general name for all those elastic fluids that preserve their aeriform state at ordinary temperatures. See Chemistry.
Gasket, a cord or piece of plaited stuff by which a sail, when it is furled, is secured to the yard or boom.
Gasket also denotes yarn of flax or hemp saturated with tallow. It is used as packing for the stuffing-boxes of engines, to make tight the joints of iron pipes, &c. Gas-Light.
Light, whether obtained from natural or artificial sources, is so necessary for the correct and successful execution of almost every operation of human industry, that whatever is calculated to simplify the means of procuring it, or to increase its intensity, cannot fail to be attended with the most beneficial consequences to civilized society. For every purpose to which it is applied, it must be admitted that the light of day, when it can be enjoyed freely and without interruption, is by far the most suitable; but in large and crowded cities, as well as in situations less favourable in point of climate, where the sun is sometimes shrouded for days together in dense and impenetrable clouds, it becomes expedient to compensate for the absence of his rays by artificial substitutes, which, however inferior in brilliancy and general usefulness, may nevertheless answer sufficiently well in those cases where a less ample supply of light is requisite.
Some substances, denominated phosphorescent, have the property of absorbing the solar rays, on being exposed for a short time to their influence, and of emitting the light which they thus imbibe when they are afterwards placed in the dark; but the feeble and transient illumination which they shed, though sufficient to indicate their luminous condition, is totally unfit to afford such a supply of light as is necessary for conducting any of the operations of art, which require care and precision for their performance.
There are, however, a variety of inflammable substances, both of animal and vegetable origin, which, during the process of combustion, give out light as well as heat; and hence, from the earliest periods of human society, it has been customary to burn substances of that description for the purpose of obtaining artificial light. These substances, which were generally of a fatty or oleaginous nature, are composed chiefly of carbon and hydrogen. When they are exposed to a certain high temperature, they are resolved into some of the compound gases which result from the union of these elements, particularly carburetted and bicarburetted hydrogen, or olefiant gas, both of which are highly inflammable, and yield, during their combustion, a fine white light. In order to facilitate the decomposition, and to carry on the combustion with due economy, a quantity of some fibrous substance, in the form of a wick, is connected with the oleaginous matter, for the purpose of causing it to burn slowly and effectually. Accordingly, if the flame be suddenly extinguished, the inflammable gas which is formed by the decomposition of the matter in immediate contact with the wick is observed to escape from it, and may be again set on fire by the application of a lighted taper.
When it is required to convey from place to place the light obtained from these substances, no arrangement is found to be more convenient for their decomposition than that which is effected by means of the wick; but if the light is to remain in a permanent position, it will frequently be more advantageous to resolve the oleaginous matter into gas, and then to transmit it, in that state, through pipes, to the various points where it is to be consumed.
Although the different substances which have been used from the earliest times for yielding artificial light have always been actually resolved into gas before they underwent the process of combustion, that fact was entirely unknown until pneumatic chemistry unfolded the properties of the aerial bodies, which perform so many important functions in the economy of nature, as well as in the processes of the arts. It was then discovered that hydrogen, one of the component parts of water, was a highly inflammable gas, capable of being produced under a great variety of circumstances: from vegetable matter decaying in stagnant water, forming what is called light carburetted hydrogen, a stream of which, when ignited, produces the natural phenomenon known as "ignis fatuus, or Will-of-the-Wisp:" from coal, oils, and fatty substances, when, in combination with larger proportions of carbon, it forms gases of high illuminating powers. The use of gas for the purpose of illumination is therefore of recent date; but although late in its origin, the successive improvements which the invention has received, and continues to receive, from the joint labours of chemists and practical engineers, have tended greatly to simplify the processes for producing the gas, and for improving its quality and means of distribution.
In many parts of the world there are certain deposits of Discovery petroleum or naphtha which furnish gaseous matter; and of coal-gas, this issuing from some fissure in the earth, becomes ignited by lightning or some other cause, and continues to burn for a long period. Such a flame is regarded by an ignorant people with superstitious reverence, and has been sufficient to found a religious sect of fire-worshippers. Deposits of coal, or of bituminous schist, sometimes furnish the gaseous matter for such flames. The practical Chinese, about thirty miles from Pekin, are said to make use of this gas in the boiling and evaporating of salt brine, and for lighting their streets and houses. "Burning fountains," as they are sometimes called, are not uncommon, and their origin is the same. In 1851, in boring for water on Chat Moss, on the line of railway between Manchester and Liverpool, a stream of gas suddenly issued up the bore, floated along the surface of the ground, and caught fire on the application of flame. A pipe was inserted into the bore, and a flame eight or nine feet long was thus produced.
In 1667, Mr Shirley describes in the Philosophical Transactions of the Royal Society a burning spring in the coal district of Wigan in Lancashire: he traced its origin to the underlying beds of coal. In 1726, Dr Hales, in his work on Vegetable Statics gives an experiment on the distillation of coal, by which it appears that 158 grains of Newcastle coal yielded 180 cubic inches of inflammable air. In 1733, Sir James Lowther sent to the Royal Society specimens of inflammable air from a coal-mine near Whitehaven. The gas was collected in bladders, and a number of experiments were tried on it.
It appears, however, that the Rev. John Clayton had performed some experiments on the distillation of coal some years previous to the publication of Dr Hales's book; but he did not publish an account of them until 1739, and this account consists of an extract from a letter written by Clayton to the Hon. Robert Boyle, who died in 1691, and was probably written some time before this year. It is inserted in the Transactions of the Royal Society for the year 1739; and is probably the earliest evidence of the possibility of extracting from coal, by means of heat, a permanently elastic fluid of an inflammable nature. We shall therefore give the account of the discovery in his own words: Having introduced a quantity of coal into a retort, and placed it over an open fire, he states that "at first there came over only phlegm, afterwards a black oil, and then likewise a spirit arose which I could noways condense; but it forced my lute, or broke my glasses. Once when it had forced my lute, coming close thereto in order to try to repair it, I observed that the spirit which issued out caught fire at the flame of the candle, and continued burning with violence as it issued out in a stream, which I blew out and lighted again alternately for several times. I then had a mind to try if I could save any of this spirit; in order to which I took a Gas-Light.
turbinate receiver, and putting a candle to the pipe of the receiver whilst the spirit arose, I observed that it caught flame, and continued burning at the end of the pipe, though you could not discern what fed the flame. I then blew it out and lighted it again several times; after which I fixed a bladder, squeezed and void of air, to the pipe of the receiver. The oil and phlegm descended into the receiver, but the spirit still ascending, blew up the bladder. I then filled a good many bladders therewith, and might have filled an inconceivable number more; for the spirit continued to rise for several hours, and filled the bladders almost as fast as a man could have blown them with his mouth; and yet the quantity of coals I distilled were inconsiderable.
"I kept this spirit in the bladders a considerable time, and endeavoured several ways to condense it, but in vain. And when I had a mind to divert strangers or friends, I have frequently taken one of these bladders, and pricking a hole therein with a pin, and compressing gently the bladder near the flame of a candle till it once took fire, it would then continue flaming till all the spirit was compressed out of the bladder; which was the more surprising, because no one could discern any difference in the appearance between these bladders and those which are filled with common air."
It is evident from this narrative, related with so much simplicity, that an accident which happened to Mr Clayton's apparatus was the means of leading to the discovery of coal-gas; but it does not appear that he or any other individual thought of applying the discovery to any practical purpose until the year 1792, when Mr Murdoch, who then resided at Redruth, in Cornwall, commenced a series of experiments upon the properties of the gases contained in different substances. In the course of his researches he found that the gas obtained by distillation from coal, peat, wood, and other inflammable substances, yielded a fine bright light during its combustion; and it occurred to him, that by confining it in proper vessels, and afterwards expelling it through pipes, it might be employed as a convenient and economical substitute for lamps and candles.
Mr Murdoch's attention to the subject having been interrupted for some time by his professional avocations, he resumed the consideration of it in 1797, when he exhibited publicly the results of his more matured plans for the preparation of coal-gas. The following year (being then connected with Messrs Boulton and Watt's engineering workshop), he constructed an apparatus at the Soho foundery for lighting that establishment, with suitable arrangements for the purification of the gas; and these experiments, Dr Henry states, "were continued with occasional interruptions until the epoch of the peace in 1802, when the illumination of the Soho manufactory afforded an opportunity of making a public display of the new lights; and they were made to constitute a principal feature in that exhibition."
In this brief sketch of the progress of gas-lighting, it may be noticed that the Lyceum theatre in London was lighted with gas in the course of the years 1803-4, under the direction of Mr Winsor, who is entitled to no small commendation for the warm interest which he took in drawing the public attention to the subject; and in 1804-5 Mr Murdoch had an opportunity of carrying his plans into effect on a Gas-Light, still larger scale, by means of the apparatus erected under his superintendence in the extensive cotton mills of Messrs Philips and Son of Manchester.
It has been alleged that gas-lights were used in France before they were known in this country; but as the earliest exhibition of these lights, on which the claim of priority of discovery is founded, took place at Paris in 1802, it is evident, from the foregoing statements, that the exhibition alluded to was ten years subsequent to the first experiments of Mr Murdoch on the subject.
The practicability of lighting by means of coal-gas having been demonstrated by Mr Murdoch, a number of scientific men applied their talents to the further development of the art. Dr Henry, the celebrated chemist, lectured on the subject in 1804 and 1805, and furnished many hints for the improvement of the manufacture. Mr Clegg, an engineer in the employment of Boulton and Watt, was a worthy successor of Murdoch, and for many years was the most eminent gas-engineer of this country. A good deal of the machinery of the gas-house in its present form was contrived by Mr Clegg, and to him also we are indebted for the ingenious wet gas-meter. In 1813 Westminster bridge was first lighted with gas, and in the following year the streets of Westminster were thus lighted, 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 it was first introduced, 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, not entirely groundless, of the danger attending the use of it; 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. These inconveniences have been in a great measure, if not wholly, removed by a more enlarged knowledge of the management of gas; and at present there are few private houses in large towns which are not either partially or entirely lighted up by it. As the demand for gas increased, various improvements were from time to time introduced both in the mechanical arrangements, and in the chemical operations of the manufacture. The rapid increase in the population of the metropolis, and of all large towns, has naturally led to an increased consumption of gas, and the application of gas to the purposes of warming and cooking has also further increased the demand for it. Hence it has been not only necessary that new gas-works should be erected for the supply of new districts, but that the resources of old works should be enlarged. It is only a few years ago that a gas-holder, capable of storing 250,000 cubic feet of gas was regarded as of enormous size; at the present time, gas-holders are made of double that capacity, and we occasionally hear of them of the capacity of upwards of a million cubic feet. There is one such at Philadelphia; it is 140 feet in diameter and 70 feet in height. Nor will such dimensions as these be regarded as superfluous when it is stated that some of the large metropolitan works send out each from a million to a million and a half cubic feet of gas in one night in midwinter. The Westminster gas-works alone are accustomed
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1 Mr Clayton also alludes to the discovery of the gas which he obtained from coal, in a letter to the Royal Society, dated May 12, 1688.
2 The manufacture of gas has now become of such vast extent and importance, and so many persons are interested in it commercially, practically, and theoretically, that a journal devoted to its details has been found necessary. The Journal of Gas-Lighting is published twice a-month, and reports the progress of the manufacture. It also publishes a share list of the companies for the United Kingdom, which will give some idea of the extent of the manufacture. Many of the continental cities are also lighted with gas by English companies. A few years ago Mr Hume, in the House of Commons, moved for a return, which has been published under the following title:—"Return or statement from every gas company established by act of parliament in the United Kingdom, stating the several acts of parliament under which established, the rates per 1000 cubic feet at which each company or corporation have supplied gas in each of the three years since 1848 to 1849, and the average prices of the coals used by the company in each year for the same period; also stating the amount of fixed capital invested by each gas company, and the rate per cent. of dividend to the shareholders or proprietors on their shares in each year since that date (in continuation of Parliamentary Paper No. 734 of Session 1847)." It appears from this document that the fifteen Of the Site and general Arrangement of the Apparatus for the Production and Purification of Coal-Gas.
In describing the site and general arrangements of a gas establishment, it is not easy to give directions respecting points which must be regulated in every case by circumstances of a local nature; but when a choice of ground is in our power, a spot ought to be selected having a central situation with regard to the buildings, streets, &c., which are to be supplied with light, and standing as nearly as possible on a medium level with them. When the manufactory is placed considerably below that level, the gas is apt to be propelled with too much velocity through the burners; and when above it, an opposite inconvenience is experienced, the gas being in that case necessarily subjected to an extra pressure, by which the chance of its escape through any imperfection of the pipes is proportionally increased. Of the two evils, therefore, the least objectionable is that in which the situation of a gas-work is below the mean level of the streets.
But besides the conditions favourable to an equable and uniform distribution of the gas at the different points to which it may be conducted, there are other considerations scarcely less important, which in selecting a proper site for the erection of the establishment ought not to be disregarded. Among these may be reckoned a regular supply of water for the various manipulations of the work; and facility of access for the delivery of coal and the removal of the coke, tar, and other products of the distillation. Railways are now so common that they are often as valuable to a gas-work as the vicinity of navigable water. In the Central Gas-consumer's works at Bow Common, which were laid out under the skilful scientific direction of Mr. Croll, a branch railway is connected with the lines which supply the coal, and is actually continued into the retort-house, so that the coal wagons only arrive at their final destination at the mouths of the retorts which are to be fed. But in fixing the situation of an establishment which is professedly erected for the public benefit, the comfort or the interest of individuals ought not to be entirely overlooked; for although a gas-work may not prove, under proper management, a nuisance, it can never be considered to be any advantage to the neighbourhood in which it is placed.
The apparatus for the production and purification of coal gas consists, in the first place, of suitable vessels for decomposing by heat the coal from which the gas is to be procured; secondly, of a series of pipes for conveying off the gas, and conducting it into proper receptacles, where it may be separated from the grosser products, which tend to impair the brilliancy of the light; thirdly, of the condensing apparatus, for removing more effectually the tar and other condensible substances that come over with the gas; fourthly, of the purifying apparatus, for abstracting the sulphuretted hydrogen, carbolic acid, &c., by which the gas is contaminated, and which if allowed to remain, would be injurious to the gas-fittings, to the books and furniture of rooms, or to the health of the consumer; and, fifthly, of the gasometer or gas-holder, with its tank, into which the gas is finally received in a purified state.
Of the Retorts, or Vessels for decomposing the Coal.
The vessels employed for the decomposition of coal and other substances capable of yielding carburetted hydrogen, by their destructive distillation, are formed of cast iron, of Gas-Light clay, of brick, or of wrought iron, and are termed retorts. 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 erected for their reception. In many instances they have been constructed of a cylindrical shape, varying in length and diameter. Those first employed were placed with their axis in a vertical direction, but experience soon showed that this position was extremely inconvenient, on account of the difficulty which it occasioned in removing the coke, and other residuary matters, after the coal had been carbonized. Attempts were made to remedy this inconvenience, by enlarging the size of the retort, and introducing the coal inclosed in a proper grating of iron, having the form of a cage. The increased dimensions of the retort, from which the principal advantage to be derived from this arrangement was expected, were found, however, to present great obstacles to the complete carbonization of the coal; for although the disengagement of gas during the first stages of the process was sufficiently copious, it diminished rapidly the longer the distillation was continued, in consequence of a crust of coke being formed next to the heated metal, which not only opposed the transmission of the heat to the internal mass of coal, but gradually prevented, by its accumulation, the extrication of the gas from the undecomposed portion of it.
The retorts were, therefore, next placed in a horizontal mode of 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, after the carbonization was carried to a due extent. 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 it, 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 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 continued for a long time in use, and seemed to admit of little improvement, unless with respect to the shape and dimensions of the retorts. The cylindrical form a, fig. 1, has the advantage of possessing great durability, but it is not so well fitted for rapid decomposition of the coal (on which depends much of the good qualities of the gas) as the elliptical shape b. Flat-bottomed, or D-shaped retorts, d, have also been long in use: the small London D is about 12 inches wide by 12½ inches deep, while the York D varies from 20 to 30 inches in width, and from 9 to 14 inches in height. 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 incloses them. The length of retorts formerly varied from 6 to 9 feet: they are now in some cases made 19½ feet in length. Gas-Light, and 12½ inches in internal diameter, and are charged at both ends.
Iron retorts of from 6 to 9 feet in length carry a charge of from 120 to 200 lb. of coal, which is usually renewed every six hours. Instead of the old method of charging with the shovel, which occupied at least half an hour, and entailed a great loss of gas, the whole charge is now deposited in an iron scoop, with a cross handle at the end, and it is lifted by three men, pushed into the retort, turned over, and the whole charge deposited at once, a contrivance which does not occupy more than 30 or 40 seconds. Indeed it is not uncommon for a bench of 7 retorts to be emptied and recharged in the brief space of 20 minutes. When square-backed retorts are used, the backs are apt to wear much more quickly than any other part, in consequence of the fierce heat which plays upon them; it is therefore sometimes usual to throw in a few shovels-full of coal to the extreme end before depositing the charge with the scoop. This occupies more time in the charging, but it has the effect of preserving the backs. The objection does not apply to retorts with circular ends.
Every retort is furnished with a separate mouthpiece, usually of cast iron, with a socket b, fig. 2, for receiving the stand-pipe, and there is a moveable lid attached to the mouth, together with an ear-box cast on each side of the retort for receiving the ears which support the lid. Fig. 2 shows the mouthpiece attached to the retort a, and also the method of screwing the lid to the mouthpiece. The ears hold a cross-bar through which is passed a screw which presses on the lid, and secures it to the mouthpiece. That part of the lid which comes in contact with the edge of the mouthpiece has applied to it a lute of lime mortar and fire-clay, and when the lid is screwed up, a portion of this lute oozes out round the edges and forms a gas-tight joint.
In some cases the screw is got rid of by a more expeditious contrivance, shown in figs. 4 and 5, in which the ears support an axis, which carries a lever formed at one end into a sort of cam, and bearing at the other end a ball of cast iron about 4 inches in diameter. On lowering this ball the cam presses with great force against the back of the lid, and holds it securely; and if more force be required, a weight can be attached to the iron ball.
In attaching a mouthpiece to a clay retort, the end is notched with grooves for the purpose of holding the binding cement more securely. The mouthpiece is attached by means of bolts with T heads let into the body of the retort. Iron cement is used, in which fire-clay takes the place of sulphur; this being spread over the joint, the mouthpiece is attached and screwed up.
The temperature best suited for the production of gas from coal, being what the workmen term a bright red, was found to be very destructive to the retorts when they were exposed to the direct action of the fuel; and accordingly means were employed to protect them from the rapid oxidation which they suffered under these circumstances, by interposing between them and the furnace a partition of fire tiles or arched bricks, with side flues for the admission of the heated air.
With the view of occupying less room, and saving the expense of fuel, several retorts are sometimes set together in an oven of brickwork, and heated by a smaller number of furnaces than there are retorts. By this arrangement the fuel is certainly economized; but the plan is liable to the objection, that when any one of the retorts is worn out, those connected with it cannot be used till the faulty one is replaced; and though various expedients have been proposed for obviating that inconvenience, none of them can be said to have effectually answered the purpose. Figs. 4 and 5 represent an arrangement of the oven plan. A is the retort, g the furnace, ff the flues. Mr Croll's arrangement is represented in fig. 3.
The fuel required for carbonizing a given quantity of coal may be stated to be, in general, from one-third to one-fourth of its weight for Newcastle coal. It is stated, that under Mr Croll's method of setting, the carbonization is carried on by the combustion of only 12 per cent. of fuel, or that 100 tons of coal are carbonized by 12 tons of coke.
Various attempts have been made to render the retorts more independent of the labourers. In Mr Brunton's retort, a hopper containing the charge of coal is attached to the mouthpiece. The charge is introduced by removing a slide, and a piston is then advanced for the purpose of pushing forward the coal, and ejecting the coke, the latter falling through a shoot at the further end of the retort, and thence into a cistern of water into which the lower end of the shoot dips. This retort is not of equal section throughout; it is 15 inches in diameter at the mouth, and 21 inches at the other end, the length being 4½ feet. The advantages of this arrangement, independently of the saving of labour, are said to be an increased production of gas, and a consequent diminution in the amount of tar, naphtha, and ammoniacal liquor, this diminution being stated at 50 per cent. less than the ordinary yield of those secondary products. Moreover, a good deal of bituminous vapour, and minutely divided carbon, which, under the usual arrangement, go to swell the increase of tar, become decomposed under the higher temperature of Mr Brunton's retorts by passing over the red-hot coke, and forming illuminating gas. Indeed, it is now generally admitted as an axiom in gas-making, that the most productive yield of gas is under a high temperature; for it is possible under low heats to distil off the volatile parts of the coal as bituminous vapour only, without any production of carburetted hydrogen gas. By exposing the coal in a thin layer to a very high heat, the distillation is effected most rapidly and most profitably. Mr Clegg describes a retort into which the coal is introduced by means of an endless web formed of iron plates, each 2 feet long, and 14 inches wide, and linked together by iron rods. The coal, broken small, is placed in a hopper, to which is attached a feeder with six radial projections. Each of the six partitions thus formed supplies sufficient coal to cover one plate of the web, with about 120 cubic inches of coal to the depth of ⅝ths of an inch. The hopper, which contains 24 hours' charge of coal, is luted after each charge. The endless web is moved by passing over drums, one revolution of which every 15 minutes conveys the web through the retort, and effects the distillation of the coal. The coal is carried on the upper surface of the web, and as the web turns over the second drum the coke is discharged by a pipe into a vessel below, and the empty portion of the web returns to the hopper, and passing over the surface of the first drum receives another charge. The charge is so regulated, that about 100 squares inches of heated surface in the retort is allowed for every pound of coal, which is said to yield 5·36 cubic feet of gas, or 12,000 cubic feet per ton of Wallsend coal. The charge for each retort is about 18 cwt. of coal for 24 hours, or about double the quantity under the old plan in retorts of similar dimensions. The coke is also said to be in much greater quantity. In the course of time the plates of the iron web become converted into steel, the value of which is sufficient for the purchase of a new web. Mr Lowe has also introduced an arrangement for increasing the yield of gas by making the products of a new change pass For this purpose the reciprocating retort, as it is called, is made of thrice the usual length, and is charged at both ends; but the dip pipe at one end is made to enter to a greater depth into the tar of the hydraulic main than at the other end; so that supposing both the dip pipes to be open, the products of distillation will of course be discharged into the main by the shorter pipe, where there is less pressure to be overcome. This pipe, however, is furnished with a cup-valve, which can be closed at pleasure; and when so closed, the products of distillation must escape by the longer dip-pipe. When the charge has been half worked off in one-half of the retort, a fresh charge is introduced into the other half, and the products of distillation of the new charge are made to pass over the incandescent coal, or that which has been about three or four hours under distillation. This is readily effected by closing or opening the shorter dip-pipe, according to the end of the retort last charged. The principle of the reciprocating retort has been adopted at different works, with variations in the practical details.
Of late years clay retorts have been largely introduced into gas works, and they are said to be more durable, and to stand a higher temperature than iron retorts, the latter working best at a cherry-red heat, and the former at a white heat, which is more favourable to the increased production of gas than the lower temperature. It is stated, that where a clay retort has yielded a million and a half cubic feet of gas, an iron one has furnished only 800,000 cubic feet. Clay retorts appear, from their greater porosity, to leak more than iron ones; but after working some months, the pores become clogged with carbon, and the porosity is thus greatly diminished, and the leakage is even less than in iron retorts working under the same pressure. As the demand for clay retorts increased, the manufacture of them improved, an example of which improvement is well illustrated in the case of the retorts in the Great Exhibition of 1851, exhibited by Messrs Cowen of Blaydon Burn, near Newcastle-on-Tyne. When this firm first manufactured retorts about twenty years ago, each retort was made in ten pieces, which number was reduced to four, then to three, and then to two; and in 1844 the retort was made complete in one piece of the dimensions of 10 feet in length, and 3 feet in internal width. The clay of which these retorts are manufactured is exposed to the weather for some years, and is frequently turned over, and the fragments of fossils picked out, by which means most of the iron is got rid of, which in other fire-clays is so injurious. Some of these retorts are stated to have continued in active use for 38 months, thus exhibiting four times the durability of iron ones.
Brick retorts, or rather ovens, have also been introduced, and are said to be very durable, and to work satisfactorily. In one case the charge is 5 or 6 cwt. of coal every twelve hours, and the yield 9000 cubic feet of gas for one ton of Welsh coal, and from 10,000 to 12,000 cubic feet from one ton of Newcastle coal. The fuel required for the carbonization of the coal is said to be unusually large. Wrought-iron retorts, made of thick boiler plates firmly riveted together, have also been tried to a limited extent.
When clay retorts came into general use, the circumstance that they required a much higher heat than iron retorts suggested the economical plan of heating the clay retorts by the direct action of the furnace, and arranging the iron retorts in a separate oven, heated by the same furnace, or within a system of return flues, where they would be submitted to a less intense heat. By this means Mr Croll has found, that with two furnace grates of 252 square inches in each, he has been able to carbonize in 24 hours five tons of coal in the clay retorts of one bench, and three tons and a half in the iron retorts of the same bench, with such an economy of fuel, that only 12 per cent. of all the coke made is required for the furnaces; whereas, in most of the London works, nearly one-third of the coke made is consumed in heating the retorts.
Fig. 3 represents one form of Mr Croll's method of combining iron and clay retorts. In this cross section cc are the clay retorts; the two uppermost are elliptical in form, and 16 inches by 12 in dimension; four are circular, each being 15 inches in inside diameter. Each clay retort is made in four pieces jointed with fire-clay. The furnaces are at each end of the retorts, one such furnace being shown at g, with its ash-pit a, and the retorts are arranged so as to receive an equal share of the radiated heat; ii are iron retorts in the lower oven, which receives the hot air through openings 14 inches by 10, in the lower arch, which separates the two ovens; ff are openings from the lower oven into the horizontal flues, which extend along the whole length of the retorts, and they are furnished with tile-dampers for the adjustment of the draught.
The quantity of gas produced during the time the coal is undergoing decomposition is extremely variable. From a small retort, exposed for eighty-five minutes to a bright red heat, which was kept up with the utmost possible uniformity, the following results were obtained from eight pounds of the Wemyss coal:
| Time | Cub. Ft. | |---------------|----------| | In 1st ten minutes | 6 | | 2d do | 8 | | 3d do | 8 | | 4th do | 5 | | 5th do | 4 | | 6th do | 3 | | Last twenty-five minutes | 6 |
Total: 43
1 One of the greatest sources of loss in the manufacture of gas arises from the leakage, not only of the retorts and other apparatus within the works, but also of the mains, a loss amounting to from 10 to 30 and upwards per cent. Mr Croll estimates the loss at one-sixth of the gas sent out. The porosity of cast-iron pipes, not at their joints merely, but throughout their whole length, is evident from the saturation of the soil with gas in the immediate vicinity of the mains. Not only does the gas escape by exomose into the air, but, by the reverse process of endomose, air enters the pipes in some cases, as Professor Graham has found, to the extent of 25 per cent. Professor Brande thinks that the feilod odour of the soil in contact with the gas mains is due to the exomose of ammonia, rather than of tar and naphtha, to which the ill odour is generally attributed. At the time the process was terminated the extraction of sériform matter had nearly ceased, so that the quantity of gas yielded by a pound of the coal was about five and a half cubic feet. The same coal carbonized on the large scale yielded, when the process was carried on for four hours, at the rate of four and one-third cubic feet of gas per pound. The weight of the coke in the above experiment was 32,050 grains; and as the weight of the gas, the specific gravity of which was .65, must have been 15,026-grains, the tar and other residuary products, including the sulphurated hydrogen abstracted by the process of purification, must have amounted to 8924 grains.
When the decomposition is effected on the large scale, the quantity of gas is found to vary with the quality of the coal, and the manner in which the operation is conducted. According to Mr Peckton, a chaloner of Newcastle Wallsend coal yields 10,000 cubic feet, being at the rate of 370½ cubic feet per hundredweight. The different kinds of Newcastle coal yield from 8000 to 12,500 cubic feet of gas per ton; the parrot or cannel coals furnish from 9000 to 15,000 feet per ton, the last-named quantity being obtained from the Boghead cannel, in which case the specific gravity of the gas is .752, and as much as 866 avoirdupois lb. of gas are obtained from each ton of coal. The Wallsend Newcastle, known as Berwick and Craister's, only yields 449 lb., and of the specific gravity .470. Of the Derbyshire, Staffordshire, Welsh, and other varieties of coal, the yield varies from 6500 to about 11,000 cubic feet of gas per ton of coal. So that under the best methods of working it is of great importance to obtain a coal that is rich in bituminous matter.
It must not, however, be supposed that anything like the above quantities of gas are obtained from coal in the practical working of it in the gas-house. The manufacturer is exposed to losses from a variety of causes, such as leakage, as already noticed, and also from the tendency of the carbon of the gas, or of the hydro-carburets distilled from the coal, to form deposits of charcoal which may attain an inch or more in thickness on the inner surface of the retorts, not only producing a loss of gas, but causing the retorts to burn out more quickly, and leading to expense and delay in removing the deposit. It was formerly supposed that this deposit was owing to the overheating of the retort, or to an excess of heating surface. It was found, however, by Mr Grafton that the pressure to which the gas is subjected in the retort is the cause of the deposit. It is scarcely necessary to remark, that when an elastic body is generated in a close vessel, the pressure which it exerts upon such vessel depends greatly upon the resistance to which it is exposed in seeking to escape. In endeavouring to force its way by the dip-pipe through several inches of tar into the hydraulic main, the resistance thus offered produces a considerable pressure on the inner surface of the retorts. The passage of the gas through the washing vessels and lime purifiers increases this pressure, thereby promoting the deposit complained of, and causing an increased production of tar at the expense of the gas. Mr Grafton found that by working the retorts under a pressure of 14 inches of water, a deposit of carbon one inch in thickness was formed within the retorts in one week, and in the course of two months it filled up nearly one-fourth of the retort. On working the retorts with no other pressure than that produced by the insertion of the dip-pipe half an inch into the fluid of the hydraulic main, little or no deposit took place in the retorts in four months with the same kind of coal. It is now common at many gas works to introduce some kind of pumping apparatus, known under the name of the exhaustor or extractor, between the hydraulic main and the condenser, or between this and the lime purifiers, by which means the pressure of the gas within the retorts can be reduced to any amount. It is, however, found desirable not to carry this reduction too far, lest atmospheric air should find its way into the retorts, and thus form an explosive mixture with the gas.
The quality of the gas yielded by coal varies greatly at different periods of the carbonizing process. The first gas during the carbonization process, when the coal has not been previously well dried, consist almost entirely of aqueous vapour and carbonic acid; these are succeeded by light carburetted hydrogen, olefiant gas, and sulphuretted hydrogen, which gradually diminish in quantity till towards the close of the process, when almost the only products are carbonic oxide and hydrogen. Hence, if the process be carried on too long, the gases obtained in the latter stages of it will not only be useless for the purpose of yielding light, but the fuel employed for their production will be expended in wasting the retorts to produce substances which are calculated to impair the illuminating power of the gases with which they are mixed. In the case of cannel coal, the interval between the charges of the retorts should not exceed three and a half or four hours; nor in the case of the Newcastle coal, which is not so easily decomposed, ought that interval to extend beyond six hours.
The Condensing Main and Dip Pipes.
From the retorts the gas, after its production, ascends by means of pipes, called stand-pipes, BB, figs. 4 and 5, into what is termed the condensing main, HH, which is a large cast-iron pipe, about twelve or fifteen inches in diameter, placed in a horizontal position, and supported by columns in front of the brickwork which contains the retorts. A wrought-iron hydraulic main are now coming into use, and are preferable on account of their superior lightness and strength. This part of a gas apparatus is intended to serve a twofold purpose: First, to condense the tar and grosser products of distillation; and, secondly, to allow each of the retorts to be charged singly without permitting the Gas-Light, gas produced from the rest; at the time that operation is going on, to make its escape. To accomplish these objects,
The first contrivances employed for the purpose of condensation were all constructed on the supposition that the object would be best attained by causing the gas to travel through a great extent of pipes, surrounded by cold water, and winding through it like the worm of a still, or ascending upwards and downwards in a circuitous manner. An improvement on this form of condenser, and the one now in general use, is represented in fig. 6. It consists of a series of upright pipes connected in pairs at the top by semicircular pipes ee, and terminating at the bottom in a trough X Y containing water, and divided by means of partitions in such a way that as the gas enters the trough from one pipe it passes up the next pipe and down into the next partition, and so on to the end of the condenser. The cooling power of this air-condenser, as it is called, is sometimes assisted by allowing cold water to trickle over the outer surface of the pipes. In passing through these pipes the gas is considerably reduced in temperature, and the tar and ammoniacal liquor condense, the tar subsiding to the bottom, and the ammoniacal liquor floating on the surface. In the course of time the water in the trough is entirely displaced by these two gaseous products, and as these accumulate they pass off into a tar-tank, from which either liquor can be removed by means of a pump adapted to the purpose.
Of the Tar Apparatus.
After emerging from the lower end of the dip-pipe, the gas, now bereft of a considerable portion of the vapour of water, tar, and oleaginous matter, which ascends with it from the retort, is conveyed by pipes, for the purpose of being completely freed from these impurities, into contrivances where a more perfect condensation takes place. As the subsequent purification of the gas depends, in no small degree, upon the perfect separation of the tar and other condensible products by which it is accompanied, the construction of the vessels best calculated for attaining that end is a matter of the utmost importance; and indeed it may be justly affirmed, that unless that separation be effectually accomplished, the action of the chemical agents to which the gas is afterwards exposed, must be limited and imperfect.
Of the purifying Apparatus for separating the Gases unfit for the purposes of Illumination.
With the two compounds of hydrogen and carbon, viz., Impurities olefiant gas and light carburetted hydrogen, which are of coal-gas, yielded by coal during its destructive distillation by heat, several other products are obtained, which are not only useless for the purpose of illumination, but are calculated Gas-Light. to diminish the brilliancy of the light which is afforded by these gases, and even to prove a source of serious nuisance during their combustion. Among these products of a deleterious nature are carbonic acid and sulphuretted hydrogen; and, in smaller quantity, carbonic oxide, nitrogen, and hydrogen. The first two are by far the most objectionable of these impurities; and fortunately their separation can be effected more easily than that of the others, the presence of which is of less importance.
Carbonic acid is readily absorbed by any of the alkalies or earthy bodies in a caustic state; and sulphuretted hydrogen, which possesses many of the properties of an acid, unites not only with the alkalies and alkaline earths, with which it forms a species of salts termed hydro-sulphurets, but also with the metallic oxides, most of which it reduces.
The alkalies being too expensive to be used for separating carbonic acid and sulphuretted hydrogen from coal-gas, a more economical substitute, and which answers the purpose almost equally well, is found in quicklime. This substance is accordingly used in every gas establishment on the large scale, in some form or another, in purifying the gas. It is employed in two states; either in the condition of a thin paste, which the workmen call the cream of lime, or of a moistened powder, such as lime assumes when it is slaked with a little more than the usual quantity of water. The apparatus must therefore be accommodated, in its construction and arrangement, to these different conditions of the purifying material.
When the lime is used in a liquid state, the gas is made to pass through it so as to be as much as possible exposed to its action; and it being highly conducive to the success of the purifying process that a succession of fresh portions of the liquid lime should be brought in contact with the gas as it passes through it, the material is kept in a state of constant agitation by means of machinery. Fig. 7 represents an arrangement of that kind in which aaa is a flat cylindrical vessel in which the purification is performed, the gas entering by the pipe e and escaping by the pipe d. In the inside of this vessel is another of a similar form ff, terminating at the bottom in a broad horizontal flanch or circular plate gg, and having revolving within it an agitator hh, which works in an air-tight stuffing box. The gas being forced by the pressure from the retorts or from the exhauster to descend below g, is more effectually exposed in its progress to the action of the lime by the commotion produced by the agitator, and finally ascends, in a purified state, through the pipe d. Fresh portions of the purifying material are supplied from the vessel k by means of the connecting pipe b; and the cream of lime, after being saturated with the impurities, is withdrawn at e. In large establishments the gas is forced in succession through several vessels of the kind described, the cream of lime Gas-Light being changed in each of them at different times, to render its action more uniform and regular.
One of the objections against the method of purifying by objections the cream of lime, or lime in a liquid state, is, that unless to the use the gas be previously freed entirely from tar, that substance of lime in enveloping it with a thin film of oleaginous matter, which, the liquid has little tendency to unite with water, carries the gas along with it in rolling bubbles, so that the internal parts of it can thus scarcely ever come into contact with the purifying materials. In some arrangements mechanical contrivances are employed to agitate and disperse the gas, with the view of exposing every portion of it, more or less, to the action of the lime; but these modes of promoting the efficacy of the process cannot be resorted to without the aid of some moving power, which, in many cases, must necessarily be attended with considerable trouble, as well as additional expense.
There is another objection to which this method of purification, even if it required not the assistance of machinery, must always be liable; namely, that the oleaginous gas, upon which the illuminating power mainly depends, is largely absorbed by water, insomuch that either oil or coal gas, standing a few days over that fluid, suffers a great deterioration of its quality, and becomes in every respect less fit for the purposes of illumination. When lime is used in the dry state, or rather in the state of a moistened powder, for purifying coal-gas, neither of these objections is applicable; and when the arrangements for that mode of purification are contrived with a due regard to the simplicity and convenience of the manipulations, the separation of the useless and noxious gases is effected more easily, and not less effectually, than by the method of liquid lime. The abstraction of the sulphuretted hydrogen becomes more perfect by adding to the lime a small portion of the peroxide of manganese, which, being a cheap substance, adds very little to the expense of the process. This method of purifying coal-gas having been adopted with success in most gas establishments, we shall now proceed to explain the nature of the apparatus by which it is carried into effect.
Fig. 8 represents an elevation in section of a dry lime Apparatus for dry lime. The lime is fresh slaked and slightly moistened, and is placed on each shelf to the depth of about 3 inches; it is then sprinkled with water from a watering-pot. It is stated that a bushel of quick lime is sufficient for the purification of 10,000 cubic feet of gas. By slaking and reducing it to powder its bulk is more than doubled: two bushels of hydrate of lime thus formed cover a surface of 25 square feet to a depth of 2½ inches. At some works a bushel of slaked lime, or half a bushel of unslaked lime, is allowed for every ton of coals distilled. Some engineers estimate that 40 lb. of lime are required for every 10,000 cubic feet of gas from average Newcastle coal. If more lime is required the coal must have been damp, or have contained more than the usual proportion of sulphur. Good Newcastle coal contains about one per cent. of sulphur; some kinds of cannel only one-half per cent. The capacity of dry lime purifiers is calculated on the assumption that 25 square feet of surface are required for 10,000 feet of gas. The purifiers are generally arranged in a set of four, three of which are usually at work while the fourth is being emptied. In fig. 8, D is the exit pipe, leading to the second purifier. In emptying one of the purifiers the cover is raised by means of the chain k, attached to the light rods e e, and the shelves are also moveable, the upper ones being taken out while the lower are charged. The spent lime contains hydrosulphuret of ammonia, and when exposed to the air it evolves sulphuretted hydrogen, carbonic acid taking its place. The poisonous liberated gas thus becomes a nuisance to the neighbourhood, but it is sometimes got rid of before the purifier is emptied, by connecting each purifier with a large horizontal pipe which opens into the chimney-shaft of the retort-house, the powerful draught of which draws off all volatile matter from the lime, air instead of gas being let in at the bottom. The cover of the purifier can then be raised, and the lime be removed without annoyance to any one. The lime is burned in ovens, and is used a second time in the purifiers, after which it becomes refuse.
An ingenious form of purifier, contrived by Mr Malam, consists of a central valve and cover, and four vessels, A, B, C, D, fig. 9, placed around it. The central valve consists of an iron cylinder, 4 feet 6 inches in diameter and 3 feet deep, supported on brick piers; the bottom is perforated with ten holes for the reception of pipes leading to and from the four purifiers, and also for the main inlet and outlet pipes. Each purifier is 5 feet square and 3 feet 6 inches deep—it contains seven layers of lime, supported on sieves of wire, or slotted plates of iron. Each purifier has a cover with short sides dipping into a water groove, and the central cylinder has also a cover fitting within it in such a way as to communicate with the pipe a, and either of the 4 inlet pipes, and also to communicate between one of the outlet pipes and the pipe h, which carries off the purified gas. The inlet pipes, b, d, f, admit the gas from the central case to the bottom of the purifiers; and the outlet pipes, c, e, g, return the gas from the purifiers back to the case, after having passed up through the layers of lime, and descended at the back of a partition plate in each purifier to the outlet pipes at the bottom. a is the main inlet pipe for conveying the gas from the scrubber or the condenser, and h is the main outlet pipe for conveying the gas to the gas-holder. The central cylinder contains water to the depth of 10 inches, and the ten pipes rise up through the bottom to the height of 12 inches, so that the mouth of each is 2 inches above the surface of the water. The cover which fits into the cylinder is 4 feet 3 inches in diameter, and is divided into five parts, the first of which, 1, fits over the inlet pipe a, and over either of the inlet pipes leading to the purifiers. The partitions 2, 3, and 5 fit each over one inlet and an outlet pipe, while one partition, 4, fits over one outlet pipe from one purifier, and over the pipe h, which leads to the gas-holder. In fig. 9 the arrangement is such as to open a communication between the inlet pipe a and the purifier A. Now supposing the gas to have passed from the scrubber or the condenser into the centre of the cylinder, its only means of escape is to pass down the pipe b into the purifier A, where it ascends through the layers of lime, and passing over the top of a dividing plate, descends and escapes from the bottom of the purifier by the pipe c back to the cylinder. Here its only means of escape is by the pipe d, which conducts it to the purifier B, in which it ascends and descends as before, returning by the pipe e to the cylinder, whence it proceeds by the pipe f into the purifier C, then along the pipe g, which is shut off from communication with any pipe except h, by which it is conveyed away to the gas-holder. By this arrangement the three purifiers A B C are being worked, while a fourth purifier D is being emptied and re-charged with lime. When it is found, on testing the gas, that the lime is unfit for its office, the purifier A is thrown out of work, and D is brought in. The frame is then shifted so as to bring the triangular division 1 over d, by which means B C D will be the working purifiers, and A will be thrown out of use. In this way, by shifting the frame round its centre over each of the four outlet pipes, any three of the purifiers can be brought into action. At the top of the frame is an upright shaft with a screw cut upon it, which works the frame up, by turning a lever which has a corresponding thread. The position of the triangular partition is then ascertained, and the communication from one purifier to another can be changed in a few minutes.
The quantity of lime necessary for purifying a given volume of coal-gas varies, as already stated, with the quantity of sulphur contained in the coal from which the gas is produced. It is proper, however, to examine at intervals, during the progress of the purification, the state of the gas by such chemical tests as are calculated to detect the presence of any of the deleterious substances with which it is usually contaminated. Thus carbonic acid is readily discovered by agitating a small portion of the coal-gas with lime water in a limpid state, the solution being quickly rendered turbid when the most minute quantity of that gas is present. Sulphuretted hydrogen is discovered with equal Gas-Light facility by causing a small current of coal-gas to play against a slip of paper moistened with a weak solution of acetate of lead, or nitrate of silver, both of which instantly become black when they are exposed to the action of sulphuretted hydrogen.
Of late years a variety of improvements have been introduced for purifying gas, which we now proceed briefly to notice. They are at present only in partial use, but are likely to lead to important results. Indeed the chemistry of the manufacture is just now in a transition state, and is receiving considerable attention from scientific men.
After the tar and ammonia have been for the most part extracted from the gas by the condenser, a further separation of ammonia is now frequently effected by passing the gas through layers of coke dust, cinder or breeze, or brick-dust, placed in trays or sieves, six or eight inches apart, in a vertical hollow shaft, and as the gas streams up through the porous column the ammonia is retained. This scrubber, as it is called, is sometimes used in conjunction with a washing vessel, and sometimes the latter only is employed, with the advantage of separating a portion of sulphuretted hydrogen and carbonic acid as well as the ammonia; but the wash-vessel is said to remove much of the olefiant gas, the illuminating power of which is very high; an objection which does not apply to the scrubber. Mr Croll has patented a method of separating ammonia by means of chloride of manganese, which has the effect also of removing much of the sulphide of carbon, of producing a saving of one-half or one-third of the lime required in the subsequent process, while a valuable product is formed by the chlorine of the manganese uniting with the ammonia, to form sal-ammoniac. Ammonia has also been separated by passing the gas through dilute sulphuric acid, the resulting sulphate of ammonia being also a valuable secondary product. The ammonia may also be separated by means of sulphate of manganese, chloride, or sulphate of zinc.
Formerly a good deal of ammonia passed off with the gas to the consumer, to the great injury of the gas meter, the gas fittings, and the furniture of houses. After the ammonia has been separated, the gas is passed into the dry-lime purifiers, which are preferable to the wet-lime, not only for the reasons already stated, but on account of the less amount of pressure required to force the gas through them. The objection to dry lime is on account of the volatile nature of the offensive hydrosulphide of ammonia, which is only mechanically combined with it, so that when the purifiers are opened, and the spent lime taken out, the oxygen of the air combines with the hydrosulphide, evolving great heat, and filling the neighbourhood with noxious odours. This serious objection is now obviated by getting rid of the ammonia between the condenser and the purifier: the salts separated by the dry lime are then no longer volatile, but, on the contrary, the spent lime becomes in some cases a valuable manure, consisting, as it does, of sulphate, carbonate, and cyanide of lime.
A method of purifying the gas, patented by Mr Hills of Deptford, is now attracting considerable attention. It is based upon the property of the hydrated oxide of iron to decompose sulphuretted hydrogen, a portion of the sulphur forming a sulphide with the iron. Quicklime is also used to separate carbonic acid, and the oxide of iron is mixed with sawdust or cinders (breeze) for the purpose of increasing the surfaces of contact, and this mixture is placed in the purifiers. When a sufficient quantity of gas has passed through it the purifiers are opened, and the mixture is exposed to the air, under which new condition it combines with oxygen, and again becomes fitted for use in the purifiers. The chemical changes which occur in these operations are the following:—The mixture of hydrated oxide of iron, &c., absorbs sulphuretted hydrogen $Fe_2O_3 + 3HS = Fe_2S_3 + 3HO$. The sulphide of iron, by exposure to the air, absorbs oxygen, and the sulphur is separated in an uncombined form $Fe_2S_3 + O_2 = Fe_2O_3 + S_n$. The mixed material can be again employed in the purification of the gas, and the process may be repeated until the accumulation of sulphur mechanically impairs the absorbent powers of the mixture. The sulphi-cyanogen which accompanies the gas is retained by the oxide of iron, and gradually accumulates in the mixture.
Chemists have also sought for substitutes for lime, or for means of diminishing the amount usually required. M. Penot recommends sulphate of lead for separating sulphide of hydrogen. Professor Graham proposes to add to the slaked lime one equivalent of crystallized sulphate of soda, which would absorb sulphide of hydrogen until two equivalents thereof were absorbed by one equivalent of lime; the lime is converted into sulphate, and the soda becomes bi-hydro-sulphuret, which might be readily washed out of the lime, and again be converted into soda by roasting, and thus be used over and over again to mix with the lime. The secondary product formed in the manufacture of chloride of lime, viz. the mixture of chloride of manganese with sulphate of soda, has also been used as an efficient gas-purifier.
Gasometers for receiving and containing the Gas before it is consumed.
As many disadvantages would be experienced by attempting to adjust the production of the gas to the rate of its consumption, it is found to be more convenient, as well as more economical, to store up such a portion of it during the day as shall compensate for the deficiency of the supply that may be furnished during the time the gas is being consumed in the course of the evening. The capacity of the vessels used for this purpose, which are incorrectly called gasometers (for they do not measure the gas, but only act as gas-holders), must be regulated by a regard to that consideration.
The form of the gasometer is generally that of an inverted Best form cylindrical cup, the diameter of which, when economy is studied, ought to be double of its depth, or at least not more than two or three inches less. Gasometers were formerly composed of sheet iron varying in weight from two to three lbs. to the square foot, well rivetted at the joints, and kept in shape by means of stays and braces formed of cast or bar iron. The sheet iron was made to overlap at the joints—a slip of canvas, well besmeared with white lead, being interposed to secure perfect tightness. The prismatic shape was also formerly adopted, but it was not found to be so convenient as the cylindrical, partly on account of the difficulty of making it retain its form, and partly on account of the greater quantity of material, compared with the capacity, that is necessary for its construction.
The gasometer on the old construction was furnished with a tank, of the same form with itself, but a little larger in dimensions, for containing the water, in which it was suspended at different altitudes, by means of a chain and counterpoise moving over pulleys. The tank was sometimes built of stone, but more frequently constructed of cast-iron plates bolted together by flanges, with an interval between them of about three-eighths of an inch, which was afterwards filled up with iron cement.
As the gasometer, when it is immersed in the water of Gas-Light.
the tank, suffers a loss of weight equal to that of the portion of fluid which it displaces; it is evident that unless some arrangement be made to counteract the varying pressure which must thus result from the different depths to which it may be immersed, the gas contained in the gasometer will be expelled, at different times, with a varying force. If, however, the weight of the chain of suspension, or rather the weight of that portion of it whose length is the same as the height to which the gasometer ascends, be equal to half the loss of weight which the gasometer sustains by immersion in water, a perfect compensation will be made, and an equilibrium will hold between the gasometer and its counterpoise at all altitudes. Thus, if the weight of the gasometer were five tons, or 11,200 lbs., and it lost by immersion a seventh part of its weight, or 1600 lbs., then the weight of that portion of the chain equal in length to the highest ascent of the gasometer would require to be 800 lbs., and the weight of the counterpoise 11,200 - 800, or 10,400 lbs.
For, the gasometer being immersed, its virtual weight is 11,200 - 1600, or 9,600.
Weight of portion of chain now acting with the gasometer 800
Sum is the weight of counterpoise 10,400
Again,
The gasometer being elevated out of the water, its weight is 11,200.
Weight of chain now acting in opposition to it 800
Difference is the weight of counterpoise 10,400
Although the compensation, by this adjustment of the weight of the chain, answers the purpose in the most effectual manner, the following method is by some deemed preferable. Let the counterpoise consist of a long cylindrical or prismatic body, having the area of its horizontal section equal to the area of a similar section of the plates of the gasometer, and be allowed to descend into the water as the gasometer rises out of it. Also let the chain be of a weight equal (length for length) to a column of water of equal bulk with the counterpoise. Then, if the weight of the gasometer be, as already supposed, 11,200 lbs., the weight of the counterpoise must be the same; but the weight of that portion of the chain which, by the above arrangement, was only equal to half the loss of weight sustained by the gasometer when immersed, must now be equal to the whole of that weight.
Then, the weight of the gasometer in the water is, as before 9,600
Weight of the chain now acting with the gasometer 1,600
Weight of counterpoise, now out of the water 11,200
Again,
The weight of the gasometer, out of the water, is 11,200
Weight of the chain, now acting in opposition to the gasometer 1,600
Weight of the counterpoise, in water 9,600
Though we have only shown, in both these modes of compensation, that an equilibrium between the gasometer and its counterpoise holds in the extreme cases, it would be easy to prove that the same thing must subsist at all the intermediate elevations of the gasometer. At the same time, it must be obvious that these contrivances, however well calculated they may be to secure the equilibrium alluded to, can have no effect in expelling the gas; and therefore, when it is wished that the contents of the gasometer shall issue from it under a certain pressure, the weight of the counterpoise must be diminished to a suitable extent. Thus, if it were required that the pressure employed for expelling the gas should be equal to that produced by a column of water three-fourths of an inch deep, then it would be necessary to diminish the weight of the counterpoise by the weight of a column of water having the same diameter with the gasometer, and an altitude of three-fourths of an inch. If the diameter of the gasometer above mentioned were, for example, 35 feet or 420 inches, the weight of a cylindrical portion of water having that diameter, and a depth of three-fourths of an inch, would be, in grains,
\[420^2 \times 7854 \times \frac{3}{4} \times 252.5 = 26236876\] grs. or 3748 lb.
Hence it would be necessary to make the counterpoise 3748 lb. lighter than it was supposed to be according to the above-mentioned arrangements, in order that the gas might issue from the gasometer under a pressure of three-fourths of an inch of water. If the calculation were conducted with extreme accuracy, the specific gravity of the gas ought also to be taken under consideration; but the object to be attained is not of so delicate a nature as to require an attention to such minute circumstances. Besides, we shall afterwards find that the value of the arrangements we have described for obtaining a uniform and equable pressure is greatly diminished; and these are even entirely superseded by a contrivance called the governor, to be afterwards explained.
Such is the old method of constructing gasometers. Of late years, however, a different system has prevailed. Instead of making them of heavy plate iron, strengthened by angle iron and stays, and of so great a weight as to require the above-described complex system of equilibrium chains and counterbalancing weights to relieve the gas from the great pressure to which it would otherwise be subjected, the gas-holders are now made so light that they actually require to be loaded in order to supply the required pressure. The practice has even been introduced of not suspending the gas-holders at all, but regulating their rise and fall by means of guide rods placed round the tank. Fig. 10 represents a section of a modern gas-holder, in which will be seen a contrivance for getting rid of a large body of water in which the gasometer was formerly sunk, and which was found inconvenient in frosty weather, when, to prevent freezing, it was necessary to introduce steam. The objections to the contact of a large body of water with the gas have been already referred to. By using a central core of masonry, brickwork, earth, or sheet iron, the large volume of water is got rid of—the only water required being that contained in the ring-shaped space or tank, TT, round the core. Small Gas-Light. Gas-holders on this construction may be suspended from the centre by a chain, while to the pillars of the triangular or polygonal cast-iron frame, i.e., guiding rollers are attached to facilitate the motion of the cylinder, and to keep it horizontal. In forming the tank, T.T., the brick-work and the mortar joints must be perfect, or the core will soon be injuriously acted on by the water. When the whole of the core is not built up, but a solid mass of earth is left in the middle, it is first dressed to the proper shape, and the slope is usually puddled, as at p. The brick-work, b, is carried up in Roman cement, or in the best hydraulic mortar, and it is recommended that the puddle behind the brick-work be clay, with a portion of sand or silt. The surface of the core is covered with concrete, c.c. On one side of the tank is a well, containing the inlet and exit pipes, mm. The inlet pipe enters at the top of the well, and passes to the bottom of the tank, where there is a vessel for the reception of any tar or moisture that may by chance be conveyed by the gas; this pipe then passes horizontally through the brick-work, and rises up vertically through the water, so that its mouth may be about two inches above the surface. The outlet pipe stands by the side of the inlet pipe, and, descending through the water, passes out through the wall of the tank into the stand-pipe well, where there is another receptacle for tar and water; this pipe proceeds up the well either to the governor or to the main.
In situations where a sufficient depth of the tank cannot be easily obtained, an expedient may be resorted to, such as is represented in fig. 11, and known as the telescope gasometer. It consists of two, or even three parts, separable from each other; the one having the form of the common gasometer, and the other being open at the top as well as the bottom, but connected with the other by a channel running around the bottom, which is filled with water. The lower part has a recurved form at the top; and this portion of it entering into the channel becomes water-luted, by which the entire gasometer is rendered air-tight at the line of junction. When the gas passes in by the inlet pipe the inner cylinder rises first, and when its lower edge nearly reaches the surface of the water its curved flange catches the flange of the next cylinder, which also rises, and when this has risen to the proper height it catches the third cylinder if there be one, and raises it also. The water contained in the lower flange of each cylinder effectually prevents the escape of the gas or the entrance of atmospheric air.
The pipes by which the gas is commonly introduced and conducted off being in many cases considerably below the level of the street pipes with which they communicate, are apt to be filled up in the course of time with the condensed water which passes off in a vaporous state with the gas. To remedy this inconvenience, it is necessary to place vessels for receiving that water in connection with the entrance and exit pipes, so contrived that the accumulated water may be easily removed from them when required. One of these vessels, which are improperly termed syphons, is represented by fig. 12. When the water rises to the height d, e, it is drawn off by means of a small suction pump, introduced into the pipe ae, which reaches within about an inch of the bottom of the syphon. To prevent too much water being removed, and thus allowing the gas to escape at ae, the lower end of the pipe of the suction pump should be perforated with small holes to as great a distance as the water is to be left in depth; or, if deemed preferable, the upper end of the pipe ae may be rendered air-tight by a screwed cap.
When access to the bottom of the tank is difficult, swivel or flexible-jointed tubes, disposed so as to rise and fall with the gasometer, are sometimes resorted to. Jointed tubes, connected by means of water-lutes, are also occasionally employed, and they are found to answer the purpose in situations where the water is not exposed to frost.
Of the Main and Service Pipes.
The gas being duly purified and prepared for combustion, the next point to be considered is the transmission of it from the gasometer to the various places where it is to be consumed. As it must sometimes be conveyed, particularly in the case of large establishments, to the distance of several miles, it is evident that unless the diameters of the various pipes through which it is to be conducted have a due relation to the quantity of gas to be transmitted, there will be a danger either of incurring an unnecessary expense, by making the pipes too large; or, what is still worse, of being exposed to a deficiency of supply, by making them too small. The first object, therefore, to be ascertained by the engineer, is the probable number of lights that may be required in the various streets and lanes in which these pipes are to be laid; and these being known, the corresponding quantity of gas, according to the quality of it, may be afterwards computed. With regard to the relative dimensions of the pipes at different distances from the gaswork, the only general rule to be observed is, that the sum of the areas of the sections of the main pipes proceeding immediately from the gasometer should be equal to the sum of the areas of the sections of the various branch-pipes which they supply with gas; and this rule, with some little modification, should be followed in the case of the subordinate ramifications.
In the case of good coal gas, we may safely reckon that Dimen. one-fourth of a cubic foot of it will furnish the light of a tons of moulded candle for an hour, of which one pound will, when Pipes. the candles are burnt in succession, last forty hours. On this supposition, and assuming that the pressure upon the gas in the gasometer is equal to three-fourths of an inch of Gas-Light. water, the diameters of pipes necessary for conveying various quantities of gas may be stated as follows:
| Diameter of Pipe in Inches | Quantity of Gas in Cubic Feet per Hour | Equivalent Number of Candles | |---------------------------|----------------------------------------|----------------------------| | 1 | 4 | 16 | | | 20 | 80 | | | 50 | 200 | | | 90 | 360 | | | 120 | 1,520 | | | 1,580 | 6,320 | | | 2,480 | 9,920 | | | 3,580 | 14,320 | | | 4,880 | 19,520 | | | 6,380 | 25,520 | | | 8,090 | 32,360 | | | 10,000 | 40,000 |
This table has been deduced partly from theoretical considerations, and partly from the results of experiment. Peckstone affirms, in his work on Gas-lighting, that a pipe ten inches in diameter, is capable of transmitting 50,000 cubic feet of gas per hour, under a pressure of one inch of water; while, according to the statement of Mr Creighton, such a pipe would scarcely convey the tenth part of that quantity, under a pressure of from four-eighths to three-fourths of an inch of water. It is impossible to reconcile these discordant statements either by an allowance for the difference of pressure or the difference of the specific gravities of the gases; for it ought to be kept in view that the discharge of gas is directly proportioned to the square root of the height of the column of water by which it is pressed, and inversely as the square root of the specific gravity of the gas. Both of these propositions, however, must be greatly modified by friction, and consequently by the length of the pipes through which the gas is conveyed.
In the supply stated to be furnished by pipes of different dimensions, we have deemed it safest rather to underrate the quantity than overrate it.
The main-pipes are usually made of cast-iron, joined together with socket-joints, in lengths of three yards. The depths of the sockets vary in pipes of different sizes from three to six inches, part of them being fitted with gasket to bring the centres of the pipes into line, and the remainder with lead after the gasket has been driven home with suitable chisels or caulking irons. The depth of lead to secure a good joint should not be less than an inch and a half; the interval between the spigot and socket being from three-eighths to seven-eighths of an inch, according to the diameter of the pipe. Joints are now frequently made without lead. One plan is to caulk into the bottom of the socket, to the depth of two inches, white rope-yarn covered with putty, and to nearly fill up with tarred gaskets, leaving a gate into which is poured a composition of melted tallow and vegetable oil. Another plan is to bore the socket of the pipe with a slightly conical opening, the small end being similarly turned to fit the socket. The two ends of the pipe are coated with a mixture of white and red lead, and being brought together, are driven home by a mallet. Such a joint is said to be quite tight. Rings of vulcanized India rubber have also been recommended for the joints of gas and water pipes.
As a considerable quantity of water is carried off by the gas in the state of vapour, which is afterwards condensed in the pipes, some arrangement must be made for its collection and occasional removal; and accordingly, in laying the pipes, care must be taken to give them a regularity of declivity towards one or more points, where proper syphons, close vessels, and cocks must be placed, to receive and discharge the collected water. When these precautions are neglected, or when the levels are inaccurately taken, much annoyance is experienced; and as the evil can only be corrected by lifting and rejoining the pipes, the utmost attention should be paid to guard against it at first.
For stopping off the gas in large pipes, valves fitted with water or mercury, according to the degree of pressure to be resisted, are usually employed. One of the most simple forms of these valves is represented in fig. 13, where aa is the valve, having the shape of an inverted cylindrical cup, which is raised and depressed by a rod e passing through an air-tight stuffing-box; and bb an annular cavity or interval, partially filled with water or mercury, into which the valve descends when it is shut.
To convey the gas from the main-pipes, and distribute it through the various apartments of dwelling-houses, pipes made of block-tin are generally used; these being more durable and better adapted to the purpose than pipes composed of copper or any other metal. In arranging the interior fittings, the same precautions must be observed as were recommended in the case of the main-pipes, viz. to give the various branches a due degree of inclination, so as to cause all the condensed water to flow to one or more points, where proper cocks must be placed for its removal. Unless this be done, the lights will be apt to flicker, or be extinguished at times altogether. Nor is it of trivial moment to enjoin the workmen, when they are soldering the service-pipes, to avoid with the utmost care allowing any of the melted metal to find its way into the inside of the pipes; it being in a great measure to this circumstance that the deficiency in the supply of gas, so frequently complained of, is owing.
Of the Governor or Regulator.
The quantity of gas consumed in large towns varying greatly at different times, it is evidently a matter of some importance to the public, as well as to the manufacturers of gas, that the supply of it should be duly adjusted to the consumption; so that when the lamps are once regulated to a proper height of flame, they may continue afterwards to burn with the same steady light throughout the whole of the evening. Any contrivance that can accomplish so desirable an object must save a great deal of trouble to the consumer of gas, and much unnecessary waste of it to the manufacturer; and such is the design of the governor or regulator. Fig. 14 represents one of these contrivances, d being the pipe proceeding from the gasometer, by which the gas is admitted, and e the pipe by which it escapes; f is a valve of conical form, fitted to the seat i, and raised and depressed by means of the weight attached to a cord passing over a pulley; bb is a cylindrical vessel formed of sheet iron, which ascends and descends in the exterior vessel aa, in which water is con- Gas-Light, tained to the level represented. The gas, entering at \(d\), passes through the valve, fills the upper part of the inverted vessel \(bb\), which it thus partially raises, and escapes by \(e\). If the pressure from the gasometer be unduly increased or diminished, the buoyancy of \(bb\) will be increased or diminished in like proportion, and the valve being by this means more or less closed, the quantity of gas escaping at \(e\) will be unaltered. And not only will the governor accommodate itself to the varying pressure of the gasometer, but also to the varying quantities of gas required to escape at \(e\) for the supply of the burners. Thus, if it were necessary that less gas should pass through \(e\), in consequence of the extinction of a portion of the lights, the increased pressure which would thus be produced at the gasometer would raise the governor, and partially shut the valve, till the state of it was duly adapted to the requisite supply of gas.
When a large district is supplied by a single gas company, and different parts of the same district consume variable quantities of gas, variable pressures are required. One part of the district where there are numerous shops will consume more than another part which consists chiefly of private houses, so that the pressure for the former must be greater than that required for the latter. For example, the Westminster district has about 20 such divisions, comprising nearly 150 miles of main, and the varying pressures required for each division are managed as follows:
—In the superintendent's room there are a number of small gasometers called pressure indicators, and over each is the name of the sub-district to be supplied. Each gasometer (A, fig. 15) is about 12 inches in diameter. It is supported in a tank of water in such a manner that it can rise and fall with the varying pressure in the mains with which it is connected by the pipe B. At the upper part of the gasometer is a rod C, carrying a black lead pencil, which bears upon a cylinder D, which is covered with a sheet of paper, along the top of which are marked the twenty-four hours of the day. From these hours perpendicular lines are drawn to the bottom of the sheet, and there are also horizontal lines, and the bottom is divided into tenths. The cylinder is connected with a time-piece, so as to rotate on its axis, by which means the pencil draws a line opposite the hour when it is set going. If the pressure be constant for a number of hours, the pencil will of course describe a portion of the circle round the cylinder parallel with the top and bottom edges of the paper, or a straight line when the paper is unrolled; if the pressure vary, the line will be diagonal or zig-zag. At the end of twenty-four hours the paper is taken off the cylinder, and replaced by a new one. A collection of these papers for each district furnishes an index to the supply of gas at any hour of the day to the sub-district to which it refers.
It is often necessary to ascertain the pressure to which the gas is subjected in the various forms of apparatus used in the manufacture. For this purpose a simple gauge is attached thereto, consisting of a bent graduated glass tube, containing a portion of water or of mercury, as shown in fig. 16. If one end of the tube be screwed into a vessel or an upright pipe as in the figure, containing gas of the same pressure as that of the external air, the liquid will stand at the same height in the two limbs of the gauge. If the pressure of the gas be greater than that of the external air, the liquid will rise in the open limb, and the pressure of the gas will be 1, 2, or more inches, according to the height to which the liquid rises. But if the pressure of the gas be less than that of the atmosphere, the atmospheric pressure, which always acts at the open end of the tube, will prevail, and the liquid will be depressed in the open limb, and rise in the other.
The Gas-Meter.
The gas-meter is a simple but ingenious mechanical contrivance, the design of which is to measure and record the quantity of gas passing through a pipe in any given interval of time. Experience has proved it to be no less advantageous to the consumer than to the manufacturer of gas, by allowing the former to use gas without any unnecessary waste of it, and securing to the latter a fair and regular price for the quantity of it actually consumed.
There are two forms of meter in actual use, viz., the wet and the dry. The former, the invention of Mr Clegg, is represented in the following figures. In the sections, figs. 17, 18, \(ee\) represent the outside case, having the form of a flat cylinder; \(a\) is a tube which enters at the centre for admitting the gas, and \(b\), fig. 17, is another for conveying it off to the burners; \(g, g\) are two pivots, one supported by the tube \(a\), and the other by an external water-tight cup, projecting from the outside casing, and in which is contained a toothed wheel \(h\), fixed upon the pivot, and connected with a train of wheel-work (not shown in the figure) to register its revolutions. The pivots are fixed to and support a cylindrical drum-shaped vessel \(ddd\), having openings \(e, e, e\), internal partitions \(e', e', e'\), and a centre piece \(fff\). The machine is filled with water, which is poured in at \(h\) up to the level of \(e\), and gas being admitted under a small pressure at \(a\), it enters into the upper part of the centre piece, and forces its way through such of the openings \(f\) as are from time to time above the surface of the water. By its action upon the partition which curves over the opening \(a\), a rotary motion is communicated to the cylinder; the gas from the opposite chamber being at the same time expelled by one of the openings \(e\), and afterwards escaping at \(b\), as already mentioned.
As the quantity of gas which passes through the machine in any given time depends not only upon its internal dimensions and the number of revolutions which it performs, but also upon the level of the surface of the water in which the Gas-Light cylinder revolves, due care must be taken to maintain the water at the same level, for the regular action of the meter. This is easily accomplished, by pouring in water when necessary, till the superfluous quantity is discharged by an orifice properly placed for the purpose.
One great objection to the wet meter is, that the water is liable to freeze in winter, by which means the supply of gas is stopped; it has been proposed to use a solution of caustic potash or soda instead of water, as being less liable to freeze, and exerting a beneficial action on the gas by removing traces of carbolic acid or sulphide of hydrogen. A second objection is, that if the water level be lowered so that one compartment may at the same time communicate with the central and outer spaces \( f \) and \( d \), more gas will pass than can be registered, an effect sometimes produced by the dishonest consumer tilting forward the meter. In the dry meter, as its name implies no liquid is used, and the gas is measured by the number of times that a certain bulk of it will fill a chamber constructed so as to contract and expand for the passage of the gas. These alternate contractions and expansions give motion to certain valves and arms, which, with the aid of a train of wheels, turn the hands of the dials as in the wet meter. The two forms of dry meter which have attracted most attention are Defries's and Croll and Glover's. Defries's meter consists of three measuring chambers separated by leather partitions partially covered by metal plates, and as they expand by the pressure of the gas they assume the form of a cone on one side or other, the motion of which backwards and forwards drives the measuring machinery, and by an action somewhat similar to that of a three-throw pump, a continuous stream of gas is ejected. This incessant bending of the leather backwards and forwards causes it to wear rapidly, while the efficiency of the meter obviously depends on the soundness of the leather. In Messrs Croll and Glover's meter the leather is applied in perhaps a less objectionable form. This meter consists of two short metal cylinders, each closed at one end; \( AA \), fig. 19, representing one such end attached to a fixed central plate \( BB \), by means of broad bands of leather, which act as hinges, allowing one side to swell out with gas, while the other parts with its gas by being pressed in towards the centre plate. The to-and-fro Gas-Light motion of the discs which close the short cylinders affords means for measuring the gas. Each disc is kept in place by a hinge joint \( S \) attached to upright rods, \( RR' \). There are also parallel motions \( ezy \) attached to each disc, and to the top plate of the meter. As the gas passes into each cylinder and distends it, the rods \( RR' \), one on each side, are made to move each through half of a circle by means of jointed levers \( S \) attached to them. At the top of each rod are two arms \( Rad, R'ad \), fig. 20, each of which partakes of the motion of the rods \( RR' \) describes alternately the arc of a circle, and a rotatory motion is obtained by means of connecting rods attached to these arms, and also to two other arms \( rr \) which work two \( D \) valves \( DD \), each of which is made to slide backwards and forwards over three apertures, the two outer of which lead to the inside and outside of the cylinders respectively, and the middle aperture to the exit pipe \( E \). It is the function of these valves to regulate the flow of gas into and out of the two chambers of each division of the meter. While the gas is flowing into one cylinder and distending it, the gas on the other side of this cylinder disc is expelled to the exit pipe \( E \); as soon as this is done, the valve is reversed and gas enters on the side of the disc from whence it was last expelled. The process is then repeated by the other disc, and in this way a continuous flow of gas is obtained by means of the two valves \( DD \), which being placed at right angles to the double-cranked shaft, and the two cranks on the shaft being at an angle of 45° to each other, it follows that as one valve closes the other opens, but the closed valve always begins to open before the other is quite shut. In fig. 20, the dotted portion represents one of the short cylinders \( A' \) distended with gas, and the other cylinder \( A \) collapsed.
We will now trace the course of the gas in its passage through the meter. Suppose a continuous stream of gas under pressure to be passing down the inlet pipe \( I \). On arriving at \( i \) it meets with a horizontal tube which conducts it by the aperture \( o \), fig. 20, in the direction of the arrow into a triangular chamber \( VV \). It then passes down an open slit of one of the valves, which we will call No. 1, and entering one of the cylinders, distends it and forces the gas which was on the outside of the disc to escape through slit No. 2, and so along a tube \( k \) leading to the exit pipe \( E \). While this action is going on, that is, while the cylinder on one side is being distended, the cylinder on the other side is already full, the gas is shut off from it by the sliding valve \( D \), and is made to pass on the outside, where exerting its pressure on the disc \( AA \), it forces it inwards, and the gas escapes along a short pipe attached to either side of the partition \( BB \) into slit No. 2, and so escapes to the exit pipe. The triangular chamber \( VV \) has no connection with the cylinders, &c., situated below it except through the tubes already indicated, and the train of wheels \( W \), fig. 20; and the dials are also so boxed in as not to be exposed to the corrosive action of the gas. The rods \( R' \) pass into this Gas-Light.
The cylinders are inclosed in an oblong box of iron plate or galvanized iron, so as to be completely concealed from view. The pressure to which the gas is subjected in order to force it along the mains is amply sufficient to work this meter. If the gas were subjected to the pressure of only half an inch of water, this quantity multiplied into the area of the disc, which in a ten-light meter is ten inches in diameter, amounts to many pounds.
The circular motion of the double crank is transmitted by means of an endless screw c, fig. 19, and a spur-wheel b along a wire b, fig. 20, to a train of wheels W, which record their revolutions on the face of the dials G, also shown separately in fig. 21, registering the number of cubic feet of gas consumed, in units, tens, hundreds, thousands, &c. The top circle marks the units, the left hand circle hundreds. The motion of the hand from 0 to 1 shows that 100 cubic feet of gas have passed through the meter, while a whole revolution of this hand registers ten times that quantity, or 1000 cubic feet. The motion of the hand of the centre circle from 0 to 1 indicates 1000 feet, and a whole revolution 10,000 feet. The right hand circle, in a similar manner, indicates in a whole revolution 100,000 feet.
In reading off the numbers on the circles, we take the number at which the hand is pointing, or the lower of the two numbers that the hand is between. In fig. 21, beginning with the right-hand dial, the hand is between 9 and 0, showing that nearly a whole revolution has been accomplished; we therefore write down,
\[ \begin{align*} 90,000 & \text{ for the right-hand dial,} \\ 8,000 & \text{ for the middle dial,} \\ 700 & \text{ for the left-hand dial.} \end{align*} \]
If the collector, in taking the register three months before, had recorded the quantity as 73,200, this quantity, deducted from 98,700, gives 25,500 cubic feet as the consumption of gas for three months. The top or units dial is not used in registering, but it serves to indicate to the collector as well as to the consumer that the dial is acting properly, the more rapid motion of the hand facilitating this object.
Burners.
The most economical mode of consuming gas, so as to obtain from a given volume of it the greatest possible quantity of light, both in degree and duration, is a problem of no less importance than that of the most suitable arrangements for its production and purification. The presence of oxygen, in some form or another, being essentially necessary to produce ordinary combustion, it follows, that from whatever cause that principle may be deficient in quantity, the combustion must be imperfect; and when this is the case, the light yielded by the combustible body is also diminished in a proportional degree. On the other hand, if the quantity of oxygen brought into contact with the combustible body be more than sufficient for its entire combustion, the superfluous quantity of that gas, instead of augmenting the effect, can only lower the temperature, and diminish, it may be presumed, in a corresponding degree, the intensity of the light. This must be the consequence if the brilliancy of the light yielded by a combustible body depends at all upon the temperature to which it is exposed during its combustion; and that this is the case may be inferred from the simple fact of causing the flame of a jet of gas to play, first against a sheet of ice, and then against a bar of red-hot iron, when the difference of the light will be such as to leave no doubt of the influence of temperature upon its intensity. A similar result is obtained by bringing the flames of two separate jets into contact, when an obvious increase of light is perceived. From these simple facts it may be inferred, that though a certain quantity of common air must be brought into contact with the inflamed gas to produce the greatest intensity of light, whatever exceeds that quantity will not only be useless, but, by diminishing the temperature of the flame, must tend to impair the brilliancy of its light.
But although the immediate cause of the light is probably the high temperature to which the carbonaceous portion of the gas is exposed, the condition in which the carbon exists at the time it is so exposed is of the utmost importance to the effect. According to the opinion of Sir Humphry Davy, as adopted by Drs. Christison and Turner, "a white light is emitted only by those gases which contain an element of so fixed a nature as not to be volatilizable by the heat caused during the combustion of the gas; and that in coal-gas this fixed element is charcoal, formed by the gas undergoing decomposition before it is burnt. The white light is caused by the charcoal passing into a state, first of ignition, and then of combustion. Consequently no white light can be produced by coal or oil gas without previous decomposition of the gas."
"That the gas undergoes decomposition before it burns, and that the carbonaceous matter is burnt in the white part of the flame in the form of charcoal, is shown by placing a piece of wire-gauze horizontally across the white part of the flame, when a large quantity of charcoal will be seen to escape from it unburnt. And that this previous change is necessary to the production of a brilliant white light will appear, if we consider the kind of flame which is produced when decomposition does not previously take place. For example, if the gauze be brought down into the blue part, which always forms the base of the flame, no charcoal will be found to escape. Or, if the gauze be held at some distance above the burner, and the gas be kindled not below but above it, by which arrangement the air and the gas are well mixed previous to combustion, the flame is blue, and gives hardly any light. The reason is obviously, that in both cases the air is at once supplied in such quantity in proportion to the gas that the first effect of the heat is to burn the gas, not to decompose it." (Edin. Phil. Journ. No. xxv.)
To these statements it may be added, that if a jet of oil circumstances in air, be suddenly surrounded with an atmosphere of oxygen gas, the colour instantly changes into a pale blue, yielding the most feeble light; nor does the flame recover its brilliancy until the oxygen is largely diluted with carbolic acid, when it burns for a short time with greater splendour than at first. For although the light is greatly enfeebled when the combustion of the gas takes place in pure oxygen, it becomes much more vivid when the combustion is carried on in air that is more largely charged with oxygen than common air. Hence the brilliancy of the light appears to depend upon two conditions; 1st, the perfect combustion of a portion of the gas in an undecomposed state; 2dly, the temperature produced by that combustion upon the re- Gas-Light. sidual part in a decomposed state. When a large portion of the gas is consumed in the first condition, the temperature is higher; but the undecomposed part is then too small in quantity to yield an intense light, in consequence of the attenuated state of the carbon; and, on the other hand, when a small portion of the gas is consumed in the undecomposed state, the temperature produced is too feeble to raise the temperature of the now partially decomposed part to a sufficient pitch for the full ignition of the carbon.
The conditions which thus seem to be necessary for obtaining the greatest portion of light from the combustion of a given quantity of gas, while they are perfectly consistent with the most anomalous facts presented by that process, so they appear to afford the only sure principles upon which we can proceed in the construction of gas-burners. One of the most obvious conclusions deducible from these principles is, that whatever be the form of the gas-burner its construction should be such that while it admits as much air as is necessary for the perfect combustion of the gas, it should never admit more than is barely sufficient for that purpose.
According to the experiments of Drs Christison and Turner, the diameter best fitted for single-jet burners appears to be about one twenty-eighth of an inch for coal-gas, and one forty-fifth for oil-gas. As these dimensions, however, must vary with the quality of the gas, we consider one thirty-sixth of an inch to be more applicable to the gas obtained from cannel coal, if its specific gravity be not less than '65. Every form of burner composed of separate jets, in which the gas is made to issue in a horizontal or oblique direction, gives a consumption which increases in a much faster ratio than the light which it yields; and consequently, however beautiful such burners may be in appearance, they are far from being economical.
One of the most useful forms of a burner with single jets, is where there are two holes, and their directions are so inclined as to cause the streams of issuing gas to cross, and exhibit during their combustion a broad continuous flame. This burner, which is termed a scalloped-tail, is well adapted for street-lights, as it gives a powerful light, and consumes a small quantity of gas. When the gas is emitted by a narrow slit at the top of the burner, the burner receives the name of a bent-wing. Specimens of common gas flames are represented in figs. 22, 23, 24.
But of all the forms of the burner, that upon the Argand principle, in which the holes are arranged in a circle, d, fig. 25, so as to allow the air to have access to the flame internally as well as externally, is the most economical, and the best calculated to secure the complete combustion of the gas. The diameter of the holes should, in this burner, be about the fortieth part of an inch for coal-gas of an ordinary good quality, and the distance between them should be such as to allow the separate flame of the different jets to unite together and form a continuous hollow cylinder of light. In fig. 25, a is the pipe which supplies the gas, and b the channel up which it passes to the holes shown in the lower figure.
The construction of burners, and the most economical mode of consuming gas, having been examined with much philosophical precision by Drs Christison and Turner, we shall extract from their elaborate dissertation on the subject the most valuable and important conclusions which they have deduced from their experiments; and this we do with greater confidence, because the results they obtained coincide very exactly with those which the writer of this article procured when engaged in the same inquiry. The three leading points to which they directed their attention were, 1st, the length of flame most suitable for different burners; 2dly, the form, magnitude, and position of the orifices through which the gas is discharged; and, 3dly, the modifications of the light produced by the glass chimney of the Argand burner.
With regard to the length of flame which afforded the greatest light compared with the expenditure of gas, they found that, in the case of the jet, the best length for coal-gas was about five inches, and for oil-gas about four inches. When the flame was kept shorter, the quantity of gas consumed was greater in comparison of the light which it yielded; but no advantage was gained by increasing the length beyond that mentioned as the most suitable for each gas; the combustion becoming less perfect, and beginning to be accompanied with the escape of the carbon in the form of smoke. Thus they found that, in the case of coal-gas having the specific gravity '602, while the lights emitted from a two-inch and a five-inch flame were as 556 to 178, the corresponding expenditures were to each other as 605 to 1437. But the light, in an economical point of view, must be estimated inversely as the quantity of gas from which it is obtained; and hence the ratio of the lights, in reference to the expenditure, was as 556 to 178, being as 100 to 150.
In the case of Argand burners, the augmentation of the light in a ratio greater than the expenditure was exemplified in a still more remarkable degree. Thus the following results were obtained with coal-gas of the specific gravity '605, by elevating the flame of a five-holed burner, successively from half an inch to five inches.
| Length of Flame | Half-Inch | One-Inch | Two-Inch | Three-Inch | Four-Inch | Five-Inch | |-----------------|-----------|----------|----------|------------|-----------|----------| | Light | 18.4 | 92.5 | 259.9 | 308.9 | 332.4 | 425.7 | | Expenditure | 83.7 | 148 | 203.3 | 241.4 | 265.7 | 318.1 | | Ratio of Light to expenditure | 100 | 292 | 590 | 582 | 582 | 604 |
Hence the light is increased about six times for the same expenditure by raising the flame from half an inch to three or four inches; but very little is gained by any additional increase of the flame beyond that length, in the description of burners with which the experiments were made.
These facts receive a satisfactory explanation from the general principles which we have already laid down with respect to the combustion of the luminiferous gases. When the flame is short, the supply of oxygen for the combustion is too great; almost the whole of the gas is thus consumed before any portion of it can undergo the decomposition which is necessary for the evolution of light; while the temperature of the flame being reduced by the superfluous air which brushes along its surface, the intensity of ignition, and with it the splendour of the light, is... Gas-Light proportionally diminished. This explanation is well illustrated by partially shutting the central part of the burner, and thus interrupting the supply of air to the internal surface of the flame; the moment this is done, the length of the flame is increased, and a visible improvement of the light takes place, thus indicating that more air was previously brought in contact with the gas than was requisite for its perfect combustion.
The second point to which Drs Christison and Turner directed their attention was the construction of the burner itself, particularly the magnitude and position of the orifices at which the gas is emitted during its combustion. The same principles which explained the relation between the light and the expenditure in the case of flames of different lengths, suggested the rule for regulating the dimensions of the orifices; and accordingly they justly inferred that, in a single jet, the diameter of the aperture ought to be such as to ensure the complete combustion of the gas, without rendering it more vivid than is necessary for that effect. If the orifice be too small, the greater portion of the gas is liable to be consumed without suffering a previous decomposition, and thus the light is extremely feeble; and, on the other hand, if the orifice be too large, the surface of the flame exposed to the action of the air being too small in comparison of the discharge of gas, the combustion is imperfect, and the carbon, after being separated from the hydrogen, either burns at a low temperature with a dusky flame, or, what is still worse, a large portion of it passes off in the state of smoke. In conformity with these views, they recommend, as we have already stated, a twenty-eighth of an inch for coal-gas, and a forty-fifth for oil-gas, as the most suitable dimensions for single jets. They acknowledge, however, that their experiments with coal-gas were too limited to justify them in using very confident language on the subject; and we have therefore the less hesitation in stating that we consider an orifice varying in diameter from a thirty-second to a thirty-sixth of an inch as better adapted to coal-gas of a specific gravity between '62 and '70.
In Argand burners the diameter of the orifices ought to be a little smaller. Drs Christison and Turner state that the diameter which appeared to answer best for coal-gas of the specific gravity '6, when the holes are ten in a circle of three-tenths of an inch radius, was a thirty-second of an inch. We consider this, however, to be too great for coal-gas of a better quality, and would recommend, in preference, apertures varying in diameter from a thirty-sixth to a fortieth of an inch.
The distance between the jet-holes of Argand burners is a matter of no less importance than the diameter of the orifices, and must be regulated by the same principles. When they are so far asunder that the flames of the separate jets do not coalesce, no advantage is derived from the Argand form; but when they unite, and compose a uniform and unbroken surface of flame, the light is considerably greater, compared with the expenditure of gas, than is obtained from detached jets. In order to determine the most suitable distance at which the orifices of Argand burners should be placed, Drs Christison and Turner employed burners six-tenths of an inch in diameter, which they caused to be drilled with eight, ten, fifteen, twenty, and twenty-five holes, a fifth of an inch in diameter; and having determined with each of these burners the light and expenditure in the case of oil-gas, they obtained the following results:
| Burners | VIII | X | XV | XX | XXX | |---------|------|---|----|----|-----| | Light | 350 | 350| 391| 409| 882 | | Expenditure | 367 | 318| 296| 289| 275 | | Ratio of light to expenditure | 99 | 113| 132| 141| 139 |
As the standard of comparison was a single jet, burning with a four-inch flame, the ratio of the light yielded by which to the expenditure was expressed by 100, it was inferred that no advantage is gained by giving the jets the Argand arrangement with a burner of the dimensions above mentioned if the holes are only eight in number; and that the gain does not increase after the number reaches to twenty. In the former case the distance of the holes must have been 2356 inch, or nearly one-fourth of an inch, and, in the latter, .0945; so that the most advantageous distance for jet-holes of a fifth of an inch in diameter would seem to be about \( \frac{1}{8} \)ths of an inch. For coal-gas burners, however, the distance between the jet-holes ought to be increased in a ratio varying inversely with the quality of the gas, or directly as the diameters of the orifices themselves. Hence, if the coal-gas were of an ordinary quality, the jet-holes should not be less than one-eighth nor more than one-sixth of an inch from each other.
The distance between the orifices being once assumed, serves to determine the diameter of the circle of holes, of circle Thus, in a burner of eighteen holes, each a seventh of an inch asunder, the circumference ought to be \( 18 \times \frac{1}{7} = 2.57 \) inches, and consequently the diameter of the circle of holes should be \( \frac{2.57}{3.1416} = 0.818 \) inch. If the breadth of the rim be supposed to be a tenth of an inch, and perhaps it ought not to exceed that quantity, it may be proper, in the case of the larger burners, to contract the lower part of the central air-hole, on account of the supply of air to the inside surface of the flame increasing in a faster ratio than the number of jets.
The only remaining point to be considered with respect to the burner is the glass chimney, which serves at once to protect the flame from irregular currents of air, and to convey to the gas a due supply of it during combustion. When the interval between the chimney and the external part of the flame is too great, the tendency of the air to flow through the air-hole is diminished, and the flame contracts towards the top, where it yields a dusky light, and indicates a disposition to smoke. The diameter of the chimney should therefore be reduced until it is perceived that the upper part of the flame is enlarged, and acquires the same diameter as the lower part. When this is the case, the colour of the flame is improved in brightness, and none of the carbon is useless wasted in the formation of smoke. On the other hand, if the supply of air to the external surface of the flame be diminished beyond a certain extent, either by reducing the diameter of the glass chimney, or by any other means, the flux of air through the central air-hole is unduly increased, the flame diverges in the form of a tulip till it touches the chimney, and the supply of air to the outside of the flame being thus interrupted, smoke is again produced. Hence the greatest degree of light, in relation to the expenditure of gas, may be expected to be obtained when the supply of air to the external and internal surface of the flame is so adjusted by the diameter of the chimney that the flame is perfectly cylindrical, neither burning with too much vivacity, nor showing any tendency to smoke. The length of the glass chimney is of much less importance than its diameter, and may vary from five to six inches.
A cylindrical chimney, however, is the least advantageous form that can be adopted. If the chimney be tall and narrow, and contracted towards the top, as in fig. 26, or suddenly contracted near the bottom, as in b, the draught is increased and the light improved. It is also useful to contract the Gas-Light. diameter of the glass chimney about a couple of inches above the burner, as at c, so as to form a shoulder a few lines in width, the effect of which is to change the direction of the draught, and project it on the flame at a certain angle.
Bude Light. In the Bude light proposed by Gurney, oxygen gas instead of air was passed through the flame, the effect of which was greatly to increase its brilliancy. In the Bude light as now constructed, there are two, three, or more concentric burners with chimneys supplied with common air, and a dioptric apparatus.
Ventilation. Attempts have been made of late years to ventilate gas burners so as to get rid of the injurious products of combustion. One part by weight of good coal-gas produces nearly three parts by weight of carbonic acid, which produces many distressing symptoms when breathed with the air of the room. Sulphurous acid, and other compounds which are not entirely removed in the purification of the gas, form deleterious products during the combustion of the gas. The sulphurous acid forms sulphuric acid, which exerts a corrosive action on the walls and furniture, books, pictures, &c., while the hydrogen of the gas produces vapour of water which serves as a vehicle for some of the other products. To get rid of these noxious fumes a bell-shaped vessel is sometimes suspended over the chimney, and is connected with a tube leading into the open air. Unless this tube be judiciously arranged the condensed water of the gas may accumulate in it, and cause inconvenience. By a contrivance of Dr Faraday, a copper tube of about the same diameter as the flue is conducted from its summit out of the apartment; the heat of this tube establishes a rapid current, which serves to convey away the products. The same distinguished chemist invented another contrivance, by which the ventilating current is made to descend between two concentric glass chimneys of different heights, the outer one being the taller, and this is covered with a disc of talc. When the current reaches the bottom of the space between the two glasses, it is conveyed away by a ventilating tube which bends upwards. The descending current is first established by applying heat to the bend of the ventilating tube where it begins to ascend; when this current is established the gas is lighted, and the plate of talc is put on: the products of combustion are conveyed into a box, from which proceeds a pipe for conveying the vapours outside. A globe of ground glass open only at the bottom is placed over the lamp. The accumulation of condensed water in different parts of this apparatus is said to have greatly interfered with its successful action.
Mr R. Brown of Manchester has a contrivance for ventilating by means of gas. Through an opening in the ceiling a wide tube is passed, one end of which conveys the foul air outside, and the other projects a little below the level of the ceiling. The gas pipe enters on one side, and is bent so as to hang perpendicularly in the centre of the tube, and has an annular burner at the lower extremity, surrounded by a glass chimney, which is supported on the top on a metal cone piece, secured to the lower extremity of the tube by screws. This arrangement is surrounded by a hemispherical glass shade with its mouth uppermost, and a few inches below the level of the ceiling. The air of the apartment passes off in the strong draught occasioned by the burner, and a fresh supply of air is admitted at the lower part of the room.
Oil-Gas, Resin-Gas, and Water-Gas.
When tallow or oleaginous matter of any kind is raised to a certain temperature, it is resolved into various gases. Gas-Light, of which the compounds of carbon and hydrogen, viz. olefiant gas or bicarburetted hydrogen, and light carburetted hydrogen, are the principal, both in point of quantity and quality, for the purposes of illumination. As oil contains in its composition a portion of oxygen, existing most probably in union with hydrogen in the state of water, that substance also yields, during its destructive distillation, a considerable quantity of carbonic oxide, as well as traces of carbonic acid, hydrogen, and even nitrogen. With these products, all of which are of a determinate character, is found in greater or lesser abundance a quantity of a very inflammable vapour, which seems to be a compound of carbon and hydrogen.
Oil-gas owes its illuminating power chiefly to the proportion of olefiant gas which it contains, and the oleaginous nature of the vapour which is diffused through it; and as both of these ingredients vary in quantity with the temperature at which the decomposition is effected, the quality of oil-gas is extremely fluctuating. When the temperature is too high, a portion of the olefiant gas and oleaginous vapour is resolved, by the deposition of carbon, into light carburetted hydrogen; and though the quantity of gas from a given portion of oil is thus increased, the quality of it is diminished in a still higher ratio. On the other hand, if the temperature be rather too low, a larger quantity of olefiant gas, mixed with a greater proportion of oleaginous vapour, is obtained; but as the latter is gradually and rapidly condensed when the gas is allowed to stand over water, the higher illuminating power of this richer gas is more than counterbalanced by the deficiency in its quantity, and the deterioration to which it is liable by keeping.
We are indebted to Dr Henry of Manchester for the first analysis of the aërial compounds obtained by the decomposition of oil by heat; and though his elaborate researches have scarcely been said to have led to the determination of the precise products of that decomposition, they furnish data from which their true nature may be inferred, with a probability nearly as great as that which belongs to the results of direct experiment. The principal difficulty of the analysis consists in determining the condition in which the elementary principles of carbon and hydrogen exist in union with each other, and reconciling the various suppositions that may be made respecting the compounds thus formed, with the specific gravity which belongs to the original gas, supposed to be produced by their mixture.
The results of Dr Henry's first experiments were published in 1805; but it was not till about ten years after that period that an apparatus for decomposing oil, on a large scale for economical purposes, was constructed. In 1815 a patent was obtained by Mr Taylor, in which he was afterwards joined by Mr Martineau, for producing gas from oil, by means of an apparatus which was employed with little variation in all the oil-gas establishments which were subsequently formed in various parts of the country. The general arrangements adopted in that apparatus will be readily understood from figs. 27, 28, and 29.
The retort aa has the form of a horse-shoe, being recurved and furnished with two mouth-pieces. The grate of the furnace is represented by b. The oil, which is
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1 The oleaginous vapour alluded to consists, according to the experiments of Mr Faraday, of two distinct compounds of carbon and hydrogen. One of these he terms bicarburetted hydrogen, which, by his analysis, is composed of six proportions of carbon and three of hydrogen. The other compound, to which Dr Thomson has given the name of quadro-carburetted hydrogen, consists of four proportions of carbon and four proportions of hydrogen, existing in a different state of aggregation from that in which they exist in olefiant gas, the elementary constituents of which are in the same proportion. Gas-Light contained in the cistern e, is conveyed by means of a tube, furnished with a stop-cock for regulating the discharge, and allowed to drop into the retort upon fragments of brick or coke heated to redness, to promote the decomposition. The gas, after its production, ascends from the retort, at the opposite limb through the upright pipe ff, by which it is conveyed into a close vessel hh, which is surrounded with cold water in the cistern ii. From the bottom of the vessel hh, a worm or spiral tube kk ascends, and after a few convolutions around hh, terminates in the pipe l, which descends into the air-tight chest mm. The gas, leaving the vessel hh, is conducted by the spiral tube kk, depositing in its progress any portion of the oil that may have come over with it in a volatilized state, and escapes from the lower end of the pipe l, into the chest mm, which contains water to the depth represented. After quitting the pipe l, the gas is forced to traverse the inclined partition n, in a zig-zag manner, by means of diagonal ribs attached to its lower side, till gradually ascending, it escapes at the upper end, and rising through the water, finally passes off at g to the gasometer.
Oil being decomposed at a loss of nearly fifty per cent., the conversion of it into gas, after a protracted but ineffectual competition with coal, was gradually abandoned on the large scale, even in those places where, from the interest they had in the whale-fisheries, there was the strongest inducement to foster the prejudices which prevailed for some time against the use of coal-gas. The exaggerated advantages which it was pretended would be derived from compressing oil-gas and thus rendering it portable, served to prolong the delusions on the subject; nor were these delusions fully removed until a demonstration was given of the failure of the scheme, in the decay of the costly edifices and expensive apparatus which had been constructed for carrying it into effect. The late Professor Daniell of King's College, London, also contrived an ingenious form of apparatus for making gas from resin; but the plan did not succeed on account of the impossibility of competing with the coal-gas works.
Of late years a new process of gas-making has been much discussed, and has formed the subject of a variety of patents. It is known as the hydrocarbon process of gas-making, or more briefly water-gas. The principle of the manufacture is to pass steam over red-hot coke, by which it is resolved into hydrogen and carbonic oxide, and then to supply these inflammable gases with the carbon required for their illuminating power, by passing them through a retort in which oil, resin, tar, naphtha, cannel coal, or some other carbonaceous substance, is undergoing decomposition by heat. The process does not appear to have been successful with resin, but better results seem to have been attained with cannel coal.
Methods for determining the Illuminating Power of the Gases.
Having described the various manipulations by which gas is prepared, both from coal and oil, we now proceed to explain the methods which have been adopted for determining their respective illuminating powers; it being by these methods that we acquire a knowledge of one of the most important tests by which the comparative value of the gases can be ascertained.
The first and most obvious of these tests is to determine the intensity of the light which the gases are capable of diffusing during their combustion, upon a white and smooth surface directly exposed to its emanations. The determination of that intensity is obtained with a considerable degree of accuracy, not by a direct comparison of the degree of illumination shed on two separate surfaces, but by means of a contrivance, first proposed by Count Rumford, which allows the illuminated surfaces to be contrasted with each other on the same ground, and so closely adjoining that the eye can readily detect a slight difference between them. This contrivance is as follows: Let A and B, fig. 30, be two luminous objects; EF a smooth and white surface, having the same inclination to the rays of light emitted by A and B; and CD an opaque cylindrical rod parallel to the surface EF; then it is evident that aa and bb will be the shadows of CD, in reference to the lights A and B. But the shadow aa being illuminated by the light B, and the shadow bb by the light A, it follows that if these shadows be perfectly the same in point of intensity of shade, the light yielded by A and B must be the same in degree. If the shadows, however, be different, one of the lights must be removed either further from EF or brought nearer to it, until the shades seem to be exactly alike, when the light shed upon EF by A and B must, in point of intensity, be, as before, the same. But the intensity of light, like that of other emanations proceeding in straight lines from a central point, being inversely as the square of the distance, the relative degrees of light emitted by A and B must, in con- Gas-Light.
Gas-light, conformity with that principle, be proportional to the squares of their respective distances from the surface on which the shadows are projected. Thus, if the light A were at the distance of fifteen feet, and the light B at the distance of twenty-five feet, their relative illuminating powers would be as the square of fifteen to the square of twenty-five; that is, as 225 to 625, or as 9 to 25. As the quantity of gas consumed in the same time to yield the supposed lights might be different, it is evident that a correct estimate of the absolute value of the gases for the purpose of illumination would not be duly determined unless that circumstance were also taken into account. But the economical value of the gases, yielding equal degrees of light, being inversely as the quantities consumed, it follows that that value will be directly as the squares of the distances at which the shadows are the same, and inversely as the rate of consumption. Thus, if we now suppose that the gas yielding the light A consumed three cubic feet in the same time that the gas yielding the light B consumed five cubic feet, the value of the former would be to that of the latter as \( \frac{3}{5} \) is to \( \frac{5}{3} \), or as three to five. In obtaining the necessary data for determining the ratio of the lights, it may be proper to add that the screen on which the shadows are projected should be guarded with the utmost care from all extraneous light. If it be desired to contrast the illuminating power of a gas-light with that of a candle, the comparison is easily made. If, for example, the gas-light give a shadow equal to that of a candle placed at one-third the distance, the light of the gas is equal to the light of nine candles. If the candle be placed at one-fourth the distance of the gas-light, the latter is equal to sixteen candles, and so on.
Professor Bunsen of Marburg has contrived a photometer which is now in common use in gas-works. The principle of this instrument is not the comparison by shadows, which forms a delicate experiment, but a comparison of light transmitted through a translucent surface with light reflected from an opaque surface. For this purpose a disc of paper, TO, fig. 31, is placed between the two lights to be compared; an annular portion of this paper T is made translucent by means of melted spermaceti, or that substance dissolved in oil of naphtha, while a central disc of the paper O being left untouched by the composition, remains opaque. Fine cream-coloured letter-paper answers the purpose very well, and the central opaque disc may be about the size of half-a-crown. Now it is evident that the translucent ring will be illuminated by a light behind the disc, while the opaque portion is illuminated by a light in front. The frame on which the disc is mounted is moved backwards and forwards on a graduated bar BB between the two lights until the transmitted and reflected lights appear of the same intensity. The pointer P then shows the division over which the disc stands. Under such circumstances, the lights are to each other in the ratio of the squares of their distance from the disc. See PHOTOMETER.
The determination of the intensity of light by the above simple means is capable, under careful management, of all the precision which the nature of the problem requires; it is even preferred by engineers to the more elaborate method of chemical analysis. The latter method has for its object to ascertain the relative value of the gases used for illumination, by finding the quantity of olefiant gas which they contain under equal volumes; it being assumed that the illuminating power of the compound combustible gases derived from the decomposition of oil and pit-coal is directly proportional to the quantity of that gas existing in their constitution. Though that supposition is by no means a matter of certainty, or even of probability, we shall nevertheless briefly explain the mode of analysis which has been recommended. According to the experiments of Dr Henry, chlorine has no action upon any of the gases obtained from oil or coal when the influence of light is carefully excluded, with the exception of olefiant gas; and as chlorine and olefiant gas unite together in equal volumes, this property affords an easy mode of determining the quantity of the latter which may exist in any compound gas of which it forms a constituent part. All that is required for the purpose is to add somewhat more chlorine than is absolutely necessary for uniting with the olefiant gas, and to allow the mixture to remain about fifteen minutes completely excluded from light. The extent of the absorption being thus observed, half the quantity of the gas which has disappeared of the whole mixture will be olefiant gas. Thus, if twenty parts of chlorine by measure were added to twenty-five of coal-gas, and if the mixture, after being allowed to remain a sufficient length of time in the dark, were found to occupy thirty-six measures, the absorption would be nine measures, and consequently the coal-gas must have contained four and a half measures of olefiant gas, or eighteen per cent. The quantity per cent. of olefiant gas is determined without calculation, by adding to fifty measures of the gas to be analysed an equal volume of chlorine; when the diminution of volume, in the graduated jar, is the quantity which the gas contains per cent. of olefiant gas. Dr Fyle states that the illuminating power of the different specimens of oil and coal gas which he subjected to this test bore a pretty exact ratio to the quantity of olefiant gas which they contained. One great advantage to be derived from this method of testing the quality of any species of carburetted hydrogen containing olefiant gas in its composition, is, that it admits of a comparison being made between gases in different places and at different times, without the necessity of transporting them to a distance, and making a simultaneous examination of their illuminating properties.
Of late years, bromine has been substituted for chlorine in the above analysis. The gas is passed up into a eudiometer tube, and the carbonic acid is removed by means of caustic potash: a small portion of bromine is dropped in and shaken in contact with the gas. Potash is again added to remove the bromine vapours, and the absorption is then noted. It is stated that some of the highly illuminating cannel-coal gases are condensed by this process as much as 12 or 14 per cent.; while some of the poorer gases not more than 4 or 5 per cent.
The specific gravity of oil and coal gas, and the quantity of oxygen which they require for their perfect combustion, have also been proposed as means of ascertaining their illuminating powers. The latter, however, even if it were a perfect test, is determined with considerable difficulty; and of their that little reliance can be placed on the former may be inferred from the fact that some of the gases which are component parts of oil and coal gas have a great specific gravity without possessing any illuminating power. This will readily be perceived from the subjoined table.
| Gas | Specific Gravity | Quantity of Oxygen for 100 volumes | |--------------------|------------------|----------------------------------| | Olefiant gas | 970 | 300 | | Carburetted hydrogen| 556 | 300 | | Hydrogen | 609 | 50 | | Carbonic oxide | 972 | 50 | | Carbonic acid | 1538 | None | Of these gases, carbonic oxide and carbonic acid possess the greatest specific gravity; while the latter is not only destitute of illuminating property, but calculated, as we shall afterwards show, to deteriorate to a great extent the quality of the luminiferous gases with which it may happen to be mixed.
There are cases, however, in which it is necessary to determine accurately the composition of a sample of coal-gas, and the following is the now generally adopted method of conducting the analysis. The ingredients or impurities which may be present in the gas are—1. Common hydrogen; 2. olefiant gas and other hydrocarbons; 3. light carburetted hydrogen; 4. carbonic oxide; 5. carbonic acid; 6. sulphuretted hydrogen; 7. ammonia; 8. oxygen and nitrogen derived from the atmosphere. A qualitative examination is made thus—the proportion of ammonia and of sulphuretted hydrogen is usually very minute, and in most cases these gases must be sought for by placing the tests for their presence for some time in a current of the gas. In searching for ammonia a piece of moistened litmus paper feebly reddened is placed for a minute in a jet of the issuing gas. If the blue colour be restored, ammonia is present. Paper soaked in a solution of acetate of lead may be subjected to a similar trial. If it turn brown, sulphuretted hydrogen is present. The presence of oxygen is detected by admitting a bubble of the deutoxide of nitrogen into a tube filled with the gas under trial, and looking through the tube obliquely upon a sheet of white paper; very small traces of oxygen may thus be detected by the red tinge produced, owing to the formation of peroxide of nitrogen. The presence of carbonic acid may be readily detected by throwing up a little lime water, or solution of sub-acetate of lead, into the gas whilst standing in a tube over mercury. The existence of the other gases may be assumed, as they are certain to be present in greater or less quantity. The sulphuretted hydrogen and ammonia being neglected, and supposing that oxygen and carbonic acid are found to be present, seven different gases are therefore supposed to exist in the mixture. The following method may be adopted for their quantitative determination:
1. Oxygen.—A volume of the gas is confined over mercury, and its bulk is measured with due attention to temperature and pressure. A piece of moist phosphorus, which has been melted upon the end of a long platinum wire to serve as a handle, is introduced from below through the mercury into the tube. After twenty-four hours the phosphorus is withdrawn, when the amount of absorption indicates the proportion of oxygen which was present.
2. Carbonic Acid.—This gas is determined in a similar manner, substituting a ball of caustic potash for the phosphorus; the second diminution in bulk shows the proportion of carbonic acid.
3. Olefiant Gas and Heavy Hydrocarbons.—These gases are absorbed by introducing a third ball, consisting of porous coke, moistened with fuming sulphuric acid. It is necessary, however, before reading off the volume of the gas, to introduce a ball of potash a second time, to withdraw the vapour of anhydrous sulphuric acid, which possesses sufficient volatility to introduce a serious error by dilating the bulk of the gas, unless it be completely removed. The total amount of absorption will indicate the proportion of olefiant gas, together with the vapours of condensible hydrocarbons.
4. Carbonic Oxide.—The separation of carbonic oxide from the other gases is not easily done with accuracy. The gas may be divided into two portions, one of which is to be carefully measured as it stands over mercury, and a small quantity of a solution of subchloride of copper in hydrochloric acid is to be added, and the mixture briskly agitated: the gas is then transferred to a second graduated tube, also standing over mercury, and a ball of potash is introduced for the purpose of absorbing the vapours of hydrochloric acid with which the gas is saturated; the bulk of the gas may then be read off, and the volume of carbonic acid may be known by the loss in bulk.
5. Nitrogen, Carburetted Hydrogen, and Hydrogen.—In determining the proportion of these gases, that of the carbonic oxide may also be ascertained, for which purpose a portion of coal-gas, in which the carbonic oxide is still present, is transferred to a siphon-endoimeter, and its bulk is measured: it is then mixed with twice its volume of oxygen, and the bulk of the mixed gases is again measured; the mixture is then exploded by means of the electric spark, and the bulk is a third time measured: call this diminution in bulk \(a\), next inject a small quantity of a strong solution of potash; and the resulting condensation due to the absorption of carbonic acid may be called \(b\); the remaining gases, \(c\), consist of oxygen in excess and nitrogen; the quantity of oxygen in excess is ascertained by mixing the residual gas with twice its bulk of pure hydrogen, and a second time causing the electric spark to pass; one-third of the condensation observed will be due to the excess of oxygen; on deducting this excess from the residue \(c\), the difference gives the quantity of nitrogen. The difference between the amount of the oxygen thus found to be in excess, and that originally introduced, will of course represent the quantity of oxygen consumed; call this \(d\). We have now all the data for calculating the proportion of carburetted hydrogen, of hydrogen, and of carbonic oxide, which are present in the mixture. Let \(x\) represent the quantity of light carburetted hydrogen; this gas requires twice its own volume of oxygen for complete combustion, and furnishes its own volume of carbonic acid, which requires an equal volume of oxygen for its formation, or half the amount consumed; the other half of the oxygen being required by the hydrogen, which condenses in the form of water, \(2x\) will be the diminution in bulk of oxygen which occurs on detonation. Again, when hydrogen is converted into water, it requires half its bulk of oxygen, and both are condensed entirely. If \(y\) represent the bulk of the hydrogen, \(\frac{3y}{2}\) will be the diminution in bulk of the mixed gases on detonation, which is occasioned by the hydrogen in the mixture. Let \(z\) represent the volume of carbonic acid present; carbonic oxide, for conversion into carbonic acid, requires half its bulk of oxygen, the carbonic acid produced occupying the same bulk as the carbonic oxide. \(z\) will therefore indicate the condensation which occurs on firing the mixture. The total condensation in bulk \((a)\) which occurs on firing a mixture of light carburetted hydrogen, hydrogen, and carbonic oxide, will consequently admit of thus being represented—
\[ (1.) \quad a = 2x + \frac{3y}{2} + z \]
Further, the quantity of the carbonic acid formed by detonation, \(b\), is composed of a volume of carbonic acid equal in bulk to the light carburetted hydrogen, and a volume equal to that of the carbonic oxide, so that the quantity of carbonic acid may be thus indicated—
\[ (2.) \quad b = x + z \]
And lastly, the oxygen consumed, \(d\), will be composed of the following quantities:—Light carburetted hydrogen, twice its bulk, \(2x\); hydrogen, half its bulk, \(\frac{y}{2}\); carbonic oxide, half its bulk, \(\frac{z}{2}\); or the total quantity of oxygen consumed will be the following—
\[ (3.) \quad c = 2x + \frac{y}{2} + \frac{z}{2} \]
---
1 Abridged from Elements of Chemistry, by Professor Miller, of King's College, London. Gas-Light. From these three equations the values of \( x, y, z \) are determined:
\[ x = c - \frac{a + b}{3} \\ y = a - c \\ z = \frac{a + 4b}{3} - c. \]
Hints respecting the Improvement of Coal-Gas.
Of all the combustible bodies having an elementary character, carbon and hydrogen are not only the most widely and copiously diffused throughout the three kingdoms of nature, but best adapted for the evolution of light during their combustion. It is only, however, when they are united together in due proportion that they answer the purpose most effectually; and, indeed, in a separate state their illuminating powers are so feeble, that even when their combustion is accelerated and rendered more perfect by the presence of oxygen, the light which they yield is yet unfit for many of the useful ends to which light is subservient. The substances in which carbon and hydrogen are united in the best proportion for the production of light are pit-coal in the mineral kingdom, and oils and fatty matter in the animal and vegetable.
The great abundance of coal, and the comparative cheapness at which it can be obtained, give it a decided advantage in point of economy over oleaginous matter, whether of animal or vegetable origin; while the processes of decomposing it, with the view of converting it into a volatile and elastic product, have been so much improved as to render the gas which it yields equally fit for the purpose of illumination with the more costly gases obtained from the oils.
The gas produced by the decomposition of coal and oleaginous matter at a high temperature is a compound of carbon and hydrogen, and consists chiefly of two gases, in which these elementary substances exist in definite proportions. One of these gases is termed carburetted hydrogen, and the other olefiant gas or bicarburetted hydrogen. The former contains one atom of carbon united with two atoms of hydrogen, and the latter an atom of each of these elements.
Of these two compounds of hydrogen and carbon, that which contains the largest proportion of the latter element is found to yield during its combustion the most brilliant light, and that too for a longer period of time. And, indeed, so great is the difference in these respects, that the hydrogen may not improperly be regarded as the mere solvent or vehicle of the carbon, acting the part of wick, and thus presenting that substance in a state sufficiently comminated for its more perfect combustion. Accordingly, the more abundantly the hydrogen is impregnated with carbon the greater may we expect to be its illuminating power, and the fitter in every respect for yielding artificial light. These views are fully supported by experiment; for not only is the brilliancy of the light modified by the quantity of carbon held in solution by the hydrogen, but the time which a given portion of the gas takes to consume away by combustion is affected by it in a still greater degree.
To determine in what ratio the illuminating power of the gases obtained both from oil and coal was reduced by diluting them in various proportions with hydrogen, we instituted a series of experiments, the results of which are of importance inasmuch as they indicate not only that the mixture is deteriorated, but that the same quantity of carbonaceous matter yields less light the more largely it is diluted with hydrogen.
In the first experiment we took a portion of coal-gas of the specific gravity '67, which we found to consume at the rate of 4400 cubic inches per hour, and yielded the light of eleven candles, being 400 cubic inches per hour for the light of one candle. This gas being diluted with a fourth part of its bulk of pure hydrogen, acquired the specific gravity '55, and wasted away at the rate of 6545 cubic inches per hour, yielding the light of ten candles. As a fifth part of the compound gas was hydrogen, the remaining four-fifths, amounting to 5236 cubic inches, was the quantity of the coal-gas which in its diluted state gave the light of ten candles for an hour; so that 524 cubic inches of the original coal-gas were requisite to give the light of one candle for the same time. But, in its unmixed state, 400 cubic inches were sufficient to give the light of one candle for an hour; and consequently, the deterioration occasioned by the dilution was in the ratio of 524 to 400, or of 100 to 76, being 24 per cent. It must be distinctly kept in view that the deterioration has been reckoned, not with respect to the whole volume of the mixture (in which case it would have been 39 per cent.), but simply in reference to the coal-gas itself; and therefore the experiment, so far as it goes, justifies us in adopting the conclusion, that had the hydrogen existed originally in union with the coal-gas, the latter would have improved in quality 24 per cent. by its abstraction; because the residuary portion would not only have lasted longer, but yielded during its combustion a superior light.
In a second experiment, conducted in a similar manner, in which the proportion of hydrogen was one-third of the quantity of the coal-gas, the deterioration was 27 per cent.; in a third experiment, the proportion of hydrogen being a half of the volume of the coal-gas, the deterioration amounted to 31 per cent.; and in a fourth experiment, the quantity of hydrogen being exactly equal to that of the coal-gas, the deterioration extended to 36 per cent.
These results indicate a progressive deterioration in the General quality of coal-gas by the admixture of hydrogen; and the conclusion to which they lead is, that the abstraction or removal of the latter, though diminishing the entire volume, would improve the nature of the residuary portion not only in a higher ratio than the loss which the whole sustained in its bulk, but render that portion capable of yielding, for a longer period of time, a greater light than it could have done in its original state. Hence it may be inferred that the illuminating power of coal-gas, whether considered with respect to the cost of its production or the intensity of its light, admits of being improved; first, by impregnating the hydrogenous element more largely with carbon; secondly, by preventing the disengagement of hydrogen in a free state during the carbonization of the coal; and lastly, by detaching a portion of that gas from coal-gas when it already exists in admixture with it.
The first of these modes of improvement seems to be Modes of practicable, at least to a certain extent, by thoroughly improving drying the coal before it is introduced into the retorts, and coal-gas modifying the pressure under which the gas is generated; the second, by preventing the gas after its formation from being exposed to a high temperature by allowing it to pass over very hot surfaces, the effect of which is to deprive it of carbon. The second object may also be assisted by arresting the process of distillation at an earlier period than is usually practised, hydrogen and carbonic oxide being the products which predominate during the last periods of decomposition. On this point, however, the interests of the public and of the manufacturer are at variance. The consumer pays by measure, and hence it is the interest of the manufacturer to carry on the process of distillation as long as possible, for, by so doing, not only does he increase the quantity of gas but he improves the quality of the coke. With respect to the third mode of improvement, we are unfortunately, in the present state of our knowledge, acquainted with no method of detaching hydrogen from the gases with which it is mixed in oil or coal gas that would not impair the illuminating power of these gases to a greater extent perhaps than the benefit that would be derived from the removal of the hydrogen. A plan has been proposed by Mr Lowe to increase the quantity of carbon in the gas by impregnating it with the vapour of coal naphtha; for which purpose it was proposed to fill the wet gas meter at the house of the consumer with purified naphtha, and to maintain it at the same height by means of a reservoir connected with the meter; by which means the gas would be measured and saturated with naphtha at the same time. A more practical plan was to pass the gas through an ornamental vase containing a sponge saturated with naphtha, and placed at some point between the meter and the burner.
To determine the diminution of the illuminating power produced by separating the particles of the inflammable gas during its combustion, and thus diminishing the temperature of the flame, it occurred to the writer of the present article that nitrogen, having neither the property of supporting combustion nor of adding to the quantity of combustible matter submitted to that process, was well fitted to answer the intended purpose; and accordingly, on mixing coal-gas of ordinary quality (which, when burnt alone, yielded the light of twelve candles when it consumed 5400 cubic inches per hour) with varying portions of nitrogen, results were obtained which implied that the diminution of the intensity of the light proceeded in a ratio much more rapid than was observed when the gas was diluted with hydrogen. Thus, when six volumes of the coal-gas were mixed with one volume of nitrogen, the expenditure per hour was 6000 cubic inches, and the light equivalent to that of nine candles, being 667 cubic inches per hour for the light of one candle. But one-sixth of the whole being nitrogen, the remaining four-sixths, amounting to 556 cubic inches, was the quantity of the coal-gas which, in its diluted state, afforded the light of a candle for an hour. On the other hand, the quantity of the coal-gas requisite, in its unadulterated state, to give an equal degree of illumination being $\frac{556}{12}$, or 450 cubic inches, it follows that the deterioration was in the ratio of 556 to 450, or 100 to 81 nearly.
By diluting the same coal-gas with other proportions of nitrogen as subjoined, and afterwards applying to each of the results the same kind of reduction as that which we have already made, we have deduced the following table, which exhibits the gradual deterioration of the illuminating power of the same quantity of coal-gas, produced by the mere separation of the atoms of the gas during its combustion.
| Volumes of Coal-gas | Volumes of Nitrogen | Illuminating Power | |---------------------|--------------------|-------------------| | 60 | 0 | 100 | | 60 | 10 | 81 | | 60 | 12 | 69 | | 60 | 15 | 55 | | 60 | 20 | 37 | | 60 | 30 | 29 | | 60 | 60 | 4 |
When carbonic acid was used instead of nitrogen, similar results were obtained; only the deterioration was considerably greater. Thus, when five volumes of the coal-gas were mixed with one volume of carbonic acid, the illuminating power was reduced from 100 to 30, whereas in the case of the nitrogen it was from 100 to 69. It is therefore a fortunate circumstance that carbonic acid, which is so apt to be generated during the production of coal-gas, and has so debasing an influence upon its illuminating power, is readily absorbed by a variety of substances; while nitrogen, the less injurious as well as the less abundant accompaniment, cannot be separated from the other gases with which it may exist in mixture by any process yet known.
**Deterioration of Gas by keeping it after it is prepared.**
Both oil and coal gas suffer, by keeping, a gradual loss in their power of illumination, which seems to increase in Gas-Light, a more rapid ratio than the time they are kept. The deterioration, though greatest when the gases are allowed to stand over water, takes place in a considerable degree even when they are kept over oil, or in air-tight vessels. Hence it may be presumed that the carbon held in solution by the gas when hydrogen is separated from that element, partly by its own gravity, and partly perhaps by solution in the water, or by condensation in the liquid form.
To whatever cause the deterioration is owing, the fact itself is undoubted. Thus, an oil-gas which, when newly prepared, had the specific gravity 1·054, gave the light of a candle for an hour when it consumed 200 cubic inches; kept two days, it gave the same light with a consumpt of 215 cubic inches per hour; and kept four days, it required for the same light 240 cubic inches per hour. In the case of a portion of coal-gas, which, when newly prepared, required 404 cubic inches to yield the light of a candle for an hour, the same gas kept two days required 430 cubic inches; and kept four days, 460 cubic inches to yield the same light. These results indicate a progressive deterioration in the quality of the gases, increasing with the length of time they are kept; and it is deserving of remark, that in both gases the diminution of the illuminating power decreases in a faster ratio than the time increases. After being kept three weeks, the oil-gas was so much debased in quality that it required 606 cubic inches of it to yield the light of a candle for an hour; and hence its illuminating power was reduced to one-third of what it was when the gas was newly made. From these experiments it may obviously be inferred that both oil and coal gas should be used as soon as possible after they are prepared.
**Economy of Coal-Gas.**
Among the advantages which have resulted from the introduction of coal-gas, we may reckon, first, its comparative cheapness; and, secondly, its superiority to all the other modes of artificial illumination.
In forming a comparative estimate of the cost of coal-gas and that of the other means employed for procuring artificial light, we may contrast it with the expense of wax, tallow, and oil, the ordinary substances used for the purpose. It deserves to be remarked, however, that while the price of coal, in consequence of the regular and abundant supply of that article, is liable to little fluctuation, the cost of wax, tallow, and oil, on account of the more precarious nature of the sources from which they are obtained, varies exceedingly in different seasons. The very extensive use, too, into which coal-gas has been brought, has produced a considerable effect upon the price of oil and tallow, as well as of wax; so that a comparative estimate of the expense of procuring the same extent of illumination from coal-gas and from these substances must appear less favourable to the former than would have been the case had the comparison been made when gas was first introduced. But by way of illustration, the approximative economy of the substances commonly employed for illumination may be contrasted as follows:—Supposing that 5 cubic feet of gas per hour give a light equal to that of 12 candles, then 1000 cubic feet, if burnt at the rate of 5 feet per hour, would give a light equal to that of 12 candles for 200 hours, at the cost of 4s. 6d., which is about the average price of gas in London per 1000 feet at the present time (1855). Suppose the candles to cost 9d. per lb., then 2 lb. of candles, 6 to the lb., would burn for 6½ hours, at the cost of 1s. 6d., or 60 lb. would burn 200 hours, at the cost of L.2, 5s. Assuming wax to be three times the price of the candles, the cost of wax candles for 200 hours would be L.6, 15s.; and taking sperm oil at 8s. per gallon, 4 gallons would give a light equal to that of 12 candles for 200 hours, at a cost of So that, by comparing the cost of these various sources of light for equal periods of time, we have—
| Source | Cost per Unit | |-----------------|---------------| | For wax candles | 6 15 0 | | For tallow candles | 2 5 0 | | For sperm oil | 1 12 0 | | For gas | 0 4 6 |
The expense of gas, as compared with that of the other sources of light, will be—
| Source | Cost per Unit | |-----------------|---------------| | Gas | 1 9 | | Candles | 10 0 | | Oil | 7 1 | | Wax | 30 0 |
In the above comparison we have taken London gas as the standard, which is scarcely fair, seeing that this gas is inferior in illuminating power to that of most other towns.
But the light obtained from coal-gas is not only procured at a smaller expense; it is also more convenient for most purposes than the light yielded by other substances. In the ordinary mode of lighting by tallow and oil, the light derived from their combustion cannot be diminished in intensity without considerable disadvantage and trouble; whereas in the case of gas, it may be reduced in an instant from the most perfect splendour to the feeblest degree of illumination by the simple adjustment of the stop-cock. The advantages arising from this easy method of regulating the light of gas, when it is used in the chambers of the sick, and indeed in all apartments where a variable but uninterrupted supply of light must be kept up, can only be duly estimated by those who have experienced them. To every branch of manufacturing industry which requires a steady and powerful light, the benefits which have resulted from the introduction of coal-gas are not less important. In many operations the light may be conveyed by means of flexible pipes, connected together with ball-and-socket joints, so as to be almost in contact with the fabric it is intended to illuminate, without the slightest risk of injury; and it may be kept in the same state for many hours in succession, or altered, as circumstances may render necessary.
For lighting churches, theatres, and other public buildings, where a strong and uniform light is required, gas answers the purpose more effectually than any other mode of illumination; partly from the facility of its application, and partly from the diversified and tasteful manner in which the jets of flame may be exhibited in various kinds of burners.
As a street light, its superiority is universally admitted; and from that application of gas it cannot be doubted that the metropolis, and other large towns, have derived great additional security against the perpetration of nocturnal crimes, as well as the means of carrying on the ordinary business of life during the evening with nearly the same convenience as during the full light of day.
Secondary Products.
The chemistry of the gas manufacture has been for some years in a state of mutation, the effect of which has been to bring about important changes in the nature and amount of the secondary products. We may, however, refer to the methods of disposing of the usual secondary products, namely, the coke, the tar, and the ammoniacal liquor. A ton of Newcastle coals of the average weight of 2240 lb. yields—
| Product | Yield (lb.) | |--------------------------|-------------| | 1 Chaldron of coke | 1494 | | 12 Gallons of tar | 135 | | 10 Gallons of ammoniacal liquor | 100 | | 9000 to 10,000 Cubic feet of gas | 291 | | Loss | 220 | | Total | 2240 |
It is found, on an average, that 1 cwt. of coals yields about 2 bushels of coke. About one-fourth of the quantity of coke produced is used as fuel for heating the retorts, and the remainder is sold. The tar and ammoniacal liquor or gas-water separate in the tar cistern, the tar forming the lower stratum. This is used in the manufacture of patent fuel Gas-Light, and of creosote, and as a rough paint for outdoor work 100 lb. of tar yield by distillation about 26 lb. of an oily liquid known as coal-oil. A light product first distills over, which is called coal-naphtha; the remaining pitch is used for paying the bottoms of ships, wooden piles, &c. The coal-naphtha is used for dissolving caoutchouc, and for burning in the naphtha-lamp. The ammoniacal liquor is used in the manufacture of sal-ammoniac, carbonate of ammonia, and prussian-blue. The presence of cyanogen in the ammoniacal liquor has led to its employment in the manufacture of ferrocyanide of iron or prussian-blue. It is stated that a gallon of ammoniacal liquor, when saturated with sulphuric acid, contains enough of cyanogen and cyanates to form, with a salt of iron, 24 grains of prussian-blue.
The secondary products of the Edinburgh gas works are turned to account at the chemical works, situate at a distance of about two miles from them, the gas works being on a lower level. They are, however, connected by a line of pipes, and the gas liquor is lifted over the shoulder of the Calton Hill by means of a force-pump. The difference of level is then sufficient to carry it to the chemical works. The liquor is left for the tar to subside, but the ammoniacal liquor, consisting of an impure solution of carbonate and hydroxysulphuret of ammonia, still contains a portion of tar, which is got rid of by distillation. The larger portion of the distilled liquid is converted into sal-ammoniac, and a portion into sulphate of ammonia. In order to obtain the sal-ammoniac, the liquor is neutralized with hydrochloric acid, and is then pumped into large cauldrons and evaporated to the crystallizing point, when it is drawn off into large vats, and on cooling deposits small feathery crystals; these are transferred to a stone chest, and are dried by the heat of a furnace below. The salt then resembles brown sugar; it is mixed with charcoal powder for the purpose of reducing any oxide of iron which may be present, and thus to get rid of the brown tint in the process of sublimation. The subliming vessels resemble a man's hat, and are arranged in the furnace with the crown downwards; they are about three feet in depth, and two and a half in diameter, and they contain sufficient for a week's charge. Each pot is covered with a leaden cupola, luted on with clay, and the salt is at first allowed to sublime away through a hole in the centre. This occasions some loss, but it appears to be a necessary precaution to prevent porosity in the sublimate. The central hole is then plugged with clay, and the sublimation is continued for a week. In this way hemispherical cakes of sal-ammoniac are produced; they are rasped on the surface to remove crust or colouring matter, and are broken into wedges, which are packed in barrels for exportation.
In preparing sulphate of ammonia the distilled ammoniacal liquor is saturated with sulphuric acid, and concentrated until small crystals are formed, which are removed by perforated ladles, dried, and packed in barrels lined with paper.
The tar, which contains a considerable portion of water, is transferred to a still, where crude naphtha and vapour of water distil over. They separate in consequence of their different densities, and the naphtha is digested with sulphuric acid in a leaden trough. This separates ammonia and other substances; the acid is removed by means of quicklime, the naphtha is washed with water, distilled, and is ready for the market. The remaining tar is raised to a higher temperature, and a liquid less volatile than naphtha is produced; it is termed pitch-oil, and is used for impregnating wood, &c. The pitch in the still is then run out, when it settles into a soft solid, for which at Edinburgh no market has yet been found, but it may probably be turned to account as a cheap fuel.
Scarcely any market is found for the tar, which was formerly largely consumed at Continental seaports. The in-