is very frail, and would otherwise be liable to fall in pieces by slight shocks. Fig. 351d is a full-sized representation of the completed mantle, and Fig. 351e shows it mounted on the burner, where a rather small flame is allowed for the first time to play upon it, by which the collodion is quickly burnt off, and then the chimney-glass is placed over it, as in Fig. 351a. In the earlier forms of lamp the lighting of the gas was a matter requiring some delicacy of manipulation, for a rude shock, or an awkward touch might cause the mantle to crumble into ruin, but now the makers fit their lamps with a by-pass by which a very small flame is maintained within the lamp ready to light up the gas when that is fully turned on. (Fig. 351c.) The makers have also now made the lamps available for street lighting, to which the fragility of the mantle was formerly an obstacle, as it was liable to collapse by the tremor of the traffic. This risk has been obviated by providing a spring to support the mantle at the base. (Fig. 351b.)

Fig. 351e.