MERITS OF LIGHTING BY INCANDESCENT MANTLES.--It has already been shown that acetylene bases its chief claim for adoption as an illuminant in country districts upon the fact that, when consumed in simple self- luminous burners, it gives a light comparable in all respects save that of cost to the light of incandescent coal-gas. The employment of a mantle is still accompanied by several objections which appear serious to the average householder, who is not always disposed either to devote sufficient attention to his burners to keep them in a high state of efficiency or to contract for their maintenance by the gas company or others. Coal-gas cannot be burnt satisfactorily on the incandescent system unless the glass chimneys and shades are kept clean, unless the mantles are renewed as soon as they show signs of deterioration, and, perhaps most important of all, unless the burners are frequently cleared of the dust which collects round the jets. For this reason luminous acetylene ranks with luminous coal-gas in convenience and simplicity, while ranking with incandescent coal-gas in hygienic value. Very similar remarks apply to paraffin, and, in certain countries, to denatured alcohol. Since those latter illuminants are also available in rural places where coal-gas is not laid on, luminous acetylene is a less advantageous means of procuring artificial light than paraffin (and on occasion than coal-gas and alcohol when the latter fuels are burnt under the mantle), if the pecuniary aspect of the question is the only one considered. Such a comparison, however, is by no means fair; for if coal- gas, paraffin, and alcohol can be consumed on the incandescent system, so can acetylene; and if acetylene is hygienically equal to incandescent coal-gas, it is superior thereto when also burnt under the mantle. Nevertheless there should be one minor but perfectly irremediable defect in incandescent acetylene, viz., a sacrifice of that characteristic property of the luminous gas to emit a light closely resembling that of the sun in tint, which was mentioned in Chapter 1. Self-luminous acetylene gives the whitest light hitherto procurable without special correction of the rays, because its light is derived from glowing particles of carbon which happen to be heated (because of the high flame temperature) to the best possible temperature for the emission of pure white light. The light of any combustible consumed on the "incandescent" system is derived from glowing particles of ceria, thoria, or similar metallic oxides; and the character or shade of the light they emit is a function, apart from the temperature to which they are raised, of their specific chemical nature. Still, the light of incandescent acetylene is sufficiently pleasant, and according to Caro is purer white than that of incandescent coal-gas; but lengthy tests carried out by one of the authors actually show it to be appreciably inferior to luminous acetylene for colour-matching, in which the latter is known almost to equal full daylight, and to excel every form of artificial light except that of the electric arc specially corrected by means of glass tinted with copper salts.

CONDITIONS FOR INCANDESCENT ACETYLENE LIGHTING.--For success in the combustion of acetylene on the incandescent system, however, several points have to be observed. First, the gas must be delivered at a strictly constant pressure to the burner, and at one which exceeds a certain limit, ranging with different types and different sizes of burner from 2 to 4 or 5 inches of water. (The authors examined, as long ago as 1903, an incandescent burner of German construction claimed to work at a pressure of 1.5 inches, which it was almost impossible to induce to fire back to the jets however slowly the cock was manipulated, provided the pressure of the gas was maintained well above the point specified. But ordinarily a pressure of about 4 inches is used with incandescent acetylene burners.) Secondly, it is necessary that the acetylene shall at all times be free from appreciable admixture with air, even 0.5 per cent, being highly objectionable according to Caro; so that generators introducing any noteworthy amount of air into the holder each time their decomposing chambers are opened for recharging are not suitable for employment when incandescent burners are contemplated. The reason for this will be more apparent later on, but it depends on the obvious fact that if the acetylene already contains an appreciable proportion of air, when a further quantity is admitted at the burner inlets, the gaseous mixture contains a higher percentage of oxygen than is suited to the size and design of the burner, so that flashing back to the injector jets is imminent at any moment, and may be determined by the slightest fluctuation in pressure--if, indeed, the flame will remain at the proper spot for combustion at all. Thirdly, the fact that the acetylene which is to be consumed under the mantle must be most rigorously purified from phosphorus compounds has been mentioned in Chapter V. Impure acetylene will often destroy a mantle in two or three hours; but with highly purified gas the average life of a mantle may be taken, according to Giro, at 500 or 600 hours. It is safer, however, to assume a rather shorter average life, say 300 to 400 burning hours. Fourthly, owing to the higher pressure at which acetylene must be delivered to an incandescent burner and to the higher temperature of the acetylene flame in comparison with coal-gas, a mantle good enough to give satisfactory results with the latter does not of necessity answer with acetylene; in fact, the authors have found that English Welsbach coal-gas mantles of the small sizes required by incandescent acetylene burners are not competent to last for more than a very few hours, although, in identical conditions, mantles prepared specially for use with acetylene have proved durable. The atmospheric acetylene flame, too, differs in shape from an atmospheric flame of coal-gas, and it does not always happen that a coal- gas mantle contracts to fit the former; although it usually emits a better light (because it fits better) after some 20 hours use than at first. Caro has stated that to derive the best results a mantle needs to contain a larger proportion of ceria than the 1 per cent. present in mantles made according to the Welsbach formula, that it should be somewhat coarser in mesh, and have a large orifice at the head. Other authorities hold that mantles for acetylene, should contain other rare earths besides the thoria and ceria of which the coal-gas mantles almost wholly consist. It seems probable, however, that the composition of the ordinary impregnating fluid need not be varied for acetylene mantles provided it is of the proper strength and the mantles are raised to a higher temperature in manufacture than coal-gas mantles by the use of either coal-gas at very high pressure or an acetylene flame. The thickness of the substance of the mantle cannot be greatly increased with a view to attaining greater stability without causing a reduction in the light afforded. But the shape should be such that the mantle conforms as closely as possible to the acetylene Bunsen flame, which differs slightly with different patterns of incandescent burner heads. According to L. Cadenel, the acetylene mantle should be cylindrical for the lower two- thirds of its length, and slightly conical above, with an opening of moderate size at the top. The head of the mantle should be of slighter construction than that of coal-gas mantles. Fifthly, generators belonging to the automatic variety, which in most forms inevitably add more or less air to the acetylene every time they are cleaned or charged, appear to have achieved most popularity in Great Britain; and these frequently do not yield a gas fit for use with the mantle. This state of affairs, added to what has just been said, makes it difficult to speak in very favourable terms of the incandescent acetylene light for use in Great Britain. But as the advantages of an acetylene not contaminated with air are becoming more generally recognised, and mantles of several different makes are procurable more cheaply, incandescent acetylene is now more practicable than hitherto. Carburetted acetylene or "carburylene," which is discussed later, is especially suitable for use with mantle burners.

ATMOSPHERIC ACETYLENE BURNERS.--The satisfactory employment of acetylene in incandescent burners, for boiling, warming, and cooking purposes, and also to some extent as a motive power in small engines, demands the production of a good atmospheric or non-luminous flame, i.e., the construction of a trustworthy burner of the Bunsen type. This has been exceedingly difficult to achieve for two reasons: first, the wide range over which mixtures of acetylene and air are explosive; secondly, the high speed at which the explosive wave travels through such a mixture. It has been pointed out in Chapter VIII. that a Bunsen burner is one in which a certain proportion of air is mixed with the gas before it arrives at the actual point of ignition; and as that proportion must be such that the mixture falls between the upper and lower limits of explosibility, there is a gaseous mixture in the burner tube between the air inlets and the outlet which, if the conditions are suitable, will burn with explosive force: that is to say, will fire back to the air jets when a light is applied to the proper place for combustion. Such an explosion, of course, is far too small in extent to constitute any danger to person or property; the objection to it is simply that the shock of the explosion is liable to fracture the fragile incandescent mantle, while the gas, continuing to burn within the burner tube (in the case of a warming or cooking stove), blocks up that tube with carbon, and exhibits the other well-known troubles of a coal-gas stove which has "fired back."

It has been shown, however, in Chapter VI. that the range over which mixtures of acetylene and air are explosive depends on the size of the vessel, or more particularly on the diameter of the tube, in which they are stored; so that if the burner tube between the air inlets and the point of ignition can be made small enough in diameter, a normally explosive mixture will cease to exhibit explosive properties. Manifestly, if a tube is made very small in diameter, it will only pass a small volume of gas, and it may be useless for the supply of an atmospheric burner; but Le Chatelier's researches have proved that a tube may be narrowed at one spot only, in such fashion that the explosive wave refuses to pass the constriction, while the virtual diameter of the tube, as far as passage of gas is concerned, remains considerably larger than the size of the constriction itself. Moreover, inasmuch as the speed of propagation of the explosion is strictly fixed by the conditions prevailing, if the speed at which the mixture, of acetylene and air travels from the air inlets to the point of ignition is more rapid than the speed at which the explosion tends to travel from the point of ignition to the air inlets, the said mixture of acetylene and air will burn quietly at the orifice without attempting to fire backwards into the tube. By combining together these two devices: by delivering the acetylene to the injector jet at a pressure sufficient to drive the mixture of gas and air forward rapidly enough, and by narrowing the leading tube either wholly or at one spot to a diameter small enough, it is easy to make an atmospheric burner for acetylene which behaves perfectly as long as it is fairly alight, and the supply of gas is not checked; but further difficulties still remain, because at the instant of lighting and extinguishing, i.e., while the tap is being turned on or off, the pressure of the gas is too small to determine a flow of acetylene and air within the tube at a speed exceeding that of the explosive wave; and therefore the act of lighting or extinguishing is very likely to be accompanied by a smart explosion severe enough to split the mantle, or at least to cause the burner to fire back. Nevertheless, after several early attempts, which were comparative failures, atmospheric acetylene burners have been constructed that work quite satisfactorily, so that the gas has become readily available for use under the mantle, or in heating stoves. Sometimes success has been obtained by the employment of more than one small tube leading to a common place of ignition, sometimes by the use of two or more fine wire- gauze screens in the tube, sometimes by the addition of an enlarged head to the burner in which head alone thorough mixing of the gas and air occurs, and sometimes by the employment of a travelling sleeve which serves more or less completely to block the air inlets.

DUTY OF INCANDESCENT ACETYLENE BURNERS.--Granting that the petty troubles and expenses incidental to incandescent lighting are not considered prohibitive--and in careful hands they are not really serious-- and that mantles suitable for acetylene are employed, the gas may be rendered considerably cheaper to use per unit of light evolved by consuming it in incandescent burners. In Chapter VIII. it was shown that the modern self-luminous, l/2-foot acetylene burner emits a light of about 1.27 standard English candles per litre-hour. A large number of incandescent burners, of German and French construction, consuming from 7.0 to 22.2 litres per hour at pressures ranging between 60 and 120 millimetres have been examined by Caro, who has found them to give lights of from 10.8 to 104.5 Hefner units, and efficiencies of from 2.40 to 5.50 units per litre-hour. Averaging his results, it may be said that incandescent burners consuming from 10 to 20 litres per hour at pressures of 80 or 100 millimetres yield a light of 4.0 Hefner units per litre- hour. Expressed in English terms, incandescent acetylene burners consuming 0.5 cubic foot per hour at a pressure of 3 or 4 inches give the duties shown in the following table, which may advantageously be compared with that printed in Chapter VIII., page 239, for the self-luminous gas:

ILLUMINATING POWER OF INCANDESCENT ACETYLENE. HALF-FOOT BURNERS.

A number of tests of the GÜntner or Schimek incandescent burners of the 10 and 15 litres-per-hour sizes, made by one of the authors in 1906, gave the following average results when tested at a pressure of 4 inches:

These figures indicate that the duty increases slightly with the size of the burner. Other tests showed that the duty increased more considerably with an increase of pressure, so that mantles used, or which had been previously used, at a pressure of 5 inches gave duties of 115 to 125 candles per cubic foot.

It should be noted that the burners so far considered are small, being intended for domestic purposes only; larger burners exhibit higher efficiencies. For instance, a set of French incandescent acetylene burners examined by FouchÉ showed:

By increasing the pressure at which acetylene is introduced into burners of this type, still larger duties may be obtained from them:

High-power burners such as these are only fit for special purposes, such as lighthouse illumination, or optical lantern work, &c.; and they naturally require mantles of considerably greater tenacity than those intended for employment with coal-gas. Nevertheless, suitable mantles can be, and are being, made, and by their aid the illuminating duty of acetylene can be raised from the 30 odd candles per foot of the common 0.5-foot self-luminous jet to 140 candles or more per foot, which is a gain in efficiency of 367 per cent., or, neglecting upkeep and sundries and considering only the gas consumed, an economy of nearly 79 per cent.

In 1902, working apparently with acetylene dissolved under pressure in acetone (cf. Chapter XI.), Lewes obtained the annexed results with the incandescent gas:

It will be seen that although the total candle-power developed increases with the pressure, the duty of the burner attained a maximum at a pressure of 10 inches. This is presumably due to the fact either that the same burner was used throughout the tests, and was only intended to work at a pressure of 10 inches or thereabouts, or that the larger burners were not so well constructed as the smaller ones. Other investigators have not given this maximum of duty with a medium-sized or medium-driven burner; but Lewes has observed a similar phenomenon in the case of 0.7 to 0.8 cubic foot self-luminous jets.

Figures, however, which seem to show that the duty of incandescent acetylene does not always rise with the size of the burner or with the pressure at which the gas is delivered to it, have been published in connexion with the installation at the French lighthouse at Chassiron, the northern point of the Island of OlÉron. Here the acetylene is generated in hand-fed carbide-to-water generators so constructed as to give any pressure up to nearly 200 inches of water column; purified by means of heratol, and finally delivered to a burner composed of thirty- seven small tubes, which raises to incandescence a mantle 55 millimetres in diameter at its base. At a pressure of 7.77 inches of water, the burner passes 3.9 cubic feet of acetylene per hour, and at a pressure of 49.2 inches (the head actually used) it consumes 20.06 cubic feet per hour. As shown by the following table, such increment of gas pressure raises the specific intensity of the light, i.e., the illuminating power per unit of incandescent surface, but it does not appreciably raise the duty or economy of the gas. Manifestly, in terms of duty alone, a pressure of 23.6 inches of water-column is as advantageous as the higher Chassiron figures; but since intensity of light is an important matter in a lighthouse, it is found better on the whole to work the generators at a pressure of 49.2 inches. In studying these figures referring to the French lighthouse, it is interesting to bear in mind that when ordinary six-wick petroleum oil burners wore used in the same place, the specific intensity of the light developed was 75 candle-power per square inch, and when that plant was abandoned in favour of an oil-gas apparatus, the incandescent burner yielded 161 candle-power per square inch; substitution of incandescent acetylene under pressure has doubled the brilliancy of the light.

When tested in modern burners consuming between 12 and 18 litres per hour at a pressure of 100 millimetres (4 inches), some special forms of incandescent mantles constructed of ramie fibre, which in certain respects appears to be better suited than cotton for use with acetylene, have shown the following degree of loss in illuminating power after prolonged employment (Caro):

_Luminosity in Hefner Units._

It will be seen that the maximum loss of illuminating power in 400 hours was 6.4 per cent., the average loss being 6.0 per cent.

TYPICAL INCANDESCENT BURNERS.--Of the many burners for lighting by the use of incandescent mantles which have been devised, a few of the more widely used types may be briefly referred to. There is no doubt that finality in the design of these burners has not yet been reached, and that improvements in the direction of simplification of construction and in efficiency and durability will continue to be made.

Among the early incandescent burners, one made by the Allgemeine Carbid und Acetylen Gesellschaft of Berlin in 1900 depended on the narrowness of the mixing tube and the proportioning of the gas nipple and air inlets to prevent lighting-back. There was a wider concentric tube round the upper part of the mixing tube, and the lower part of the mantle fitted round this. The mouth of the mixing tube of this 10-litres-per-hour burner was 0.11 inch in diameter, and the external diameter of the middle cylindrical part of the mixing tube was 0.28 inch. There was no gauze diaphragm or stuffing, and firing-back did not occur until the pressure was reduced to about 1.5 inches. The same company later introduced a burner differing in several important particulars from the one just described. The comparatively narrow stem of the mixing tube and the proportions of the gas nipple and air inlets were retained, but the mixing tube was surmounted by a wide chamber or burner head, in which naturally there was a considerable reduction in the rate of flow of the gas. Consequently it was found necessary to introduce a gauze screen into the burner head to prevent firing back. The alterations have resulted in the lighting duty of the burner being considerably improved. Among other burners designed about 1900 may be mentioned the Ackermann, the head of which consisted of a series of tubes from each of which a jet of flame was produced, the FouchÉ, the Weber, and the Trendel. Subsequently a tubular-headed burner known as the Sirius has been produced for the consumption of acetylene at high pressure (20 inches and upwards).

The more recent burners which have been somewhat extensively used include the "Schimek," made by W. GÜntner of Vienna, which is shown in Fig. 19. It consists of a tapering narrow injecting nozzle within a conical chamber C which is open below, and is surmounted by the mixing tube over which telescopes a tube which carries the enlarged burner head G, and the chimney gallery D. There are two diaphragms of gauze in the burner head to prevent firing back, and one in the nozzle portion of the burner. The conical chamber has a perforated base-plate below which is a circular plate B which rotates on a screw cut on the lower part of the nozzle portion A of the burner. This plate serves as a damper to control the amount of air admitted through the base of the conical chamber to the mixing tube. There are six small notches in the lower edge of the conical chamber to prevent the inflow of air being cut of entirely by the damper. The mixing tube in both the 10-litre and the 15-litre burner is about 0.24 inch in internal diameter but the burner head is nearly 0.42 inch in the 10-litre and 0.48 inch in the 15-litre burner. The opening in the head of the burner through which the mixture of gas and air escapes to the flame is 0.15 and 0.17 inch in diameter in these two sizes respectively. The results of some testings made with Schimek burners have been already given.

FIG. 19.--'SCHIMEK' BURNER

The "Knappich" burner, made by the firm of Keller and Knappich of Augsburg, somewhat resembles the later pattern of the Allgemeine Carbid und Acetylen Gesellschaft. It has a narrow mixing tube, viz., 0.2 inch in internal diameter, and a wide burner head, viz., 0.63 inch in internal diameter for the 25-litre size. The only gauze diaphragm is in the upper part of the burner head. The opening in the cap of the burner head, at which the gas burns, is 0.22 inch in diameter. The gas nipple extends into a domed chamber at the base of the mixing tube, and the internal air is supplied through four holes in the base-plate of that chamber. No means of regulating the effective area of the air inlet holes are provided.

The "Zenith" burner, made by the firm of GebrÜder Jacob of Zwickau, more closely resembles the Schimek, but the air inlets are in the side of the lower widened portion of the mixing tube, and are more or less closed by means of an outside loose collar which may be screwed up and down on a thread on a collar fixed to the mixing tube. The mixing tube is 0.24 inch, and the burner head 0.475 inch in internal diameter. The opening in the cap of the burner is 0.16 inch in diameter. There is a diaphragm of double gauze in the cap, and this is the only gauze used in the burner.

All the incandescent burners hitherto mentioned ordinarily have the gas nipple made in brass or other metal, which is liable to corrosion, and the orifice to distortion by heat or if it becomes necessary to remove any obstruction from it. The orifice in the nipple is extremely small-- usually less than 0.015 inch--and any slight obstruction or distortion would alter to a serious extent the rate of flow of gas through it, and so affect the working of the burner. In order to overcome this defect, inherent to metal nipples, burners are now constructed for acetylene in which the nipple is of hard incorrodible material. One of these burners has been made on behalf of the Office Central de l'AcÉtylÈne of Paris, and is commonly known as the "O.C.A." burner. In it the nipple is of steatite. On the inner mixing tube of this burner is mounted an elongated cone of wire wound spirally, which serves both to ensure proper admixture of the gas and air, and to prevent firing-back. There is no gauze in this burner, and the parts are readily detachable for cleaning when required. Another burner, in which metal is abolished for the nipple, is made by Geo. Bray and Co., Ltd., of Leeds, and is shown in Fig. 20. In this burner the injecting nipple is of porcelain.

FIG. 20.--BRAY'S INCANDESCENT BURNER

ACETYLENE FOR HEATING AND COOKING.--Since the problem of constructing a trustworthy atmospheric burner has been solved, acetylene is not only available for use in incandescent lighting, but it can also be employed for heating or cooking purposes, because all boiling, most warming, and some roasting stoves are simply arrangements for utilising the heat of a non-luminous flame in one particular way. With suitable alterations in the dimensions of the burners, apparatus for consuming coal-gas may be imitated and made fit to burn acetylene; and as a matter of fact several firms are now constructing such appliances, which leave little or nothing to be desired. It may perhaps be well to insist upon the elementary point which is so frequently ignored in practice, viz., that no stove, except perhaps a small portable boiling ring, ought ever to be used in an occupied room unless it is connected with a chimney, free from down- draughts, for the products of combustion to escape into the outer air; and also that no chimney, however tall, can cause an up-draught in all states of the weather unless there is free admission of fresh air into the room at the base of the chimney. Still, at the prices for coal, paraffin oil, and calcium carbide which exist in Great Britain, acetylene is not an economical means of providing artificial heat. If a 0.7 cubic foot luminous acetylene burner gives a light of 27 candles, and if ordinary country coal-gas gives light of 12 to 13 candles in a 5-foot burner, one volume of acetylene is equally valuable with 15 or 16 volumes of coal-gas when both are consumed in self-luminous jets; and if, with the mantle, acetylene develops 99 candles per cubic foot, while coal-gas gives in common practice 15 to 20 candles, one volume of acetylene is equally valuable with 5 to 6-1/2 volumes of coal-gas when both are consumed on the incandescent system; whereas, if the acetylene is burnt in a flat flame, and the coal-gas under the mantle, 1 volume of the former is equally efficient with 2 volumes of coal-gas as an artificial illuminant. This last method of comparison being manifestly unfair, acetylene may be said to be at least five times as efficient per unit of volume as coal-gas for the production of light. But from the table given on a later page it appears that as a source of artificial heat, acetylene is only equal to about 2-3 times its volume of ordinary coal-gas. Nevertheless, the domestic advantages of gas firing are very marked; and when a properly constructed stove is properly installed, the hygienic advantages of gas-firing are alone equally conspicuous--for the disfavor with which gas-firing is regarded by many physicians is due to experience gained with apparatus warming principally by convection [Footnote: Radiant heat is high-temperature heat, like the heat emitted by a mass of red-hot coke; convected heat is low-temperature heat, invisible to the eye. Radiant heat heats objects first, and leaves them to warm the air; convected heat is heat applied directly to air, and leaves the air to warm objects afterwards. On all hygienic grounds radiant heat is better than convected heat, but the latter is more economical. By an absurd and confusing custom, that particular warming apparatus (gas, steam, or hot water) which yields practically no radiant heat, and does all its work by convection, is known to the trade as a "radiator."] instead of radiation; or to acquaintance with intrinsically better stoves either not connected to any flues or connected to one deficient in exhausting power. In these circumstances, whenever an installation of acetylene has been laid down for the illumination of a house or district, the merit of convenience may outweigh the defect of extravagance, and the gas may be judiciously employed in a boiling ring, or for warming a bedroom; while, if pecuniary considerations are not paramount, the acetylene may be used for every purpose to which the townsman would apply his cheaper coal-gas.

The difficulty of constructing atmospheric acetylene burners in which the flame would not be likely to strike back to the nipple has already been referred to in connexion with the construction atmospheric burners for incandescent lighting. Owing, however, to the large proportions of the atmospheric burners of boiling rings and stove and in particular to the larger bore of their mixing tube, the risk of the flame striking back is greater with them, than with incandescent lighting burners. The greatest trouble is presented at lighting, and when the pressure of the gas-supply is low. The risk of firing-back when the burner is lighted is avoided in some forms of boiling rings, &c., by providing a loose collar which can be slipped over the air inlets of the Bunsen tube before applying a light to the burner, and slipped clear of them as soon as the burner is alight. Thus at the moment of lighting, the burner is converted temporarily into one of the non-atmospheric type, and after the flame has thus been established at the head or ring of the burner, the internal air-supply is started by removing the loose collar from the air inlets, and the flame is thus made atmospheric. In these conditions it does not travel backwards to the nipple. In other heating burners it is generally necessary to turn on the gas tap a few seconds before applying a light to the burner or ring or stove; the gas streaming through the mixing tube then fills it with acetylene and air mixed in the proper working proportions, and when the light is applied, there is no explosion in the mixing tube, or striking-back of the flame to the nipple.

Single or two-burner gas rings for boiling purposes, or for heating cooking ovens, known as the "La Belle," made by Falk Stadelmann and Co., Ltd., of London, may be used at as low a gas pressure as 2 inches, though they give better results at 3 inches, which is their normal working pressure. The gas-inlet nozzle or nipple of the burner is set within a spherical bulb in which are four air inlets. The mixing tube which is placed at a proper distance in front of the nipple, is proportioned to the rate of flow of the gas and air, and contains a mixing chamber with a baffling pillar to further their admixture. A fine wire gauze insertion serves to prevent striking-back of the flame. A "La Belle" boiling ring consumes at 3 inches pressure about 48 litres or 1.7 cubic feet of acetylene per hour.

ACETYLENE MOTORS.--The question as to the feasibility of developing "power" from acetylene, i.e., of running an engine by means of the gas, may be answered in essentially identical terms. Specially designed gas-engines of 1, 3, 6, or even 10 h.p. work perfectly with acetylene, and such motors are in regular employment in numerous situations, more particularly for pumping water to feed the generators of a large village acetylene installation. Acetylene is not an economical source of power, partly for the theoretical reason that it is a richer fuel even than coal-gas, and gas-engines would appear usually to be more efficient as the fuel they burn is poorer in calorific intensity, i.e., in heating power (which is explosive power) per unit of volume. The richer, or more concentrated, any fuel in, the more rapidly does the explosion in a mixture of that fuel with air proceed, because a rich fuel contains a smaller proportion of non-inflammable gases which tend to retard explosion than a poor one; and, in reason, a gas-engine works better the more slowly the mixture of gas and air with which it is fed explodes. Still, by properly designing the ports of a gas-engine cylinder, so that the normal amount of compression of the charge and of expansion of the exploded mixture which best suit coal-gas are modified to suit acetylene, satisfactory engines can be constructed; and wherever an acetylene installation for light exists, it becomes a mere question of expediency whether the same fuel shall not be used to develop power, say, for pumping up the water required in a large country house, instead of employing hand labour, or the cheaper hot-air or petroleum motor. Taking the mean of the results obtained by numerous investigators, it appears that 1 h.p.-hour can be obtained for a consumption of 200 litres of acetylene; whence it may be calculated that that amount of energy costs about 3d. for gas only, neglecting upkeep, lubricating material (which would be relatively expensive) and interest, &c.

Acetylene Blowpipes--The design of a satisfactory blowpipe for use with acetylene had at first proved a matter of some difficulty, since the jet, like that of an ordinary self-luminous burner, usually exhibited a tendency to become choked with carbonaceous growths. But when acetylene had become available for various purposes at considerable pressure, after compression into porous matter as described in Chapter XI, the troubles were soon overcome; and a new form of blowpipe was constructed in which acetylene was consumed under pressure in conjunction with oxygen. The temperature given by this apparatus exceeds that of the familiar oxy- hydrogen blowpipe, because the actual combustible material is carbon instead of hydrogen. When 2 atoms of hydrogen unite with 1 of oxygen to form 1 molecule of gaseous water, about 59 large calories are evolved, and when 1 atom of solid amorphous carbon unites with 2 atoms of oxygen to form 1 molecule of carbon dioxide, 97.3 calories are evolved. In both cases, however, the heat attainable is limited by the fact that at certain temperatures hydrogen and oxygen refuse to combine to form water, and carbon and oxygen refuse to form carbon dioxide--in other words, water vapour and carbon dioxide dissociate and absorb heat in the process at certain moderately elevated temperatures. But when 1 atom of solid amorphous carbon unites with 1 atom of oxygen to form carbon monoxide, 29.1 [Footnote: Cf. Chapter VI., page 185.] large calories are produced, and carbon monoxide is capable of existence at much higher temperatures than either carbon dioxide or water vapour. In any gaseous hydrocarbon, again, the carbon exists in the gaseous state, and when 1 atom of the hypothetical gaseous carbon combines with 1 atom of oxygen to produce 1 molecule of carbon monoxide, 68.2 large calories are evolved. Thus while solid amorphous carbon emits more heat than a chemically equivalent quantity of hydrogen provided it is enabled to combine with its higher proportion of oxygen, it emits less if only carbon monoxide is formed; but a higher temperature can be attained in the latter case, because the carbon monoxide is more permanent or stable. Gaseous carbon, on the other hand, emits more heat than an equivalent quantity of hydrogen, [Footnote: In a blowpipe flame hydrogen can only burn to gaseous, not liquid, water.] even when it is only converted into the monoxide. In other words, a gaseous fuel which consists of hydrogen alone can only yield that temperature as a maximum at which the speed of the dissociation of the water vapour reaches that of the oxidation of the hydrogen; and were carbon dioxide the only oxide of carbon, a similar state of affairs would be ultimately reached in the flame of a carbonaceous gas. But since in the latter case the carbon dioxide does not tend to dissociate completely, but only to lose one atom of oxygen, above the limiting temperature for the formation of carbon dioxide, carbon monoxide is still produced, because there is less dissociating force opposed to its formation. Thus at ordinary temperatures the heat of combustion of acetylene is 315.7 calories; but at temperatures where water vapour and carbon dioxide no longer exist, there is lost to that quantity of 315.7 calories the heat of combustion of hydrogen (69.0) and twice that of carbon monoxide (68.2 x 2 = 136.4); so that above those critical temperatures, the heat of combustion of acetylene is only 315.7 - (69.0 + 136.4) = 110.3. [Footnote: When the heat of combustion of acetylene is quoted as 315.7 calories, it is understood that the water formed is condensed into the liquid state. If the water remains gaseous, as it must do in a flame, the heat of formation is reduced by about 10 calories. This does not affect the above calculation, because the heat of combustion of hydrogen when the water remains gaseous is similarly 10 calories less than 69, i.e., 59, as mentioned above in the text. Deleting the heat of liquefaction of water, the calculation referred to becomes 305.7 - (59.0 + l36.4) = 110.3 as before.] This value of 110.3 calories is clearly made up of the heat of formation of acetylene itself, and twice the heat of conversion of carbon into carbon monoxide, i.e., for diamond carbon, 58.1 + 26.1 x 2 = 110.3; or for amorphous carbon, 52.1 + 29.1 x 2 = 110.3. From the foregoing considerations, it may be inferred that the acetylene-oxygen blowpipe can be regarded as a device for burning gaseous carbon in oxygen; but were it possible to obtain carbon in the state of gas and so to lead it into a blowpipe, the acetylene apparatus should still be more powerful, because in it the temperature would be raised, not only by the heat of formation of carbon monoxide, but also by the heat attendant upon the dissociation of the acetylene which yields the carbon.

Acetylene requires 2.5 volumes of oxygen to burn it completely; but in the construction of an acetylene-oxygen blowpipe the proportion of oxygen is kept below this figure, viz., at 1.1 to 1.8 volumes, so that the deficiency is left to be made up from the surrounding air. Thus at the jet of the blowpipe the acetylene dissociates and its carbon is oxidised, at first no doubt to carbon monoxide only, but afterwards to carbon dioxide; and round the flame of the gaseous carbon is a comparatively cool, though absolutely very hot jacket of hydrogen burning to water vapour in a mixture of oxygen and air, which protects the inner zone from loss of heat. As just explained, theoretical grounds support the conclusions at which FouchÉ has arrived, viz., that the temperature of the acetylene-oxygen blowpipe flame is above that at which hydrogen will combine with oxygen to form water, and that it can only be exceeded by those found in a powerful electric furnace. As the hydrogen dissociated from the acetylene remains temporarily in the free state, the flame of the acetylene blowpipe, possesses strong reducing powers; and this, coupled probably with an intensity of heat which is practically otherwise unattainable, except by the aid of a high-tension electric current, should make the acetylene-oxygen blowpipe a most useful piece of apparatus for a large variety of metallurgical, chemical, and physical operations. In FouchÉ's earliest attempts to design an acetylene blowpipe, the gas was first saturated with a combustible vapour, such as that of petroleum spirit or ether, and the mixture was consumed with a blast of oxygen in an ordinary coal-gas blow-pipe. The apparatus worked fairly well, but gave a flame of varying character; it was capable of fusing iron, raised a pencil of lime to a more brilliant degree of incandescence than the eth-oxygen burner, and did not deposit carbon at the jet. The matter, however, was not pursued, as the blowpipe fed with undiluted acetylene took its place. The second apparatus constructed by FouchÉ was the high-pressure blowpipe, the theoretical aspect of which has already been studied. In this, acetylene passing through a water-seal from a cylinder where it is stored as a solution in acetone (cf. Chapter XI.), and oxygen coming from another cylinder, are each allowed to enter the blowpipe at a pressure of 118 to 157 inches of water column (i.e., 8.7 to 11.6 inches of mercury; 4.2 to 5.7 lb. per square inch, or 0.3 to 0.4 atmosphere). The gases mix in a chamber tightly packed with porous matter such as that which is employed in the original acetylene reservoir, and finally issue from a jet having a diameter of 1 millimetre at the necessary speed of 100 to 150 metres per second. Finding, however, that the need for having the acetylene under pressure somewhat limited the sphere of usefulness of his apparatus, FouchÉ finally designed a low-pressure blowpipe, in which only the oxygen requires to be in a state of compression, while the acetylene is drawn directly from any generator of the ordinary pattern that does not yield a gas contaminated with air. The oxygen passes through a reducing valve to lower the pressure under which it stands in the cylinder to that of 1 or 1.5 effective atmosphere, this amount being necessary to inject the acetylene and to give the previously mentioned speed of escape from the blowpipe orifice. The acetylene is led through a system of long narrow tubes to prevent it firing-back.

AUTOGENOUS SOLDERING AND WELDING.--The blowpipe is suitable for the welding and for the autogenous soldering or "burning" of wrought or cast iron, steel, or copper. An apparatus consuming from 600 to 1000 litres of acetylene per hour yields a flame whose inner zone is 10 to 15 millimetres long, and 3 to 4 millimetres in diameter; it is sufficiently powerful to burn iron sheets 8 to 9 millimetres thick. By increasing the supply of acetylene in proportion to that of the oxygen, the tip of the inner zone becomes strongly luminous, and the flame then tends to carburise iron; when the gases are so adjusted that this tip just disappears, the flame is at its best for heating iron and steel. The consumption of acetylene is about 75 litres per hour for each millimetre of thickness in the sheet treated, and the normal consumption of oxygen is 1.7 times as much; a joint 6 metres long can be burnt in 1 millimetre plate per hour, and one of 1.5 metres in 10 millimetre plate. In certain cases it is found economical to raise the metal to dull redness by other means, say with a portable forge of the usual description, or with a blowpipe consuming coal-gas and air. There are other forms of low- pressure blowpipe besides the FouchÉ, in some of which the oxygen also is supplied at low pressure. Apart from the use of cylinders of dissolved acetylene, which are extremely convenient and practically indispensable when the blowpipe has to be applied in confined spaces (as in repairing propeller shafts on ships in situ), acetylene generators are now made by several firms in a convenient transportable form for providing the gas for use in welding or autogenous soldering. It is generally supposed that the metal used as solder in soldering iron or steel by this method must be iron containing only a trifling proportion of carbon (such as Swedish iron), because the carbon of the acetylene carburises the metal, which is heated in the oxy-acetylene flame, and would thereby make ordinary steel too rich in carbon. But the extent to which the metal used is carburised in the flame depends, as has already been indicated, on the proper adjustment of the proportion of oxygen to acetylene. Oxy-acetylene autogenous soldering or welding is applicable to a great variety of work, among which may be mentioned repairs to shafts, locomotive frames, cylinders, and to joints in ships' frames, pipes, boilers, and rails. The use of the process is rapidly extending in engineering works generally. Generators for acetylene soldering or welding must be of ample size to meet the quickly fluctuating demands on them and must be provided with water-seals, and a washer or scrubber and filter capable of arresting all impurities held mechanically in the crude gas, and with a safety vent- pipe terminating in the open at a distance from the work in hand. The generator must be of a type which affords as little after-generation as possible, and should not need recharging while the blowpipe is in use. There should be a main tap on the pipe between the generator and the blowpipe. It does not appear conclusively established that the gas consumed should have been chemically purified, but a purifier of ample size and charged with efficient material is undoubtedly beneficial. The blowpipe must be designed so that it remains sufficiently cool to prevent polymerisation of the acetylene and deposition of the resultant particles of carbon or soot within it.

It is important to remember that if a diluent gas, such as nitrogen, is present, the superior calorific power of acetylene over nearly all gases should avail to keep the temperature of the flame more nearly up to the temperature at which hydrogen and oxygen cease to combine. Hence a blowpipe fed with air and acetylene would give a higher temperature than any ordinary (atmospheric) coal-gas blowpipe, just as, as has been explained in Chapter VI., an ordinary acetylene flame has a higher temperature than a coal-gas flame. It is likely that a blowpipe fed with "LindÉ-air" (oxygen diluted with less nitrogen than in the atmosphere) and acetylene would give as high a limelight effect as the oxy-hydrogen or oxy-coal-gas blowpipe.