Inflammable gas is formed in great abundance within the earth in connexion with carbonaceous deposits, such as coal and petroleum; and similar accumulations not unfrequently occur in connexion with deposits of rock-salt; the gases from any of these sources, escaping by means of fissures or seams to the open air, may be collected and burned in suitable arrangements. Thus the “eternal fires” of Baku, on the shores of the Caspian Sea, which have been known as burning from remote ages, are due to gaseous hydrocarbons issuing from and through petroleum deposits. In the province of Szechuen in China, gas is obtained from beds of rock-salt at a depth of 1500 or 1600 feet: being brought to the surface, it is conveyed in bamboo tubes and used for lighting as well as for evaporating brine; and it is asserted that the Chinese used this naturally evolved gas as an illuminant long before gas-lighting was introduced among European nations. At a salt mine in the comitat of Marmaro in Hungary, gas is obtained at a depth of about 120 feet, and is used for illuminating the works of the mine. Again at Fredonia (New York State) a natural emission of gas was discovered in a bituminous limestone, over the orifice of which a gasholder has been erected, and thus about 1000 cubic feet of a gas composed of marsh gas and hydride of ethyl has been made available for illumination. In the city of Erie (Pa.) there are 13 gas-wells, each yielding from 10,000 to 30,000 feet per day, the gas escaping from one of them at a pressure of 200 lb per square inch. At Bloomfield, Ontario co., New York, there is a spring which yields daily no less than 800,000 feet of gas of an illumin- -ating power equal to 14∣ candlβ3. The city of East Liverpool (Ohio) is entirefy illuminated, and to a large extent heated, by gas-wells which exist in and around the town. The light is of extraordinary brilliancy, and is so abundant and free that the street lamps are never extinguished, and much of the manufacturing steam-power of the town, which embraces 22 potteries, giving employment to 2000 hands, is derived from the gas. The first “well,” 450 feet deep, was opened in 1859, and up to the present year (1879) neither it nor any of those tapped at later dates show any sign of failing. In many other parts of America similar gas-wells exist; and several such natural jets of gas have been observed in England.

By general consent the merit of the discovery and application of artificial gas belongs to Great Britain, and the name most honourably connected with the beginning and early stages of gas-lighting is that of a Scotchman—William Murdoch. But previous to Murdoch’s time there occur numerous suggestive observations and experiments as to inflammable air and its sources. In the Philosophical Transactions of the Royal Society for 1667 the existence of a “burning spring” in the coal district of Wigan is noticed by Thomas Shirley, who traced its origin to the underlying coal. In the same Transactions for 1739 is printed a letter addressed to the Hon. Robert Boyle, who died in 1691, in which the Rev. John Clayton details a series of experiments he made in distilling coal in a retort, showing, not only that he had observed the inflammable gases evolved, but that he collected and stored them for some time in bladders. In Dr Stephen Hales’s work on Vegetable Stαticks, published in 1726, more precise statements are made as to the distillation of coal, he having obtained from 158 grains of Newcastle coal 180 inches of inflammable air. In 1787 Lord Dundonald, in working a patented process for obtaining coal-tar, experimented with the gas evolved in the process, and occasionally used it for lighting up the hall of Culross Abbey. None of these observations, however, led to distinct practical results; and it was not till the year 1792 that William Murdoch, then residing at Redruth in Cornwall, began the investigations into the properties of gases given off by various substances which eventuated in the establishment of coal-gas as an illuminating agent. In 1797 he publicly showed the system he had matured, and in 1798, being then employed in the famous Soho (Birmingham) workshop of Boulton & Watt, he fitted up an apparatus for the manufacture of gas in that establishment, with which it was partly lighted. Thereafter the apparatus was extended, and the gas manufactured by it was introduced to other neighbouring workshops and factories. Among others who helped most materially to develop the infant art in England were Dr Henry of Manchester, and Mr Clegg, who, succeeding Mr Murdoch at Boulton & Watt’s, introduced many improvements in gas manufacture, and ultimately became the most skilful and famous gas engineer in the United Kingdom.

In 1801 Μ. Lebon introduced gas distilled from wood into his own house in Paris, and the success of his experiment attracted so much notice and comment as to give rise to an impression that he is entitled to the credit of the invention. Lebon’s experiment came under the notice of Mr F. A. Winsor, who took up the subject with a zeal and unwearying patience which led to a recognition of the advantages of the system, and the breaking down of the powerful prejudice which existed in England against the innovation. In 1803, through Winsor’s efforts, the Lyceum Theatre was lighted with gas; but it was not till 1810 that he succeeded in forming a public company for manufacturing gas, and in obtaining an Act of Parliament for the Gas-Light and Coke Company. In 1813 Westminster Bridge was first lighted with gas, and in the following year the streets of Westminster were thus illuminated, and in 1816 gas became common in London. So rapid was the progress of this new mode of illumination that in the course of a few years after its introduction it was adopted by all the principal towns in the kingdom, for lighting streets as well as shops and public edifices. In private houses it found its way more slowly, partly from an apprehension of danger attending its use, and partly from the annoyance which was experienced in many cases through the careless and imperfect manner in which the service-pipes were at first fitted up.

Sources of Gas.

Artificial gas is now distilled from a variety of substances, among which are coal, shale, lignite, petroleum, turf, wood, resins, oils, and fats; and it is also prepared by carburetting or impregnating with volatile hydrocarbons other non-luminiferous gases. Of the very numerous systems of gasmaking which have been proposed since the early part of the century, none can compete for general purposes with the ordinary coal-gas process, when a supply of the raw material can be obtained at a moderate expense.

Coal-Gas.— Coals, varying greatly as they do in chemical constitution, differ also, as might be expected, as widely in their value and applicability for the manufacture of gas. Taking the leading varieties of coal to be included under anthracite, bituminous coal, and lignite or brown coal, we find that it is the class bituminous coal alone that yields varieties really serviceable for gas-making. Anthracite may be regarded as a natural coke from which the volatile constituents have been already driven off, and the more anthracitic any coal is, the less is it capable of yielding gas. Lignite also is rarely used for distillation, owing to the large proportion of oxygen and the amount of water in its composition. Of the bituminous coals again, it is only the caking or pitch coals, and the cannel or parrot coals, that are in practice used in gas-works. These also vary within very wide limits in their gas-making value, not only from the great difference among them in yield of gas, but also in the illuminating value of the gas they evolve. As a rule the coals which yield the largest percentage produce also the most highly illuminating qualities of gas. The cannel coals, which are specially recognized as “gas-coal,” are most abundantly developed in Scotland and in Lancashire, and the fact of the unequalled qualities of Scotch cannel and of the allied substance, bituminous shale, for gasmaking, has had the effect of rendering illumination by gas much more general and satisfactory in Scotland than in any other country. It is only a very imperfect valuation of any gas-coal that can be made from chemical analysis, the really satisfactory test being actual experiment. According to H. Fleck, the coal most available for gas-making should contain to every 100 parts of carbon 6 parts of hydrogen, of which 4 parts are available for forming hydrocarbon compounds. It is desirable that coal used for distillation in gas retorts should be as far as possible free from sulphur, that in the case of coking coal the amount of ash should be small, and the proportion of oxygen should also be low, since that element abstracts hydrogen to form injurious watery vapour. The amount of ash present, however, in the best forms of Scotch cannel is large; and consequently the resulting coke, if the residue can be so called, is of comparatively little value. Unless coal can be stored in sheds which protect it from the weather, it ought to be used as soon as possible after being raised, rain and sunshine being detrimental to its gas-making qualities. The following table exhibits the chemical analysis and gas-yielding properties of a few of the principal and typical examples of coal for gas-making :—·

Composition of Coals used in Gas-Making. [table]

Products of the Distillation of lion of Coal. [table]

When the bowl of an ordinary clay pipe is filled with small fragments of bituminous coal, luted over with clay and placed in a bright fire, immediately smoke is seen to issue from the stalk which projects beyond the fire. The smoke soon ceases, and if a light is then applied to the orifice of the stalk, the issuing gas burns with a bright, steady flame, while a proportion of a black, thin, tarry liquid oozes out from the stalk. After the combustion ceases there is left in the bowl of the pipe a quantity of char or coke. This simple operation is, on a small scale, an exact counterpart of the process by which the destructive distillation of coal is accomplished in the manufacture of gas. The products of the distillatory process classed in the gas-works as gas, tar, and ammoniacal liquor, with a solid residue of coke, are in themselves mixtures of various definite chemical compounds; and as may be evident from the following list, these substances are very numerous and complex:—

Products of the Distillation of Coal at hiqh-red heat. [table]

IV. Coks and Ash in Retort.

The proportions in which coal yields these products may be Indicated by the case of a cannel giving off 11,000 feet per ton of gas of a density of 0’600. From 100 parts of such a coal there would be yielded— [table]

The proportions, however, and even the nature of these products of distillation are greatly modified by the temperature at which the distillation is effected, a low red heat yielding a small proportion of non-condensible gas but a large amount of heavy hydrocarbon oils, whence the distillation of shales and coal in the paraffin manufacture is conducted at a low red heat. By excessive heat, on the other hand, the compounds evolved become simpler in their chemical constitution, carbon is deposited, pure hydrogen is given off, and the gain in amount of gas produced is more than counterbalanced by its poverty in illuminating properties.

Of the gases and vapours which pass out of the retorts in a highly heated condition, some portion, consisting of tarry matter and ammoniacal liquor, precipitates almost immediately by simple cooling, and other injurious constituents must be removed by a system of purification to which the gaseous products are submitted. What thereafter passes on as ordinary gas for consumption still contains some percentage of incombustible matters—aqueous vapour, oxygen, nitrogen, and carbonic acid. The combustible portion also is separable into two classes, viz., non-luminous supporters of combustion, and the luminiferous constituents,—the former embracing hydrogen, marsh gas (light carburetted hydrogen), and carbonic oxide, while the latter includes the hydrocarbon gases acetylene, ethylene (olefiant gas or heavy carburetted hydrogen), propylene, butylene, and vapours of the benzol and naphthalin series.

Formerly it was the habit to regard the proportion of heavy carburetted hydrogen (ethylene and its homologues) as the measure of the illuminating power of a gas. It has, however, been pointed out by Berthelot that the proportion of such compounds in some gas of good luminous qualities is exceedingly small; and in particular he cites the case of Paris gas, which, according to his analysis, contains only a mere trace of acetylene, ethylene, and other hydrocarbons, with 3 to 3 - 5 per cent, of benzol vapours. Subsequent experiments of Dittmar have proved that a mixture of pure ethylene and hydrogen burnt in the proportion of 3 volumes of hydrogen to 1 of ethylene yields little more light than ordinary marsh gas, while benzol vapour to the extent of only 3 per cent, in hydrogen, gives a brilliantly luminous flame. Frankland and Thorne have more recently determined the illuminating power of a cubic foot of benzol vapour burnt for 1 hour in various combinations, with the following results :— [table]

Thus it is highly probable that the illuminating value of coal-gas depends much more on the presence of benzol vapour than on the proportion of the heavy gaseous hydrocarbons,, and the estimation of benzol in the gas is a point which has hitherto been comparatively neglected. In view of the inference that the presence of benzol vapour is so intimately related to illuminating power, the fact observed by Dittmar that water readily and largely dissolves it out of any gas mixture is of great consequence. When ben-zolated hydrogen containing 6 per cent, of benzol vapour was shaken up with water, the percentage of the vapour was found on analysis to be reduced to less than 2.

The average composition of the gas supplied to London is, on the authority of the late Dr Letheby, thus stated :— [table]

Cannel gas is now, however, supplied only to the Houses of Parliament and to certain of the Government offices.

Manufacture of Coal-Gas.

The series of operations connected with the preparation and distribution of coal-gas embrace the processes of distillation, condensation, exhaustion, scrubbing or washing, purification, measuring, storing, and distribution by the governor to the mains, whence the consumers’ supply is drawn. In connexion with consumption, pressure of the gas, measurement of the amount consumed, and the burners and other arrangements for lighting are the most important considerations.

Site and Arrangement of Works.— The choice of a site for a gas establishment is necessarily conditioned by local circumstances; but the facts that a considerable area is required, and that, at best, the works do not improve the amenity of any neighbourhood, are important considerations. A central position with respect to the area to be supplied is certainly desirable, but iu the circumstances it is seldom to be obtained. Of even greater consequence for a large work is ready access to a railway or other means of transport; and most of the great establishments are now connected by. sidings with lines of railway, whereby coals, àc., are delivered direct from the waggons to the store or retorthouse, and in the same way the coke and residual products are removed. Where the arrangement is practicable, it is also desirable that the works should be erected at the lowest level of the area to be supplied, since coal-gas, being specifically lighter than atmospheric air, acquires a certain amount of pressure as it rises in pipes, which pressure facilitates its distribution, and it is much easier to control than to beget pressure. In the planning of works, regard must be given to economy of space and to labour-saving arrangements, so that the cost of manual labour may be minimized, and operations proceed in an orderly, methodical, and easily-controlled manner. The accompanying ground plan of gas-works (fig. 1) has been kindly furnished by Mr James Hislop of Glasgow, a gas engineer of known skill and experience; and while it shows arrangements of the most approved character, it will also enable the reader to recognize the position of the various erections and apparatus as they follow each other, and as they will now be described.

Retorts.— Retorts for destructive distillation of coal are formed of cast iron, clay, brick, or wrought iron. Various shapes have been adopted in the construction of these vessels; nor have their forms been more varied than the modes in which they have been disposed in the furnaces. In many instances they have been constructed of a cylindrical shape varying in length and diameter. Those first employed were of iron, with the axis vertical, but experience βoon showed that this position was extremely inconvenient, on account of the difficulty which it occasioned in removing the coke.

The retorts were therefore next placed in a horizontal position, as being not only more favourable to the most economical distribution of the heat, but better adapted to the introduction of the coal and the subsequent removal of the coke. At first the heat was applied directly to the lower part of the retort; but it was soon observed that the high temperature to which it was necessary to expose it, for the perfect decomposition of the coal, proved destructive to the lower side, and rendered it useless long before the upper part had sustained much injury. The next improvement was, accordingly, to interpose an arch of brickwork between it and the furnace, and to compensate for the diminished intensity of the heat by a more equally diffused distribution of it over the surface of the retort. This was effected by causing the flue of the furnace to return towards the mouth of the retort, and again conducting it in an opposite direction, till the heated air finally escaped into the chimney. This arrangement was continued so long as iron retorts were in use, but on the general adoption of clay retorts the furnaces were constructed to allow the fire to play freely around them.

The cylindrical form of retort a (fig. 2) was long in favour on account of its great durability, but it is not so well fitted for rapid decomposition of the. coal as the elliptical. δ, or the flat-bottomed or D-shaped retorts d, which are now principally in use. Retorts are also made of a rectangular section with the corners rounded and the roof arched. Elliptical retorts are varied into what are called ear-shaped or kidney-shaped c, and it is not unusual to set retorts of different forms in the same bench, for the convenience of filling up the haunches of the arch which encloses them.

The length of single retorts varies from 6 to 9 feet, but they are now in some cases made 19∣ feet in length and 12⅛ inches in internal diameter, these being charged from both end*.

Every retort is furnished with a separate mouthpiece, usually of cast iron, with a socket b (fig. 3) for receiving the stand-pipe orascen-Γ"^ Ί

sion-p ipe, and there ∕∕' 2 ^~"⅜

is a movable lid *Γ f attached to the ~

mouth, together ι ∖∖ ∣ ∣∣

with an ear-box ∖ ∕f ∕ cast on each side of I ∕ I ∕∖ '

the retort for re- ∕ ∕ jLUΓj τ - h ^ ∖ ∖∖ ∖

ceiving the ears J ' Ί 11 ∖

which support the ∕7∕⅛^ 11 ∣ ( lid. Fig. 3 shows ×SΓ ∖ 1 ∠Z ∕ ∕∕L) a form of mouth- . V/ ∕ piece attached to the retort a, and also the method of f ⅛∙ 3 ∙

screwing the lid to the mouthpiece. That part of the lid which comes...