The essential feature of the Naphey burner is the tip, which is shown in longitudinal section at A in Fig. 8. It consists of a mushroom headed cylinder of steatite, drilled centrally with a gas passage, which at its point is of a diameter suited to pass half the quantity of acetylene that the entire burner is intended to consume. The cap is provided with four radial air passages, only two of which are represented in the drawing; these unite in the centre of the head, where they enter into the longitudinal channel, virtually a continuation of the gas-way, leading to the point of combustion by a tube wide enough to pass the introduced air as well as the gas. Being under some pressure, the acetylene issuing from the jet at the end of the cylindrical portion of the tip injects air through the four air passages, and the mixture is finally burnt at the top orifice. As pointed out in Chapter VII., the injector jet is so small in diameter that even if the service-pipes leading to the tip contain an explosive mixture of acetylene and air, the explosion produced locally if a light is applied to the burner cannot pass backwards through that jet, and all danger is obviated. One tip only of this description evidently produces a long, jet-like flame, or a "rat-tail," in which the latent illuminating power of the acetylene is not developed economically. In practice, therefore, two of these tips are employed in unison, one of the commonest methods of holding them being shown at B. From each tip issues a stream of acetylene mixed with air, and to some extent also surrounded by a jacket of air; and at a certain point, which forms the apex of an isosceles right-angled triangle having its other angles at the orifices of the tips, the gas streams impinge, yielding a flat flame, at right- angles, as mentioned before, to the plane of the triangle. If the two tips are three-quarters of an inch apart, and if the angle of impingement is exactly 90, the distance of each tip from the base of the flame proper will be a trifle over half an inch; and although each stream of gas does take fire and burn somewhat before meeting its neighbour, comparatively little heat is generated near the body of the steatite.
Nevertheless, sufficient heat is occasionally communicated to the metal stems of these burners to cause warping, followed by a want of alignment in the gas streams, and this produces distortion of the flame, and possibly smoking. Three methods of overcoming this defect have been used: in one the arms are constructed entirely of steatite, in another they are made of such soft metal as easily to be bent back again into position with the fingers or pliers, in the third each arm is in two portions, screwing the one into the other. The second type is represented by the original Phos burner, in which the curved arms of B are replaced by a pair of straight divergent arms of thin, soft tubing, joined to a pair of convergent wider tubes carrying the two tips. The third type is met with in the Drake burner, where the divergent arms are wide and have an internal thread into which screws an external thread cut upon lateral prolongations of the convergent tubes. Thus both the Phos and the Drake burner exhibit a pair of exposed elbows between the gas inlet and the two tips; and these elbows are utilised to carry a screwed wire fastened to an external milled head by means of which any deposit of carbon in the burner tubes can be pushed out. The present pattern of the Phos burner is shown in Fig. 9, in which _A_ is the burner tip, _B_ the wire or needle, and _C_ the milled head by which the wire is screwed in and out of the burner tube.
[Illustration: FIG. 9.--IMPROVED PHoS BURNER.]
[Illustration: FIG. 10.--"WONDER" SINGLE AND TWO-FLAME BURNERS.]
[Illustration: FIG. 11.--"SUPREMA" NO. 266651, TWO-FLAME BURNER.]
[Illustration: FIG. 12.--BRAY'S MODIFIED NAPHEY INJECTOR BURNER TIP.]
[Illustration: FIG. 13.--BRAY'S "ELTA" BURNER.]
[Illustration: FIG. 14.--BRAY'S "LUTA" BURNER.]
[Illustration: FIG. 15.--BRAY'S "SANSAIR" BURNER.]
[Illustration: FIG. 16.--ADJUSTABLE "KONA" BURNER.]
In the original Billwiller burner, the injector gas orifice was brought centrally under a somewhat larger hole drilled in a separate sheet of platinum, the metal being so carried as to permit entry of air. In order to avoid the expense of the platinum, the same principle was afterwards used in the design of an all-steatite head, which is represented at D in Fig. 8. The two holes there visible are the orifices for the emission of the mixture of acetylene with indrawn air, the proper acetylene jets lying concentrically below these in the thicker portions of the heads.
These two types of burner have been modified in a large number of ways, some of which are shown at C, E, and F; the air entering through saw- cuts, lateral holes, or an annular channel. Burners resembling F in outward form are made with a pair of injector jets and corresponding air orifices on each head, so as to produce a pair of names lying in the same plane, "end-on" to one another, and projecting at either side considerably beyond the body of the burner; these have the advantage of yielding no shadow directly underneath. A burner of this pattern, viz., the "Wonder," which is sold in this country by Hannam's, Ltd., is shown in Fig. 10, alongside the single-flame "Wonder" burner, which is largely used, especially in the United States. Another two-flame burner, made of steatite, by J. von Schwarz of Nuremberg, and sold by L. Wiener of London, is shown in Fig. 11. Burners of the Argand type have also been manufactured, but have been unsuccessful. There are, of course, endless modifications of flat-flame burners to be found on the markets, but only a few need be described. A device, which should prove useful where it may be convenient to be able to turn one or more burners up or down from the same common distant spot, has been patented by Forbes. It consists of the usual twin-injector burner fitted with a small central pinhole jet; and inside the casing is a receptacle containing a little mercury, the level of which is moved by the gas pressure by an adaptation of the displacement principle. When the main is carrying full pressure, both of the jets proper are alight, and the burner behaves normally, but if the pressure is reduced to a certain point, the movement of the mercury seals the tubes leading to the main jets, and opens that of the pilot flame, which alone remains alight till the pressure is increased again. Bray has patented a modification of the Naphey injector tip, which is shown in Fig. 12. It will be observed that the four air inlets are at right-angles to the gas-way; but the essential feature of the device is the conical orifice. By this arrangement it is claimed that firing back never occurs, and that the burner can be turned down and left to give a small flame for considerable periods of time without fear of the apertures becoming choked or distorted. As a rule burners of the ordinary type do not well bear being turned down; they should either be run at full power or extinguished completely. The "Elta" burner, made by Geo. Bray and Co., Ltd., which is shown in Fig. 13, is an injector or atmospheric burner which may be turned low without any deposition of carbon occurring on the tips. A burner of simple construction but which cannot be turned low is the "Luta," made by the same firm and shown in Fig. 14. Of the non- atmospheric type the "Sansair," also made by Geo. Bray and Co., Ltd., is extensively used. It is shown in Fig. 15. In order to avoid the warping, through the heat of the flame, of the arms of burners which sometimes occurs when they are made of metal, a number of burners are now made with the arms wholly of steatite. One of the best-known of these, of the injector type, is the "Kona," made by Falk, Stadelmann and Co., of London. It is shown in Fig. 16, fitted with a screw device for adjusting the flow of gas, so that when this adjuster has been set to give a flame of the proper size, no further adjustment by means of the gas-tap is necessary. This saves the trouble of manipulating the tap after the gas is lighted. The same adjusting device may also be had fitted to the Phos burner (Fig. 9) or to the "Orka" burner (Fig. 17), which is a steatite- tip injector burner with metal arms made by Falk, Stadelmann and Co., Ltd. A burner with steatite arms, made by J. von Schwarz of Nuremberg, and sold in this country by L. Wiener of London, is shown in Fig. 18.
[Illustration: FIG. 17.--"ORKA" BURNER.]
[Illustration: FIG. 18.--"SUPREMA" NO. 216469 BURNER.]
ILLUMINATING DUTY.--The illuminating value of ordinary self-luminous acetylene burners in different sizes has been examined by various photometrists. For burners of the Naphey type Lewes gives the following table:
___________________________________________________________ Gas Candles Burner. Pressure, Consumed, Light in per Inches Cubic Feet Candles. Cubic Foot. per Hour. _________ ___________ ____________ __________ _____________ No. 6 2.0 0.155 0.794 5.3 " 8 2.0 0.27 3.2 11.6 " 15 2.0 0.40 8.0 20.0 " 25 2.0 0.65 17.0 26.6 " 30 2.0 0.70 23.0 32.85 " 42 2.0 1.00 34.0 34.0 _________ ___________ ____________ __________ _____________
From burners of the Billwiller type Lewes obtained in 1899 the values:
___________________________________________________________ Gas Candles Burner. Pressure, Consumed, Light in per Inches Cubic Feet Candles. Cubic Foot. per Hour. _________ ___________ ____________ __________ _____________ No. 1 2.0 0.5 7.0 11.0 " 2 2.0 0.75 21.0 32.0 " 3 2.0 0.75 28.0 37.3 " 4 3.0 1.2 48.0 40.0 " 5 3.5 2.0 76.0 38.0 _________ ___________ ____________ __________ _____________
Neuberg gives these figures for different burners (1900) as supplied by Pintsch:
______________________________________________________________________ Gas Candles "w Burner. Pressure, Consumed, Light in per Inches Cubic Feet Candles. Cubic Foot. per Hour. ____________________ ___________ ____________ __________ _____________ No. 0, slit burner 3.9 1.59 59.2 37.3 " 00000 fishtail 1.6 0.81 31.2 38.5 Twin burner No. 1 3.2 0.32 13.1 40.8 " " " 2 3.2 0.53 21.9 41.3 " " " 3 3.2 0.74 31.0 41.9 " " " 4 3.2 0.95 39.8 41.9 ____________________ ___________ ____________ __________ _____________
The actual candle-power developed by each burner was not quoted by Neuberg, and has accordingly been calculated from his efficiency values.
It is noteworthy, and in opposition to what has been found by other investigators as well as to strict theory, that Neuberg represents the efficiencies to be almost identical in all sizes of the same description of burner, irrespective of the rate at which it consumes gas.
Writing in 1902, Capelle gave for Stadelmann's twin injector burners the following figures; but as he examined each burner at several different pressures, the values recorded in the second, third, and fourth columns are maxima, showing the highest candle-power which could be procured from each burner when the pressure was adjusted so as to cause consumption to proceed at the most economical rate. The efficiency values in the fifth column, however, are the mean values calculated so as to include all the data referring to each burner. Capelle's results have been reproduced from the original on the basis that 1 _bougie decimale_ equals 0.98 standard English candle, which is the value he himself ascribes to it (1 _bougie decimale_ equals 1.02 candles is the value now accepted).
_____________________________________________________________________ Nominal Best Actual Consumption Maximum Average Consumption, Pressure at Stated Pressure. Light in Candles per Litres. Inches. Cubic Feet per Hour. Candles. Cubic Foot. _____________ _________ _____________________ __________ ____________ 10 3.5 0.40 8.4 21.1 15 2.8 0.46 16.6 33.3 20 3.9 0.64 25.1 40.0 25 3.5 0.84 37.8 46.1 30 3.5 0.97 48.2 49.4 _____________ _________ _____________________ __________ ____________
Some testings of various self-luminous burners of which the results were reported by R. Granjon in 1907, gave the following results for the duty of each burner, when the pressure was regulated for each burner to that which afforded the maximum illuminating duty. The duty in the original paper is given in litres per Carcel-hour. The candle has been taken as equal to 0.102 Carcel for the conversion to candles per cubic foot.
___________________________________________________________________ Nominal Best Duty. Candles Burner. Consumption. Pressure. per cubic foot. _______________________ _____________ __________ _________________ Litres. Inches. Twin . . . . 10 2.76 21.2 " . . . . 20 2.76 23.5 " . . . . 25 3.94 30.2 " . . . . 30 3.94-4.33 44.8 ", (pair of flames) 35 3.55-3.94 45.6 Bray's "Manchester" 6 1.97 18.8 " 20 1.97 35.6 " 40 2.36 42.1 Rat-tail . . . 5 5.5 21.9 " . . . 8 4.73 25.0 Slit or batswing . 30 1.97-2.36 37.0 _______________________ _____________ ___________ _________________
Granjon has concluded from his investigations that the Manchester or fish-tail burners are economical when they consume 0.7 cubic foot per hour and when the pressure is between 2 and 2.4 inches. When these burners are used at the pressure most suitable for twin burners their consumption is about one-third greater than that of the latter per candle-hour. The 25 to 35 litres-per-hour twin burners should be used at a pressure higher by about 1 inch than the 10 to 20 litres-per-hour twin burners.
At the present time, when the average burner has a smaller hourly consumption than 1 foot per hour, it is customary in Germany to quote the mean illuminating value of acetylene in self-luminous burners as being 1 Hefner unit per 0.70 litre, which, taking
1 Hefner unit = 0.913 English candle
1 English candle = 1.095 Hefner units,
works out to an efficiency of 37 candles per foot in burners probably consuming between 0.5 and 0.7 foot per hour.
Even when allowance is made for the difficulties in determining illuminating power, especially when different photometers, different standards of light, and different observers are concerned, it will be seen that these results are too irregular to be altogether trustworthy, and that much more work must be done on this subject before the economy of the acetylene flame can be appraised with exactitude. However, as certain fixed data are necessary, the authors have studied those and other determinations, rejecting some extreme figures, and averaging the remainder; whence it appears that on an average twin-injector burners of different sizes should yield light somewhat as follows:
_______________________________________________________ Size of Burner in Candle-power Candles Cubic Feet per Hour. Developed. per Cubic Foot. ______________________ ______________ _________________ 0.5 18.0 35.9 0.7 27.0 38.5 1.0 45.6 45.6 ______________________ ______________ _________________
In the tabular statement in Chapter I. the 0.7-foot burner was taken as the standard, because, considering all things, it seems the best, to adopt for domestic purposes. The 1-foot burner is more economical when in the best condition, but requires a higher gas pressure, and is rather too powerful a unit light for good illuminating effect; the 0.5 burner naturally gives a better illuminating effect, but its economy is surpassed by the 0.7-foot burner, which is not too powerful for the human eye.
For convenience of comparison, the illuminating powers and duties of the 0.5- and 0.7-foot acetylene burners may be given in different ways:
ILLUMINATING POWER OF SELF-LUMINOUS ACETYLENE.
_0.7-foot Burner._ _Half-foot Burner._ 1 litre = 1.36 candles. 1 litre = 1.27 candles.
1 cubic foot = 38.5 candles. 1 cubic foot = 35.9 candles.
1 candle = 0.736 litre. 1 candle = 0.79 litre.
1 candle = 0.026 cubic foot. 1 candle = 0.028 cubic foot.
If the two streams of gas impinge at an angle of 90, twin-injector burners for acetylene appear to work best when the gas enters them at a pressure of 2 to 2.5 inches; for a higher pressure the angle should be made a little acute. Large burners require to have a wider distance between the jets, to be supplied with acetylene at a higher pressure, and to be constructed with a smaller angle of impingement. Every burner, of whatever construction and size, must always be supplied with gas at its proper pressure; a pressure varying from time to time is fatal.
It is worth observing that although injector burners are satisfactory in practice, and are in fact almost the only jets yet found to give prolonged satisfaction, the method of injecting air below the point of combustion in a self-luminous burner is in some respects wrong in principle. If acetylene can be consumed without polymerisation in burners of the simple fish-tail or bat's-wing type, it should show a higher illuminating efficiency. In 1902 Javal stated that it was possible to burn thoroughly purified acetylene in twin non-injector burners, provided the two jets, made of steatite as usual, were arranged horizontally instead of obliquely, the two streams of gas then meeting at an angle of 180, so as to yield an almost circular flame. According to Javal, whereas carbonaceous growths were always produced in non-injector acetylene burners with either oblique or horizontal jets, in the former case the growths eventually distorted the gas orifices, but in the latter the carbon was deposited in the form of a tube, and fell off from the burner by its own weight directly it had grown to a length of 1.2 or 1.5 millimetres, leaving the jets perfectly clear and smooth. Javal has had such a burner running for 10 or 12 hours per day for a total of 2071 hours; it did not need cleaning out on any occasion, and its consumption at the end of the period was the same as at first. He found that it was necessary that the tips should be of steatite, and not of metal or glass; that the orifices should be drilled in a flat surface rather than at the apex of a cone, and that the acetylene should be purified to the utmost possible extent. Subsequent experience has demonstrated the possibility of constructing non-injector burners such as that shown in Fig. 13, which behave satisfactorily even though the jets are oblique. But with such burners trouble will inevitably ensue unless the gas is always purified to a high degree and is tolerably dry and well filtered. Non-injector burners should not be used unless special care is taken to insure that the installation is consistently operated in an efficient manner in these respects.
GLOBES, &C.--It does not fall within the province of the present volume to treat at length of chimneys, globes, or the various glassware which may be placed round a source of light to modify its appearance. It should be remarked, however, that obedience to two rules is necessary for complete satisfaction in all forms of artificial illumination. First, no light much stronger in intensity than a single candle ought ever to be placed in such a position in an occupied room that its direct rays can reach the eye, or the vision will be temporarily, and may be permanently, injured. Secondly, unless economy is to be wholly ignored, no coloured or tinted globe or shade should ever be put round a source of artificial light. The best material for the construction of globes is that which possesses the maximum of translucency coupled with non-transparency, _i.e._, a material which passes the highest proportion of the light falling upon it, and yet disperses that light in such different directions that the glowing body cannot be seen through the globe. Very roughly speaking, plain white glass, such as that of which the chimneys of oil-lamps and incandescent gas-burners are composed, is quite transparent, and therefore affords no protection to the eyesight; a protective globe should be rather of ground or opal glass, or of plain glass to which a dispersive effect has been given by forming small prisms on its inner or outer surface, or both. Such opal, ground, or dispersive shades waste much light in terms of illuminating power, but waste comparatively little in illuminating effect well designed, they may actually increase the illuminating effect in certain positions; a tinted globe, even if quite plain in figure, wastes both illuminating power and effect, and is only to be tolerated for so-believed aesthetic reasons.
Naturally no globe must be of such figure, or so narrow at either orifice, as to distort the shape of the unshaded acetylene flame--it is hardly necessary to say this now, but some years ago coal-gas globes were constructed with an apparent total disregard of this fundamental point.
CHAPTER IX
INCANDESCENT BURNERS--HEATING APPARATUS--MOTORS--AUTOGENOUS SOLDERING
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.