Between the arms are placed twelve shallow iron trays or boxes, destined to contain the coal from which the gas is to be obtained. They are formed to the segment of a circle, hence the whole series of them when arranged in the retort, exhibits a shallow circular tray, which, when motion is given to the shaft, may be made to revolve within the retort.

Fig. 12, plate III. exhibits one of the shallow trays, or coal boxes in perspective.

It will be obvious, that by the motion of the shaft, any number of the trays or coal-boxes can readily be brought from the coldest, into the hottest, and from the hottest into the coldest part of the retort.

H, fig. 1, plate II., and a, plate III., or H, plate VII., is a perpendicular pipe situated at the margin of the retort, close behind the mouth-piece, and consequently in the coldest part of the retort. It serves to carry off the distillatory products evolved from the coal, and causes part of the vaporous tar, which becomes condensed in it, to trickle back again upon the coal in the retort, in order to become converted into gas, when the coal on which it falls becomes situated over the fire-place.

This pipe is furnished at its upper extremity with a hydraulic valve, J, fig. 1, plate II. It consists simply of an inverted cup X, applied over the upper open extremity of the perpendicular pipe H, and submersed into a cup formed of a portion of larger pipe, surrounding the pipe H, containing tar. The smaller, or inner cup X, is represented in the design raised out of the liquid contained in the outer cup J, to show an aperture Y, made in the smaller or inner cup; the use of which will be mentioned hereafter. The inverted cup X, is furnished with a chain, one extremity of which is fastened to the upper extremity of the cup, the other passes over a small wheel, and descends through the roof of the building, as shown in the design.

K, fig. 1, plate II., or K K, fig. 2, plate VII., is a branch pipe proceeding laterally from the perpendicular pipe H; it communicates with the hydraulic box L, fig. 1, plate II. N, is a pipe which proceeds from the hydraulic box L; it serves to carry away the gaseous and liquid products to their places of destination. The liquid products, namely, the tar and ammoniacal fluid, become deposited in the tar cistern, fig. 3, plate II., into which the pipe N terminates. The tar cistern is furnished with two floats Y Y; the one serves to indicate the quantity of tar, and the other the quantity of aqueous ammoniacal fluid contained in the vessel. These fluids may be drawn off without admitting air into the vessel by the stop-cock and bent tube, exhibited in the figure.

The shorter pipe N, which proceeds from the tar cistern, fig. 3, plate II., and communicates with the purifying apparatus or lime machine, fig. 2, plate II., serves to convey the gaseous fluid, which accompanied the condensible liquids deposited in the tar cistern, back again into the lime machine, or purifying apparatus, fig. 2, plate II., the construction of which, together with the conveyance of the gas from this vessel to its place of destination will be stated hereafter.

L, fig. 1, plate II., or fig. 2, plate VII., is an iron flap table, placed level with the bottom of the mouth of the retort. It is convenient to hold several coal trays ready charged with coal in a state fit to be introduced into the retort.

The fire-place, flues, and ash-pit of the furnace, in which the retort is fixed, are sufficiently obvious by mere inspection of fig. 1, plate II. The front elevation of the retort is seen in fig. 2, plate VII., which exhibits three horizontal retorts; two of which have the door of the mouth-piece slided down, and one with the door in its place, or shut. The circular ring seen in this design, at the top of each retort, which rests on iron-bearing bars, the extremities of which are let into the end walls of the furnace, serves to support the roof of the retort by means of bolts, proceeding from the inner side of the roof. This arrangement is likewise shown in the section, fig. 1, plate II.[33] At the bended part of the perpendicular pipe H, fig. 1, plate II., is seen a bonnet, or cover, which closes an opening made into the pipe H, through which, by means of an iron rod, the lower extremity of the pipe H, may, from time to time, be examined, to guard against an incrustation of decomposed tar or carbonaceous matter that might happen to accumulate in that part of the pipe. The upper part of the pipe H, above the bonnet at the bended part, requires no examination.

b, fig. 2, and b, fig. 5, plate III., is the flanch of the retort; c, fig. 2, plate III., the flanch of the mouth-piece; d, the cutter, or wedge, which draws the mouth-piece close; e, the cross bar, against which the cutter d, bears, to render the mouth-piece air tight; f, fig. 2, one of the eye-bolts or arms which support the cross bar e; it is also seen at e, in the plan of the retort, fig. 5, plate II. In this figure b is the flanch of the retort, and c the door.

These few particulars will be sufficient to enable the reader to understand the construction of the retort; its action is as follows.

Action and Management of the Horizontal Rotary Retort.

When the retort is heated to the proper temperature for the decomposition of the coal, the door is slided down, and the coal boxes charged with small coal are slided into the retort from the table, L, fig. 1, plate II., one by one, so that each box rests firmly upon the concentric rings placed between the arms of the retort; the door is then slided up again into its place and rendered air-tight by means of wedges.

When the whole circle fig. 5, plate III. is thus filled with coal-boxes, (the coal should be spread in the boxes, in layers two or three inches in depth,) it is obvious that of all the twelve boxes, four only can be situated directly over the fire-place, while the remaining eight are placed right and left towards the door of the retort. The coal in the former boxes receives the full effect of the heat, (see the plan of the fire flues of the retort, fig. 1, plate VI.,) while the remaining eight boxes to which the fire does not extend, are less heated. The coal in the four boxes which are in the hottest part of the retort becomes rapidly decomposed, whilst the coal in all the other boxes is gradually heated, and consequently deprived only of moisture, previous to being subjected to the greatest heat. The box which is situated under the condensing pipe H, plate II., near the entry door, receives the condensed tar which trickles down the pipe H.

Now let us suppose that the coal in the four boxes over the fire place is fully decomposed, which will be the case if 32¹⁄₂ pounds of coal are in each box, in two hours, the workman then turns the shaft E, fig. 1, plate II., one-third part of the circumference of a circle, by pulling towards him by means of an iron hook the nearest iron arm that may happen to be opposite to the door; this moves those boxes which at the commencement of the operation were over the fire-place, towards the coldest part of the retort, namely, towards the door which is opposite to the fire-place, and a second series, or four of the adjacent boxes, are brought in turn into the hottest part of the retort, or over the fire-place, from whence the preceding boxes were removed.

When the coal in the second series of boxes has been two hours in the hottest part of the retort, its decomposition will be completed; the workman therefore turns the shaft again one-third part of a circle, and a third series advances in their place, while at the same time the first series becomes situated opposite the entry door of the retort, from whence they may be withdrawn and exchanged for an extra set of trays, ready charged with coal and placed on the iron table for that purpose.

In this manner the operation proceeds. One-third part of the whole charge of coal within the retort is always in the act of becoming decomposed; another third part is gradually heated, and totally deprived of moisture, previous to its being exposed to the temperature necessary for its decomposition; and the remaining third part placed in the coldest part of the retort, receives that portion of tar, which escapes decomposition, and trickles down the perpendicular pipe, in order to be decomposed when the coal upon which it falls becomes situated over the fire-place. Hence the quantity of tar obtained from one chaldron of Newcastle coal, when decomposed by means of an horizontal rotary retort, seldom amounts to more than sixty or seventy pounds, whereas the same quantity of coal when decomposed by means of cylindrical or parallelopipedal retorts, yields never less than from one hundred and fifty, to one hundred and eighty pounds. An horizontal rotary retort, twelve feet six inches in diameter, and fifteen inches high, furnishes in the ordinary way of working every twenty-four hours, fifteen thousand cubic feet of gas, when five trays of the retort are charged with three bushels of Newcastle coal. The weight of the retort is three tons; its capacity, one hundred and fifty cubic feet.

The hydraulic valve described page 116, serves merely to restore the equilibrium, between the gas within the retort, and the atmospheric air without, previous to the opening of the door of the mouth of the retort. To effect this the workman raises the cup X, by means of the chain, so that the small hole Y, in the cup X, becomes raised out of the tar in the cup L, and he closes it again when the retort is charged: this operation requires two minutes. We have stated already, that the door of the retort is ground air-tight, and hence it requires no luting.

Advantages of the method of manufacturing Coal Gas by means of Horizontal Rotary Retorts.

The advantages of the mode of manufacturing coal gas by means of horizontal rotary retorts, consist in a saving of fuel, time, labour, and machinery, a gain in the quantity of gas, and increase in the quantity of coke.

Saving of fuel.—The mass of coal subjected to decomposition being reduced from the dimension required in the old plan (by means of cylindrical retorts) to the narrowest available limits, there being no outward crust of coke to be kept red hot for hours to no purpose, while the decomposition of the interior mass of coal is going on;—the coke itself being as soon as formed removed from the source of heat, and applied while cooling, to warm up a fresh supply of coal next in order of becoming decomposed, instead of being discharged in a red hot state, into the open air, as requires to be done in the practice before detailed—the whole fuel in short being necessarily and beneficially expended—the saving of coal employed as fuel in this respect, is exactly the gaining of all that is lost on the plan of employing cylindrical or any of the retorts before described. Hence one chaldron of coal is decomposed at the gas establishments where horizontal rotary retorts are in action by means of twenty per cent of fuel, and at some establishments an expert stoker will work the retorts with fifteen per cent of fuel.

Saving of time.—The saving of time does not merely amount to what is consequent on the speedier decomposition of the coal, and the saving of that heat which formerly required to be kept up a length of time to no adequate purpose; it also includes all that is gained in consequence of the revolving motion to which the coal is submitted, superseding, as has been already mentioned, the necessity of discharging the coke in an ignited state from the retort.

When the coke is removed, as previously explained, page 72, red hot from the cylindrical, parallelopipedal, semi-cylindrical or ellipsoidal retorts, the charging of the distillatory vessel with fresh coal produces such a sudden reduction of temperature, that from three to four hours inevitably elapse before the retort is again in a full working state, and to this circumstance the workmen (perhaps very justly) attribute the frequent sudden injury which the distillatory cast-iron vessel sustains.

Another striking advantage of the new mode of decomposing coal is, that besides saving the time which is wasted in keeping up an intense temperature unnecessarily the revolving apparatus prevents entirely the loss occasioned by these three or four hours of unnecessary cooling of the distillatory vessel. For each series of trays, or coal boxes, containing the ignited coke, of the horizontal rotary retort, being suffered to cool within the retort before the coke is discharged, and being placed in contact with a fresh supply of coal, the temperature of the retort is kept up uniformly the same from beginning to end.

Saving of Labour.—In consequence of the superior facility with which the mode of decomposing coal in thin layers and removing the coke as fast as it is formed is effected, the saving in point of labour is very great. The charging and discharging of the retort is performed in two minutes. Hence one chaldron of coal may be decomposed by means of three horizontal rotary retorts, each twelve feet six inches in diameter, and with the attendance of two men, in eight hours, and produces from fifteen thousand, to eighteen thousand cubic feet of gas, whilst ten thousand cubic feet of gas can only be obtained from the same quantity of coal in eight hours, by means of twenty cylindrical retorts, attended by the same number of workmen.

Saving of machinery.—When we compare the original cost and wear and tear of the horizontal rotary retorts, with the cost and deterioration of a set of cylindrical, parallelopipedal, ellipsoidal, or semi-cylindrical retorts of an equal power, (that is to say to produce a like quantity of gas, in a given time,) a difference not less striking presents itself in favour of the horizontal retort.

We have stated already, that cylindrical, ellipsoidal, parallelopipedal, or semi-cylindrical retorts, when constantly kept in action, and worked to the greatest advantage, cannot be made to last longer than six months.[34]

Only one-third part of the top and bottom plates of the rotary retort being exposed to the action of heat, are alone liable to deterioration. It is only necessary therefore that these parts of the vessel be renewed, while the other parts remain uninjured for years. The new top and bottom plates being rivetted to the old and undecayed part, without deranging the rest, the retort is rendered as good as new.

Gain in the quantity of gas.—A large increase in the quantity of gas obtained, is a natural consequence of the mode in which the decomposition of coal is effected by means of the horizontal rotary retort.

Every body knows that coal, when decomposed slowly, affords a larger quantity of tar and ammoniacal liquor, but a less quantity of gas than when decomposed rapidly.

In the former case, the formation of the proximate products which coal is capable of furnishing is effected properly; the bituminous part of the coal is developed under the most favourable circumstances.

But when coal, after being previously deprived of moisture, is very suddenly heated to a high temperature, in thin strata, and small portions at a time, so that the vaporous products instead of becoming condensed, are made to come into contact with a substance (which in this case is the roof of the retort,) kept constantly at a temperature rather higher than that at which gold, silver, and copper melts, (32°, Wedgwood, or 5237°, Fahrenheit,) a very different arrangement of principles takes place.

The greatest portion of tar which the coal is capable of furnishing, instead of being produced in a liquid form, becomes then decomposed into carburetted hydrogen, and olifiant gas. That portion of tar which escapes decomposition, is condensed in the perpendicular pipe H, fig. 2, plate II., or H, fig. 2, plate VII., and falls back again into the retort, where it is also decomposed when the coal upon which it falls comes under the process of decomposition.

Hence the quantity of tar obtained by means of horizontal rotary retorts, is very small; it seldom exceeds the proportion mentioned page 123, when the retort is worked to the greatest advantage. This quantity is considerably diminished, when Newcastle coal, broken into pieces of the size of split pease is decomposed in strata, not exceeding two inches in thickness. The quantity of tar afforded by a chaldron of coal then amounts to thirty pounds, whilst at the same time the quality of the gas is improved; because coal tar furnishes olifiant gas, which the coal alone, when distilled by means of cylindrical or other shaped cast-iron retorts of the usual form, cannot produce, or at least but in a small quantity. One gallon of coal tar yields 15 cubic feet of olifiant gas, which greatly increases the illuminating power of the carburetted hydrogen.

From what has been so far stated, it will be understood why one chaldron of Newcastle coal, when decomposed by the new process, may readily be made to produce from 15,000 to 18,000 cubic feet of gas and upwards, whereas the same quantity of coal, if decomposed by the old method, yields only upon an average 10,000 cubic feet of gas.[35]

In the former case, the greater part of the essential oil and tar which the coal would have afforded is decomposed, as stated already by virtue of the high temperature to which the vapourous tar is suddenly exposed in the horizontal rotary retort, which is not the case when coal is decomposed in the retorts of the old construction.

Gain in the quantity of coke.—With the cylindrical or cast-iron retorts of the old shapes, the quantity of coke obtained from a given quantity of coal is upon an average 25 per cent. increase by measure from the best kind of Newcastle and Sunderland coal, but taking into account the waste incurred in breaking out and removing the red hot coke from the retort, which requires the application of rakers and crow bars, a considerable portion of it becomes reduced to dust or breeze, and hence no more than bulk for bulk of the coal decomposed can seldom be depended upon as the ultimate saleable quantity of coke.[36]

In the new mode of carbonizing coal by means of the horizontal rotary retorts, the increase of coke is 150 per cent. by measure, so that one chaldron of Newcastle coal produces two and a half chaldron of coke—this is the quantity produced upon an average. But when the retort is worked at a temperature to produce at the rate of 18,000 cubic feet of gas from the chaldron of coal, the increase of coke by measure is 175 per cent.; in that case, the layers of coal in the coal boxes must not exceed two inches in thickness, so that the volume of coke is in the ratio of the quantity of gas produced and the rapidity and elevation of temperature at which the decomposition of the coal is effected.

The coke being withdrawn from the place where it is formed by merely turning the boxes containing it, upside down, all waste is avoided.

With respect, again, to the quality of the coke, it will be observed that when the coal is rapidly carbonized in thin layers, and has full liberty to expand freely, as in the case of the horizontal rotary retort, it affords a light and porous coke, whereas in the cylindrical, paralellopipedal, semi-cylindrical, or ellipsoidal retorts, the coke being compressed, the intense heat to which it is so long and superfluously exposed, renders it extremely dense, and of a stony hardness.

The latter sort of coke is unquestionably preferable for the smelter, and all furnace operations, standing the blast of the bellows well. But the coke produced in the new mode of operating, is better suited for the great majority of domestic purposes, kindling more readily, and making a more cheerful fire. The combustion of the dense, or as it is now called, cylinder coke, can be only kept up when used in a common grate, by a strong draft of air, and it is therefore not so well suited for fuel for domestic purposes, to make a small fire; but the coke obtained by the horizontal rotary retort, readily maintains its own combustion, even when in small masses; hence it may be used without any trouble, either in the fire-place of the cottager, or of the prince, and accordingly it bears a higher price in the market.

Directions to workmen, with regard to the management of Horizontal Rotary Retorts.

The circumstance most essential to the economical application of the horizontal rotary retort, is, as has been repeatedly stated, that the coal shall be spread in thin layers in the boxes of the retort, not exceeding from two to four inches in thickness; and it may be laid down as a general rule, that the thinner the layers, and the higher the temperature, the greater will be the proportion of gas, the greater the bulk of coke, and the smaller the quantity of tar.

The coal before it is submitted to the distillatory process, should be as dry as possible, and the more it is comminuted the better. The very refuse of the coal called slack, provided it is perfectly free from foreign matter, answers best. It should also be spread in the trays, in even layers.

When the retort is in a good working state, the temperature should be kept up by the application of small quantities of fuel at a time. A prodigious saving of fuel may be effected by attending the fire properly, and it is this which distinguishes a careful stoker from a bungler. For in the working of this retort particularly, it is a wasteful process to clog up the fire-place with a large quantity of fuel injudiciously applied. The difference in this respect, with regard to the economy of fuel is so great, that an expert stoker will work the retort with one-third less of fuel and half the labour that would be employed by a negligent workman.

The quantity of gas produced from a chaldron of coal may be ascertained by the gas metre, or by the gas holder, if the outlet valve of the latter be shut during the distillatory process.

The heat at the same time employed for working the retort, will be best defined for the stoker’s guide, by copying carefully on paper the red tint of the retort, as seen through the sight hole, made for that purpose in the brick-work directly over the fire-place.

The first six feet of the perpendicular pipe H, fig. 1, plate II., which conveys the distillatory products from the retort, should be well cleaned out once a month, the bonnet at the bended part of the pipe H, fig. 1, is provided for that purpose, as already stated, page 119.

When the retort remains uncharged, the fire must be kept low in order to prevent its getting beyond the usual temperature, and the arms and moveable axis should be turned occasionally, and the door kept close.

The fire tiles which cover the flues under the retort should be examined about once a fortnight, and if a tile is melted or broken, it must be replaced by a new one, because the preservation of the retort greatly depends upon this precaution.

All the parts of the arms composing the moveable disc within the retort, may be taken out of the door of the retort, if they should require a repair, first taking off the cap from the perpendicular pipe E, fig. 1, plate II., surrounding the shaft of the retort, then the centre piece, or rose centre, F, fig. 2, plate II., the shaft D, fig. 2, plate III., may be drawn up through the pipe which surrounds it.

When the retort requires cleaning, which should be done once every six or eight months, a screw may be attached to the upper extremity of the shaft D, which passes through the retort; by this means, the arms and rose centre within the retort can easily be raised, to leave the bottom of the retort quite clear, in order that the lumps of coke, that may be scattered about, may be easily removed. And if an incrustation of coke should happen to be attached to the bottom of the retort, it may be readily detached by a crow bar, or other suitable instrument.

The trays or coal boxes, fig. 12, plate II., may be made by the stoker, of sheet iron, (called in commerce No. 16,) framed upon a wooden mould made for the purpose.

The temperature best suited for the decomposition of coal by means of the horizontal rotary retort depends, as has been already stated in the case of cylindrical cast-iron retorts, altogether on the price of coal, and the price which can be obtained for the coke.

In all places where the average price of coal, equal in quality to (Bewick and Craister’s Walls End) Newcastle coal, or any other species of coal, capable of producing from fifteen to eighteen thousand feet of gas from one chaldron, is not less than £ 2. 8s. the chaldron (27 Cwt.) or upwards, and where coke can be sold at the average price of £. 1 the chaldron, the horizontal rotary retort should be worked at such a temperature, that when viewed through the sight hole, it shall appear of a bright cherry redness, and at which it produces from 15,000 to 16,000 cubic feet of gas, from a chaldron of coal.

But in all other places where coal of the same quality to (Bewick and Craister’s Walls End) Newcastle coal, may be purchased at £. 1 8s. the chaldron, or at a less price, it will be more advantageous to the manufacturer, to work the horizontal rotary retort, at a lower temperature, so as to produce only at the rate of thirteen or fourteen thousand cubic feet of gas from the chaldron of coal. In the latter case the manufacturer expends coal in order to save his retort, whereas in the former case he economizes the fuel, as productive of a gain more than commensurate for the waste of the retort.

When the supply of gas required, happens at any time to be less than the retort when in action is capable of furnishing, the fire must then be kept low, but the retort should never be suffered to acquire a lower temperature, than that of a dull red heat visible by day-light.

PART VIII.

Purifying Apparatus, or Lime Machine.

Coal gas, even as obtained from the best species of coal, must be rendered pure before it is fit for the purpose of illumination. The gas in its crude state always contains a portion of sulphuretted hydrogen and carbonic acid; and when burnt, although its illuminating power is greater in an impure than in a pure state, it produces an oppressive and suffocating odour, which is speedily perceptible in confined places. The gaseous product evolved during its combustion, blackens paint and tarnishes metallic bodies; an impure gas besides strongly acts upon the copper branch pipes through which it is conveyed.

To obviate these defects the sulphuretted hydrogen and carbonic acid which are the cause of them must be removed, and to effect this, no method more economical and efficacious, has as yet been discovered, than to bring the gas confined under a pressure equal to a column of water, not less than eight or ten inches in height, into contact with quick-lime, diffused through water. Other means have been tried, but all of them have failed to be sufficiently efficacious or economical on a large scale.

Lime Machine originally employed for the Purification of Coal Gas.

In the lime-machine, until lately in use, the gas was made to pass in the apparatus, through passages which could not be guarded from being stopped up in the course of time by the concretion of a quantity of carbonate and hydro-sulphuret of lime, formed during the purification of the gas, so that when the stoppage occurred, a prodigious pressure was produced in the machine, in consequence of which, it was either found impossible to keep the distillatory apparatus air-tight, or if this was accomplished, a great part of the gas was forced through the purifying apparatus, without coming in contact with the lime, by driving the column of mixture of lime and water before it, and of course without being rendered fit for use, previous to its passing into the gas reservoir. This effect was unavoidable without the precaution of employing a very dilute mixture of quick-lime and water.

Numerous instances have also occurred where from the increased pressure which the gas exerted in the lime apparatus, the tar from the hydraulic main was driven up with a prodigious force through the dip pipe, P, fig. 2, plate IV., into the retort when the retort was opened, where it took fire to the imminent danger of the whole establishment.

The apparatus originally employed was composed of a large vessel closed on all sides to receive the gas; within this was a smaller vessel or lime trough open at top containing the quick-lime and water; and there was also a third vessel, or inverted trough into which the gas was received.

This inverted trough was open at bottom, and the edge of the open part was immersed beneath the surface of the mixture of lime and water contained in the lime-trough, so that the gas which was introduced in the last-mentioned inverted trough could not escape therefrom, except rising up through the lime and water. To facilitate this, holes or openings were made in the inverted trough near the bottom edge thereof, and beneath the surface of the purifying mixture, so that the bubbles of gas were obliged to rise up through these openings. From this construction of the machine the apertures through which the gas had to pass, were extremely liable to become stopped up, and dangerous consequences ensued.

In order to remedy in some measure the evil, a plan was adopted by Mr. Malam, for making the gas to pass in thin strata underneath a series of shelves, placed horizontally in the machine so as to expose the gas in as large a surface as possible to the contact of the lime and water, and employing the purifying mixture at the same time in a more dilute state:—this arrangement is as follows.

Fig. 4; plate V., represents a vertical section of the machine; it is made of cast-iron plates, rendered air-tight by screws, bolts, and iron cement. It consists of three separate chambers, a, a, a, destined to contain the mixture of quick-lime and water. At the under side of each chamber, is bolted a cylinder, h, h, h, the lower extremity of which is furnished with a large flanch, extending nearly to within the whole inner diameter of the machine.

From the bottom of each of the chambers, a, a, a, proceeds a pipe curved upwards, and communicating with a circular vessel, C, C, C, which serve for the purpose of charging the chambers, a, a, a, with the mixture of quick-lime and water, and regulating the level of the fluid within the chambers. The curved pipe likewise prevents the escape of the gas when the contents of the chambers a, a, a, are discharged.

The vessels, C, C, C, are provided with a waste pipe and stop-cock, as shown in the sketch, for discharging the contents of one chamber into the chamber placed below it, and lastly into the reservoir e.

b b, are pipes which convey the gas into the chambers, one extremity of each pipe communicates with the cylinders h, h, h, and the other with the chamber below it, and the lower pipe communicates with the valve M, so that by this means a communication is formed from the lower cylinder h, to the middle cylinder h, and from the middle to the uppermost cylinder. K, is the exit pipe which conveys the purified gas from the uppermost chamber into the reservoir destined to receive it. Through the centre of the machine passes a wrought-iron shaft, furnished with agitators or arms, to stir up the mixture of quick-lime and water. The arms are not immediately connected with the shaft, but proceed from cast-iron hydraulic cups, of the usual construction, by this means the escape of the gas is prevented, nor can the fluid pass from one chamber into another. The axis is put in motion, by wheel-work as shown in the design e, the handle for turning the shaft.

g, is a receiver to collect the condensible products. The contents of this vessel may be discharged by a hand pump being attached to the upper extremity of the pipe f, after the cap with which it is closed is removed.

The operation of this lime machine is obvious. The gas first passes into the lowermost chamber of the cylinder h, where it comes in contact with the purifying mixture and passes through the fluid to the top of the same chamber, and thence through the pipe b, into the cylinder above it which communicates with the lower chamber, where it is acted on again by the lime and water, and bubbles up through the fluid to the top of the chamber. From this compartment the gas passes into the third cylinder, where it bubbles up and passes through the lime and water; and lastly it makes its exit through the pipe K, into the gasholder or vessel destined to receive it.

When the mixture of quick-lime and water in the compartments a, a, a, of the machine, requires to be renewed, it is let off by the stop-cock at the bottom of the lowermost vessel into the reservoir e. The fluid contained in the upper chamber may be discharged into the chamber below it, and so on with the chambers below it, care being taken to close the stop-cock of the lower vessel. The machine may be recharged at the uppermost chamber with the purifying mixture. Fig. 5, exhibits the plan of the machine. b, b, b, the tubes connecting the chambers. B, the flanch of the cylinder h.

This machine has in part remedied the inconveniences stated pages 141, 142, but the increase in the quantity of the purifying materials which the apparatus requires, is of itself productive of most serious disadvantages.

The greater accumulation of waste lime which such a practice occasions, renders it necessary that capacious reservoirs and sewers should be constructed to receive and carry off the refuse materials, and where an outlet by such means cannot be obtained, the carting away the increased quantities of waste matter adds greatly to the cost of the gas.

If attempts are made to convey the waste substances into the common sewers or drains of the neighbourhood, the proprietors of gas works are exposed to indictments for a nuisance at the suit of the inhabitants, and when the near proximity of any river or lake induces an attempt to convey the waste materials thither, the most serious injury may be done to the water, which becoming impregnated with hydrosulphuret of lime is rendered unfit not only for domestic but for many manufacturing purposes. The latter evil indeed is one which operates also in a greater or lesser degree, even when the fœtid refuse or hydrosulphuret is discharged into the common sewers, all of which ultimately empty themselves into some water course, rivulet or lake. I would here beg to suggest, that considering how rapidly the new mode of procuring light is extending throughout Britain,[37] and how much the waters of the country are liable to be contaminated, from discharging into them the noxious refuse from the process of purifying coal gas, so as to be rendered proportionably unfit for the various purposes of domestic and manufacturing economy, it is well deserving the attention of the legislature, whether such contamination ought not to be guarded against by prohibiting enactments.

It appears to me that it would be a wise exertion of authority, to insert in every act of Parliament granted for incorporating Gas-light Companies, a clause prohibiting the proprietors from ever conveying the waste material, or any other produce from the manufacture of coal gas, either directly or indirectly into the common sewers, drains or water courses, or into rivers and lakes adjacent. The salubrity of the water we use is of as much consequence to us, as any superior excellence or saving of cost in our light can possibly be, and we ought to take care that in profiting by an improvement which science and art have discovered, we do not unnecessarily depreciate one of those primary blessings we owe to the bounty of nature.

Lime Machine lately adopted.

In the improved purifying apparatus[38] lately brought into use, of which we shall now give an account, a shaft or axis furnished with teeth or claws, is applied within the interior of the vessel, and made to act in such a manner as to scrape out the openings or slits through which the gas has to pass every time the axis is moved round, and by which regular clearance all chance of stoppage is avoided without any augmentation of the purifying mixture.