Acetylene, the Principles of Its Generation and Use Part 17

In Chapter VI. are given the explosive limits of acetylene-air mixtures as influenced by the diameter of the tube containing them. If we possessed a similar table showing the speed of the explosive wave in mixtures of known composition, the foregoing formulae would enable us to calculate the minimum speed which would insure absence of explosibility in a supply-pipe of any given diameter throughout its length, or at its narrowest part. It would not, however, be possible simply by increasing the forward speed of an explosive mixture of acetylene and air to a point exceeding that of its explosion velocity to prevent all danger of firing back in an atmospheric burner tube. A much higher pressure than is usually employed in gas-burners, other than blowpipes, would be needed to confer a sufficient degree of velocity upon the gas, a pressure which would probably fracture any incandescent mantle placed in the flame.

SERVICE-PIPES AND MAINS.--The pipes used for the distribution of acetylene must be sound in themselves, and their joints perfectly tight.

Higher pressures generally prevail in acetylene service-pipes within a house than in coal-gas service-pipes, while slight leaks are more offensive and entail a greater waste of resources. Therefore it is uneconomical, as well as otherwise objectionable, to employ service-pipes or fittings for acetylene which are in the least degree unsound.

Unfortunately ordinary gas-barrel is none too sound, nor well-threaded, and the taps and joints of ordinary gas-fittings are commonly leaky.

Hence something better should invariably be used for acetylene. What is known as "water" barrel, which is one gauge heavier than gas-barrel of the same size, may be adopted for the service-pipes, but it is better to incur a slight extra initial expense and to use "steam" barrel, which is of still heavier gauge and is sounder than either gas or water-pipe. All elbows, tees, &c., should be of the same quality. The fitters' work in making the joints should be done with the utmost care, and the sloppy work often pa.s.sed in the case of coal-gas services must on no account be allowed. It is no exaggeration to say that the success of an acetylene installation, from the consumer's point of view, will largely, if not princ.i.p.ally, depend on the tightness of the pipes in his house. The statement has been made that the "paint" used by gas-fitters, _i.e._, the mixture of red and white lead ground in "linseed" oil, is not suitable for employment with acetylene, and it has been proposed to adopt a similar material in which the vehicle is castor-oil. No good reason has been given for the preference for castor-oil, and the troubles which have arisen after using ordinary paint may be explained partly on the very probable a.s.sumption that the oil was not genuine linseed, and so did not dry, and partly on the fact that almost entire reliance was placed on the paint for keeping the joint sound. Joints for acetylene, like those for steam and high-pressure water, must be made tight by using well-threaded fittings, so as to secure metallic contact between pipe and socket, &c.; the paint or spun-yarn is only an additional safeguard. In making a faced joint, washers of (say, 7 lb) lead, or coils of lead-wire arc extremely convenient and quite trustworthy; the packing can be used repeatedly.

LEAKAGE.--Broadly speaking, it may be said that the commercial success of any village acetylene-supply--if not that of all large installations-- depends upon the leakage being kept within moderate limits. It follows from what was stated in Chapter VI. about the diffusion of acetylene, that from pipes of equal porosity acetylene and coal-gas will escape at equal rates when the effective pressure in the pipe containing acetylene is double that in the pipe containing coal-gas. The loss of coal-gas by leakage is seldom less than 5 per cent. of the volume pa.s.sed into the main at the works; and provided a village main delivering acetylene is not unduly long in proportion to the consumption of gas--or, in other words, provided the district through which an acetylene distributing main pa.s.ses is not too spa.r.s.ely populated--the loss of acetylene should not exceed the same figure. Caro holds that the loss of gas by leakage from a village installation should be quoted in absolute figures and not as a percentage of the total make as indicated by the works meter, because that total make varies so largely at different periods of the year, while the factors which determine the magnitude of the leakage are always identical; and therefore whereas the actual loss of gas remains the same, it is represented to be more serious in the summer than in the winter.

Such argument is perfectly sound, but the method of returning leakage as a percentage of the make has been employed in the coal-gas industry for many years, and as it does not appear to have led to any misunderstanding or inconvenience, there is no particular reason for departing from the usual practice in the case of acetylene where the conditions as to uniform leakage and irregular make are strictly a.n.a.logous.

Caro has stated that a loss of 15 to 20 litres per kilometre per hour (_i.e._, of 0.85 to 1.14 cubic feet per mile per hour) from an acetylene distributing main is good practice; but it should be noted that much lower figures have been obtained when conditions are favourable and when due attention has been devoted to the fitters' work. In one of the German village acetylene installations where the matter has been carefully investigated (Dose, near Cuxhaven), leakage originally occurred at the rate of 7.3 litres per kilometre per hour in a main 8.5 kilometres, or 5.3 miles, long and 4 to 2 inches in diameter; but it was reduced to 5.2 litres, and then to 3.12 litres by tightening the plugs of the street lantern and other gas c.o.c.ks. In British units, these figures are 0.415, 0.295, and 0.177 cubic foot per mile per hour. By calculation, the volume of acetylene generated in this village would appear to have been about 23,000 cubic feet per mile of main per year, and therefore it may be said that the proportion of gas lost was reduced by attending to the c.o.c.ks from 15.7 per cent, to 11.3 per cent, and then to 6.8 per cent.

At another village where the main was 2.5 kilometres long, tests extending over two months, when the public lamps were not in use, showed the leakage to be 4.4 litres per kilometre per hour, _i.e._, 1.25 cubic foot per mile per hour, when the annual make was roughly 46,000 cubic feet per mile of main. Here, the loss, calculated from the direct readings of the works motor, was 4.65 per cent.

When all the fittings, burners excepted, have been connected, the whole system of pipes must be tested by putting it under a gas (or air) pressure of 9 or 12 inches of water, and observing on an attached pressure gauge whether any fall in pressure occurs within fifteen minutes after the main inlet tap has been shut. The pressure required for this purpose can be obtained by temporarily weighting the holder, or by the employment of a pump. If the gauge shows a fall of pressure of one quarter of an inch or more in these circ.u.mstances, the pipes must be examined until the leak is located. In the presence of a meter, the installation can conveniently be tested for soundness by throwing into it, through the meter, a pressure of 12 inches or so of water from the weighted holder, then leaving the inlet c.o.c.k open, and observing whether the index hand on the lowest dial remains perfectly stationary for a quarter of an hour--movement of the linger again indicating a leak. The search for leaks must never be made with a light; if the pipes are full of air this is useless, if full of gas, criminal in its stupidity. While the whole installation is still under a pressure of 12 inches thrown from the loaded holder, whether it contains air or gas, first all the likely spots (joints, &c.), then the entire length of pipe is carefully brushed over with strong soapy water, which will produce a conspicuous "soap- bubble" wherever the smallest flaw occurs. The tightness of a system of pipes put under pressure from a loaded holder cannot be ascertained safely by observing the height of the bell, and noting if it falls on standing. Even if there is no issue of gas from the holder, the position of the bell will alter with every variation in temperature of the stored gas or surrounding air, and with every movement of the barometer, rising as the temperature rises and as the barometer falls, and _vice versa_, while, unless the water in the seal is saturated with whatever gas the holder contains, the bell will steadily drop a little an part of its contents are lost by dissolution in the liquid.

PIPES AND FITTINGS.--As a general rule it is unadvisable to use lead or composition pipe for permanent acetylene connexions. If exposed, it is liable to be damaged, and perhaps penetrated by a blow, and if set in the wall and covered with paper or panel it is liable to be pierced if nails or tacks should at any time be driven into the wall. There is also an increased risk in case of fire, owing to its ready fusibility. If used at all--and it has obvious advantages--lead or composition piping should be laid on the surface of the walls, &c., and protected from blows, &c., by a light wooden casing, outwardly resembling the wooden coverings for electric lighting wires. It has been a common practice, in laying the underground mains required for supplying the villages which are lighted by means of acetylene from a central works in different parts of France, to employ lead pipes. The plan is economical, but in view of the danger that the main might be flattened by the weight of heavy traction-engines pa.s.sing over the roads, or that it might settle into local dips from the same cause or from the action of subterranean water, in which dips water would be constantly condensing in cold weather, the use of lead for this purpose cannot be recommended. Steam-barrel would be preferable to cast pipe, because permanently sound joints are easier to make in the former, and because it is not so brittle.

The fittings used for acetylene must have perfectly sound joints and taps, for the same reasons that the service-pipes must be quite sound.

Common gas-fittings will not do, the joints, taps, ball-sockets, &c., are not accurately enough ground to prevent leakage. They may in many cases be improved by regrinding, but often the plug and barrel are so shallow that it is almost impossible to ensure soundness. It is therefore better to procure fittings having good taps and joints in the first instance; the barrels should be long, fairly wide, and there should be no sensible "play" between plug and barrel when adjusted so that the plug turns easily when lightly lubricated. Fittings are now being specially made for acetylene, which is a step in the right direction, because, in addition to superior taps and joints being essential, smaller bore piping and smaller through-ways to the taps than are required for coal-gas serve for acetylene. It is perhaps advisable to add that wherever a rigid bracket or fitting will answer as well as a jointed one, the latter should on no account be used; also water-slide pendants should never be employed, as they are fruitful of accidents, and their apparent advantages are for the most part illusory. Ball-sockets also should be avoided if possible; if it is absolutely necessary to have a fitting with a ball-socket, the latter should have a sleeve made of a short length of sound rubber-tubing of a size to give a close fit, slipped over so as to join the ball portion to the socket portion. This sleeve should be inspected once a quarter at least, and renewed immediately it shows signs of cracking.

Generally speaking all the fittings used should be characterised by structural simplicity; any ornamental or decorative effects desired may be secured by proper design without sacrifice of the simplicity which should always mark the essential and operative parts of the fitting.

Flexible connexions between the fixed service-pipe and a semi-portable or temporary burner may at times be required. If the connexion is for permanent use, it must not be of rubber, but of the metallic flexible tubing which is now commonly employed for such connexions in the case of coal-gas. There should be a tap between the service-pipe and the flexible connexion, and this tap should be turned off whenever the burner is out of use, so that the connexion is not at other times under the pressure which is maintained in the service-pipes. Unless the connexion is very short--say 2 feet or less--there should also be a tap at the burner.

These flexible connexions, though serviceable in the case of table-lamps, &c., of which the position may have to be altered, are undesirable, as they increase the risk attendant on gas (whether acetylene or other illuminating gas) lighting, and should, if possible, be avoided. Flexible connexions may also be required for temporary use, such as for conveying acetylene to an optical lantern, and if only occasionally called for, the cost of the metallic flexible tubing will usually preclude its use. It will generally be found, however, that the whole connexion in such a case can be of composition or lead gas-piping, connected up at its two ends by a few inches of flexible rubber tubing. It should be carried along the walls or over the heads of people who may use the room, rather than across the floor, or at a low level, and the acetylene should be turned on to it only when actually required for use, and turned off at the fixed service-pipe as soon as no longer required. Quite narrow composition tubing, say 1/4-inch, will carry all the acetylene required for two or three burners. The cost of a composition temporary connexion will usually be less than one of even common rubber tubing, and it will be safer. The composition tubing must not, of course, be sharply bent, but carried by easy curves to the desired point, and it should be carefully rolled in a roll of not less than 18 inches diameter when removed. If these precautions are observed it may be used very many times.

Acetylene service-pipes should, wherever possible, be laid with a fall, which may be very slight, towards a small closed vessel adjoining the gasholder or purifier, in order that any water deposited from the gas owing to condensation of aqueous vapour may run out of the pipe into that apparatus. Where it is impossible to secure an uninterrupted fall in that direction, there should be inserted in the service-pipe, at the lowest point of each dip it makes, a short length of pipe turned downwards and terminating in a plug or sound tap. Water condensing in this section of the service-pipe will then run down and collect in this drainage-pipe, from which it can be withdrawn at intervals by opening the plug or tap for a moment. The condensed water is thus removed from the service-pipe, and does not obstruct its through-way. Similar drainage devices may be used at the lowest points of all dips in mains, though there are special seal-pots which take the place of the c.o.c.k or plug used to seal the end of the drainage-pipe. Such seal-pots or "syphons" are commonly used on ordinary gas-distributing systems, and might be applied in the case of large acetylene installations, as they offer facilities for removing the condensed water from time to time in a convenient and expeditious manner.

EXPULSION OF AIR FROM MAINS.--After a service-pipe system has been proved to be sound, it is necessary to expel the air from it before acetylene can be admitted to it with a view to consumption. Unless the system is a very large one, the expulsion of air is most conveniently effected by forcing from the gasholder preliminary batches of acetylene through the pipes, while lights are kept away from the vicinity. This precaution is necessary because, while the acetylene is displacing the air in the pipes, they will for some time contain a mixture of air and acetylene in proportions which fall within the explosive limits of such a mixture. If the escaping acetylene caught fire from any adjacent light under these conditions, a most disastrous explosion would ensue and extend through all the ramifications of the system of pipes. Therefore the first step when a new system of pipes has to be cleared of air is to see that there are no lights in or about the house--either fires, lamps, cigars or pipes, candles or other flames. Obviously this work must be done in the daytime and finished before nightfall. Burners are removed from two or more brackets at the farthest points in the system from the gasholder, and flexible connexions are temporarily attached to them, and led through a window or door into the open air well clear of the house. One of the brackets selected should as a rule be the lowest point supplied in the house. The gasholder having been previously filled with acetylene, the tap or taps on the pipe leading to the house are turned on, and the acetylene is pa.s.sed under slight pressure into the system of pipes, and escapes through the aforesaid brackets, of which the taps have been turned on, into the open. The taps of all other brackets are kept closed.

The gas should be allowed to flow thus through the pipes until about five times the maximum quant.i.ty which all the burners on the system would consume in an hour has escaped from the open brackets. The taps on these brackets are then closed, and the burners replaced. Flexible tubing is then connected in place of the burners to all the other brackets in the house, and acetylene is similarly allowed to escape into the open air from each for a quarter of an hour. All taps are then closed, and the burners replaced; all windows in the house are left open wide for half an hour to allow of the dissipation of any acetylene which may have acc.u.mulated in any part of it, and then, while full pressure from the gasholder is maintained, a tap is turned on and the gas lighted. If it burns with a good, fully luminous flame it may be concluded that the system of pipes is virtually free from air, and the installation may be used forthwith as required. If, however, the flame is very feebly luminous, or if the escaping gas does not light, lights must be extinguished, and the pipes again blown through with acetylene into the open air. The burner must invariably be in position when a light is applied, because, in the event of the pipes still containing an explosive mixture, ignition would not be communicated through the small orifices of the burner to the mixture in the pipes, and the application of the light would not entail any danger of an explosion.

Gasfitters familiar with coal-gas should remember, when putting a system of acetylene pipes into use for the first time, that the range over which mixtures of acetylene and air are explosive is wider than that over which mixtures of coal-gas and air are explosive, and that greater care is therefore necessary in getting the pipes and rooms free from a dangerous mixture.

The mains for very large installations of acetylene--_e.g._, for lighting a small town--may advisedly be freed from air by some other plan than simple expulsion of the air by acetylene, both from the point of view of economy and of safety. If the chimney gases from a neighbouring furnace are found on examination to contain not more than about 8 per cent of oxygen, they may be drawn into the gasholder and forced through the pipes before acetylene is admitted to them. The high proportion of carbon dioxide and the low proportion of oxygen in chimney gases makes a mixture of acetylene and chimney gases non-explosive in any proportions, and hence if the air is first wholly or to a large extent expelled from a pipe, main, or apparatus, by means of chimney gases, acetylene may be admitted, and a much shorter time allowed for the expulsion by it of the contents of the pipe, before a light is applied at the burners, &c. This plan, however, will usually only be adopted in the case of very large pipes, &c.; but on a smaller scale the air may be swept out of a distributing system by bringing it into connexion with a cylinder of compressed or liquefied carbon dioxide, the pressure in which will drive the gas to any spot where an outlet is provided. As these cylinders of "carbonic acid" are in common employment for preparing aerated waters and for "lifting" beer, &c., they are easy to hire and use.

TABLE (B).

Giving the Sizes of Pipe which should be used in practice for Acetylene when the fall of pressure in the Pipe is not to exceed 0.1 inch. (Based on Morel's formula.)

_________________________________________________________ | | | | Cubic Feet of | Diameters of Pipe to be used up to | | Acetylene | the lengths indicated. | | which the Pipe |_______________________________________| | is required to | | | | | | | pa.s.s in | 1/4 | 3/8 | 1/2 | 3/4 | 1 | | One Hour. | inch. | inch. | inch. | inch. | inch. | |________________|_______|_______|_______|_______|_______| | | | | | | | | | Feet. | Feet. | Feet. | Feet. | Feet. | | 1 | 520 | 3960 | 16700 | ... | ... | | 2 | 130 | 990 | 4170 | ... | ... | | 3 | 58 | 440 | 1850 | ... | ... | | 4 | 32 | 240 | 1040 | ... | ... | | 5 | 21 | 150 | 660 | 5070 | ... | | 6 | 14 | 110 | 460 | 3520 | ... | | 7 | 10 | 80 | 340 | 2590 | ... | | 8 | ... | 62 | 260 | 1980 | ... | | 9 | ... | 49 | 200 | 1560 | ... | | 10 | ... | 39 | 160 | 1270 | 5340 | | 15 | ... | 17 | 74 | 560 | 2370 | | 20 | ... | 10 | 41 | 310 | 1330 | | 25 | ... | ... | 26 | 200 | 850 | | 30 | ... | ... | 18 | 140 | 590 | | 35 | ... | ... | 13 | 100 | 430 | | 40 | ... | ... | 10 | 79 | 330 | | 45 | ... | ... | ... | 62 | 260 | | 50 | ... | ... | ... | 50 | 210 | |________________|_______|_______|_______|_______|_______|

TABLE (A).

Showing the Quant.i.ties [Q] (in cubic feet) of Acetylene which will pa.s.s in One Hour through Pipes of various diameters (in inches) under different Falls of Pressure. (Based on Morel's formula.)

____________________________________________________________________ | | | | | | | | | | | | | | Diameter | | | | | | | | | | | | | of Pipe | 1/4| 3/8| 1/2| 3/4 | 1 | 1 | 1 | 1 | 2 | 2 | 3 | | [_d_] = | | | | | | 1/4 | 1/2| 3/4| | 1/2| | | inches | | | | | | | | | | | | |__________|____|____|____|_____|_____|_____|____|____|____|____|____| | | | | Length | | | of Pipe | | | [_l_] = | Fall of Pressure in the Pipe [_h_] = 0.10 inch. | | Feet | | |__________|_________________________________________________________| | | | | | | | | | | | | | | 10 | 7.2|19.9|40.8|112 |230 |405 | 635| 935|1305|2285|3600| | 25 | 4.5|12.6|25.8| 71.2|146 |255 | 400| 590| 825|1445|2280| | 50 | 3.2| 8.9|18.3| 50.3|103 |180 | 285| 420| 585|1020|1610| | 100 | 2.3| 6.3|12.9| 35.6| 73.1|127 | 200| 295| 410| 720|1140| | 200 | 1.6| 4.4| 9.1| 25.2| 51.7| 90.3| 142| 210| 290| 510| 805| | 300 | 1.3| 3.6| 7.4| 20.5| 42.2| 73.7| 116| 171| 240| 415| 655| | 400 | 1.1| 3.1| 6.4| 17.8| 36.5| 63.8| 100| 148| 205| 360| 570| | 500 | 1.0| 2.8| 5.8| 15.9| 32.7| 57.1| 90| 132| 185| 320| 510| |__________|____|____|____|_____|_____|_____|____|____|____|____|____| | | | | Length | | | of Pipe | | | [_l_] = | Fall of Pressure in the Pipe [_h_] = 0.25 inch. | | Feet | | |__________|_________________________________________________________| | | | | | | | | | | | | | | 25 | 7.2|19.9|40.8|112 |230 |405 | 635| 935|1305|2285|3600| | 50 | 5.1|14.1|28.9| 79.6|163 |285 | 450| 660| 925|1615|2550| | 100 | 3.6| 9.9|20.4| 56.3|115 |200 | 320| 470| 655|1140|1800| | 250 | 2.3| 6.3|12.9| 35.6| 73.1|127 | 200| 295| 410| 720|1140| | 500 | 1.6| 4.4| 9.1| 25.2| 51.7| 90.3| 142| 210| 290| 510| 805| | 1000 | 1.1| 3.1| 6.4| 17.8| 36.5| 63.8| 100| 148| 205| 360| 570| |__________|____|____|____|_____|_____|_____|____|____|____|____|____| | | | | Length | | | of Pipe | | | [_l_] = | Fall of Pressure in the Pipe [_h_] = 0.50 inch. | | Feet | | |__________|_________________________________________________________| | | | | | | | | | | | | | | 25 |10.2|28.1|57.8|159 |325 |570 | 900|1325|1850|3230|5095| | 50 | 7.2|19.9|40.8|112 |230 |405 | 635| 935|1305|2285|3600| | 100 | 5.1|14.1|28.9| 79.6|163 |285 | 450| 660| 925|1615|2550| | 250 | 3.2| 8.9|18.3| 50.3|103 |180 | 285| 420| 585|1020|1610| | 500 | 2.3| 6.3|12.9| 35.6| 73.1|127 | 200| 295| 410| 720|1140| | 1000 | 1.6| 4.4| 9.1| 25.2| 51.7| 90.3| 142| 210| 290| 510| 805| |__________|____|____|____|_____|_____|_____|____|____|____|____|____| | | | | Length | | | of Pipe | | | [_l_] = | Fall of Pressure in the Pipe [_h_] = 0.75 inch. | | Feet | | |__________|_________________________________________________________| | | | | | | | | | | | | | | 50 | 8.8|24.4|50.0|138 |280 |495 | 780|1145|1160|2800|4410| | 100 | 6.2|17.2|35.4| 97.5|200 |350 | 550| 810|1130|1980|3120| | 250 | 3.9|10.9|22.4| 61.7|126 |220 | 350| 510| 715|1250|1975| | 500 | 2.8| 7.7|15.8| 43.6| 89.5|156 | 245| 360| 505| 885|1395| | 1000 | 2.0| 5.4|11.2| 30.8| 63.3|110 | 174| 255| 360| 625| 985| | 2000 | 1.4| 3.8| 7.9| 21.8| 44.8| 78.2| 123| 181| 250| 440| 695| |__________|____|____|____|_____|_____|_____|____|____|____|____|____| | | | | Length | | | of Pipe | | | [_l_] = | Fall of Pressure in the Pipe [_h_] = 1.0 inch. | | Feet | | |__________|_________________________________________________________| | | | | | | | | | | | | | | 100 | 7.2|19.9|40.8|112 |230 |405 | 635| 935|1305|2285|3600| | 250 | 4.5|12.6|25.8| 71.2|146 |255 | 400| 590| 825|1445|2280| | 500 | 3.2| 8.9|18.3| 50.3|103 |180 | 285| 420| 585|1020|1610| | 1000 | 2.3| 6.3|12.9| 35.6| 73.1|127 | 200| 295| 410| 720|1140| | 2000 | 1.6| 4.4| 9.1| 25.2| 51.7| 90.3| 142| 210| 290| 510| 805| | 3000 | 1.3| 3.6| 7.4| 20.5| 42.2| 73.7| 116| 171| 240| 415| 655| |__________|_________________________________________________________| | | | | Length | | | of Pipe | | | [_l_] = | Fall of Pressure in the Pipe [_h_] = 1.5 inch. | | Feet | | |__________|_________________________________________________________| | | | | | | | | | | | | | | 250 | 5.6|15.4|31.6| 87.2|179 |310 | 495| 725|1010|1770|2790| | 500 | 3.9|10.9|22.4| 61.7|126 |220 | 350| 510| 715|1250|1975| | 1000 | 2.8| 7.7|15.8| 43.6| 89.5|156 | 245| 360| 505| 885|1395| | 2000 | 2.0| 5.4|11.2| 30.8| 63.3|110 | 174| 255| 360| 625| 985| | 3000 | 1.6| 4.4| 9.1| 25.2| 51.7| 90.3| 142| 210| 290| 510| 805| | 4000 | 1.4| 3.8| 7.9| 21.8| 44.8| 78.2| 123| 181| 250| 440| 695| |__________|____|____|____|_____|_____|_____|____|____|____|____|____| | | | | Length | | | of Pipe | | | [_l_] = | Fall of Pressure in the Pipe [_h_] = 2.0 inches. | | Feet | | |__________|_________________________________________________________| | | | | | | | | | | | | | | 500 | 4.5|12.6|25.8| 71.2|146 |255 | 400| 590| 825|1445|2280| | 1000 | 3.2| 8.9|18.3| 50.3|103 |180 | 285| 420| 585|1020|1610| | 2000 | 2.3| 6.3|12.9| 35.6| 73.1|127 | 200| 295| 410| 720|1140| | 3000 | 1.8| 5.1|10.5| 29.1| 59.7|104 | 164| 240| 335| 590| 930| | 4000 | 1.6| 4.4| 9.1| 25.2| 51.7| 90.3| 142| 210| 290| 510| 805| | 5000 | 1.4| 4.0| 8.1| 22.5| 46.2| 80.8| 127| 187| 260| 455| 720| | 6000 | 1.3| 3.6| 7.4| 20.5| 42.2| 73.7| 116| 171| 240| 415| 655| |__________|____|____|____|_____|_____|_____|____|____|____|____|____|

NOTE.--In order not to impart to the above table the appearance of the quant.i.ties having been calculated to a degree of accuracy which has no practical significance, quant.i.ties of less than 5 cubic feet have been ignored when the total quant.i.ty exceeds 200 cubic feet, and fractions of a cubic foot have been included only when the total quant.i.ty is less than 100 cubic feet.

TABLE (C).

Giving the Sizes of Pipe which should be used in practice for Acetylene when the fall of pressure in the Pipe is not to exceed 0.25 inch. (Based on Morel's formula.)

____________________________________________________________________ | | | | Cubic feet | | | of | | | Acetylene | Diameters of Pipe to be used up to the lengths stated.| | which the | | | Pipe is | | | required |_______________________________________________________| | to pa.s.s | | | | | | | | | | in One | 1/4 | 1/2 | 3/4 | 1 | 1-1/4| 1-1/2| 1-3/4| 2 | | Hour | inch.| inch.| inch.| inch.| inch.| inch.| inch.| inch.| |____________|______|______|______|______|______|______|______|______| | | | | | | | | | | | | Feet.| Feet.| Feet.| Feet.| Feet.| Feet.| Feet.| Feet.| | 2-1/2 | 1580 | 6680 | 50750| ... | ... | ... | ... | ... | | 5 | 390 | 1670 | 12690| 53160| ... | ... | ... | ... | | 7-1/2 | 175 | 710 | 5610| 23760| ... | ... | ... | ... | | 10 | 99 | 410 | 3170| 13360| 40790| ... | ... | ... | | 15 | 41 | 185 | 1410| 5940| 18130| 45110| ... | ... | | 20 | 24 | 105 | 790| 3350| 10190| 25370| 54840| ... | | 25 | 26 | 67 | 500| 2130| 6520| 16240| 35100| ... | | 30 | 11 | 46 | 350| 1480| 4530| 11270| 24370| 47520| | 35 | ... | 34 | 260| 1090| 3330| 8280| 17900| 34910| | 40 | ... | 26 | 195| 830| 2550| 6340| 13710| 26730| | 45 | ... | 20 | 155| 660| 2010| 5010| 10830| 21120| | 50 | ... | 16 | 125| 530| 1630| 4060| 8770| 17110| | 60 | ... | 11 | 88| 370| 1130| 2880| 6090| 11880| | 70 | ... | ... | 61| 270| 830| 2070| 4470| 8730| | 80 | ... | ... | 49| 210| 630| 1580| 3420| 6680| | 90 | ... | ... | 39| 165| 500| 1250| 2700| 5280| | 100 | ... | ... | 31| 130| 400| 1010| 2190| 4270| | 150 | ... | ... | 14| 59| 180| 450| 970| 1900| | 200 | ... | ... | ... | 33| 100| 250| 540| 1070| | 250 | ... | ... | ... | 21| 65| 160| 350| 680| | 500 | ... | ... | ... | ... | 16| 40| 87| 170| | 1000 | ... | ... | ... | ... | ... | 10| 22| 42| |____________|______|______|______|______|______|______|______|______|

TABLE (D).

Giving the Sizes of Pipe which should be used in practice for Acetylene Mains when the fall of pressure in the Main is not to exceed 0.5 inch, (Based on Morel's formula.)

____________________________________________________________________ | | | | Cubic feet | | | of | | | Acetylene | Diameters of Pipe to be used up to the lengths stated.| | which the | | | Main is | | | required |_______________________________________________________| | to pa.s.s | | | | | | | | | | in One | 3/4 | 1 | 1-1/4| 1-1/2| 1-3/4| 2 | 2-1/2| 3 | | Hour | inch.| inch.| inch.| inch.| inch.| inch.| inch.| inch.| |____________|______|______|______|______|______|______|______|______| | | | | | | | | | | | |Miles.|Miles.|Miles.|Miles.|Miles.|Miles.|Miles.|Miles.| | 10 | 5.05 | ... | ... | ... | ... | ... | ... | ... | | 25 | 0.80 | 2.45 | 6.15 | ... | ... | ... | ... | ... | | 50 | 0.20 | 0.60 | 1.50 | 3.30 | 6.45 | ... | ... | ... | | 100 | 0.05 | 0.15 | 0.35 | 0.80 | 1.60 | 4.95 |12.30 | ... | | 200 | ... | 0.04 | 0.09 | 0.20 | 0.40 | 1.20 | 3.05 |12.95 | | 300 | ... | ... | 0.04 | 0.09 | 0.18 | 0.55 | 1.35 | 5.75 | | 400 | ... | ... | ... | 0.05 | 0.10 | 0.30 | 0.75 | 3.25 | | 500 | ... | .. | ... | 0.03 | 0.06 | 0.20 | 0.50 | 2.05 | | 750 | ... | ... | ... | ... | 0.03 | 0.08 | 0.20 | 0.80 | | 1100 | ... | ... | ... | ... | ... | 0.05 | 0.12 | 0.50 | | 1500 | ... | ... | ... | ... | ... | 0.02 | 0.05 | 0.23 | | 2000 | ... | ... | ... | ... | ... | ... | 0.03 | 0.13 | | 2500 | ... | ... | ... | ... | ... | ... | 0.02 | 0.08 | | 5000 | ... | ... | ... | ... | ... | ... | ... | 0.03 | |____________|______|______|______|______|______|______|______|______|

TABLE (E).

Giving the Sizes of Pipe which should be used in practice for Acetylene Mains when the fall of pressure in the Main is not to exceed 1.0 inch.

(Based on Morel's formula.)

__________________________________________________________________ | | | | Cubic feet | | | of | | | Acetylene |Diameters of Pipe to be used up to the lengths stated| | which the | | | Main is | | | required |_____________________________________________________| | to pa.s.s | | | | | | | | | | | in One | 3/4 | 1 |1-1/4|1-1/2|1-3/4| 2 |2-1/2| 3 | 4 | | Hour |inch.|inch.|inch.|inch.|inch.|inch.|inch.|inch.|inch.| |____________|_____|_____|_____|_____|_____|_____|_____|_____|_____| | | | | | | | | | | | | |Miles|Miles|Miles|Mile.|Miles|Miles|Miles|Miles|Miles| | 10 | 2.40|10.13|30.90| ... | ... | ... | ... | ... | ... | | 25 | 0.38| 1.62| 4.94|12.30| ... | ... | ... | ... | ... | | 50 | 0.09| 0.40| 1.23| 3.07| 6.65|12.96| ... | ... | ... | | 100 | 0.02| 0.10| 0.30| 0.77| 1.66| 3.24| 9.88| ... | ... | | 200 | ... | 0.02| 0.07| 0.19| 0.41| 0.81| 2.47| 6.15| ... | | 300 | ... | 0.01| 0.03| 0.08| 0.18| 0.36| 1.09| 2.73|11.52| | 400 | ... | ... | 0.0 | 0.05| 0.10| 0.20| 0.61| 1.53| 6.48| | 500 | ... | ... | 0.0 | 0.03| 0.06| 0.13| 0.39| 0.98| 4.14| | 750 | ... | ... | ... | 0.01| 0.03| 0.05| 0.17| 0.43| 1.84| | 1000 | ... | ... | ... | ... | 0.01| 0.03| 0.10| 0.24| 1.03| | 1500 | ... | ... | ... | ... | ... | 0.01| 0.01| 0.11| 0.46| | 2000 | ... | ... | ... | ... | ... | ... | 0.02| 0.06| 0.26| | 2500 | ... | ... | ... | ... | ... | ... | 0.01| 0.04| 0.16| | 5000 | ... | ... | ... | ... | ... | ... | ... | 0.01| 0.04| |____________|_____|_____|_____|_____|_____|_____|_____|_____|_____|

CHAPTER VIII

COMBUSTION OF ACETYLENE IN LUMINOUS BURNERS--THEIR DISPOSITION

NATURE OF LUMINOUS FLAMES.--When referring to methods of obtaining artificial light by means of processes involving combustion or oxidation, the term "incandescence" is usually limited to those forms of burner in which some extraneous substance, such as a "mantle," is raised to a brilliant white heat. Though convenient, the phrase is a mere convention, for all artificial illuminants, even including the electric light, which exhibit a useful degree of intensity depend on the same principle of incandescence. Adopting the convention, however, an incandescent burner is one in which the fuel burns with a non-luminous or atmospheric flame, the light being produced by causing that flame to play upon some extraneous refractory body having the property of emitting much light when it is raised to a sufficiently high temperature; while a luminous burner is one in which the fuel is allowed to combine with atmospheric oxygen in such a way that one or more of the const.i.tuents in the gas evolves light as it suffers combustion. From the strictly chemical point of view the light-giving substance in the incandescent flame lasts indefinitely, for it experiences no change except in temperature; whereas the light-giving substance in a luminous flame lasts but for an instant, for it only evolves light during the act of its combination with the oxygen of the atmosphere. Any fluid combustible which burns with a flame can be made to give light on the incandescent system, for all such materials either burn naturally, or can be made to burn with a non- luminous flame, which can be employed to raise the temperature of some mantle; but only those fuels can be burnt on the self-luminous system which contain some ingredient that is liberated in the elemental state in the flame, the said ingredient being one which combines energetically with oxygen so as to liberate much local heat. In practice, just as there are only two or three substances which are suitable for the construction of an incandescent mantle, so there is only one which renders a flame usefully self-luminous, viz., carbon; and therefore only such fuels as contain carbon among their const.i.tuents can be burnt so as to produce light without the a.s.sistance of the mantle. But inasmuch as it is necessary for the evolution of light by the combustion of carbon that that carbon shall be in the free state, only those carbonaceous fuels yield light without the mantle in which the carbonaceous ingredient is dissociated into its elements before it is consumed. For instance, alcohol and carbon monoxide are both combustible, and both contain carbon; but they yield non-luminous flames, for the carbon burns to carbon dioxide in ordinary conditions without a.s.suming the solid form; ether, petroleum, acetylene, and some of the hydrocarbons of coal-gas do emit light on combustion, for part of their carbon is so liberated. The quant.i.ty of light emitted by the glowing substance increases as the temperature of that substance rises: the gain in light being equal to the fifth or higher power of the gain in heat; [Footnote: Calculated from absolute zero.] therefore unnecessary dissipation of heat from a flame is one of the most important matters to be guarded against if that flame is to be an economical illuminant. But the amount of heat liberated when a certain weight (or volume) of a particular fuel combines with a sufficient quant.i.ty of oxygen to oxidise it wholly is absolutely fixed, and is exactly the same whether that fuel is made to give a luminous or a non-luminous flame. Nevertheless the atmospheric flame given by a certain fuel may be appreciably hotter than its luminous flame, because the former is usually smaller than the latter. Unless the luminous flame of a rich fuel is made to expose a wide surface to the air, part of its carbon may escape ultimate combustion; soot or smoke may be produced, and some of the most valuable heat-giving substance will be wasted. But if the flame is made to expose a large surface to the air, it becomes flat or hollow in shape instead of being cylindrical and solid, and therefore in proportion to its cubical capacity it presents to the cold air a larger superficies, from which loss of heat by radiation, &c., occurs. Being larger, too, the heat produced is less concentrated.

It does not fall within the province of the present book to discuss the relative merits of luminous and incandescent lighting; but it may be remarked that acetylene ranks with petroleum against coal-gas, carburetted or non-carburetted water-gas, and semi-water-gas, in showing a comparatively small degree of increased efficiency when burnt under the mantle. Any gas which is essentially composed of carbon monoxide or hydrogen alone (or both together) burns with a non-luminous flame, and can therefore only be used for illuminating purposes on the incandescent system; but, broadly speaking, the higher is the latent illuminating power of the gas itself when burnt in a non-atmospheric burner, the less marked is the superiority, both from the economical and the hygienic aspect, of its incandescent flame. It must be remembered also that only a gas yields a flame when it is burnt; the flame of a paraffin lamp and of a candle is due to the combustion of the vaporised fuel. Methods of burning acetylene under the mantle are discussed in Chapter IX.; here only self-luminous flames are being considered, but the theoretical question of heat economy applies to both processes.

Heat may be lost from a flame in three several ways: by direct radiation and conduction into the surrounding air, among the products of combustion, and by conduction into the body of the burner. Loss of heat by radiation and conduction to the air will be the greater as the flame exposes a larger surface, and as a more rapid current of cold air is brought into proximity with the flame. Loss of heat by conduction, into the burner will be the greater as the material of which the burner is constructed is a better conductor of heat, and as the ma.s.s of material in that burner is larger. Loss of heat by pa.s.sage into the combustion products will also be greater as these products are more voluminous; but the volume of true combustion products from any particular gas is a fixed quant.i.ty, and since these products must leave the flame at the temperature of that flame--where the highest temperature possible is requisite--it would seem that no control can be had over the quant.i.ty of heat so lost. However, although it is not possible in practice to supply a flame with too little air, lest some of its carbon should escape consumption and prove a nuisance, it is very easy without conspicuous inconvenience to supply it with too much; and if the flame is supplied with too much, there is an unnecessary volume of air pa.s.sing through it to dilute the true combustion products, which air absorbs its own proper proportion of heat. It is only the oxygen of the air which a flame needs, and this oxygen is mixed with approximately four times its volume of nitrogen; if, then, only a small excess of oxygen (too little to be noticeable of itself) is admitted to a flame, it is yet harmful, because it brings with it four times its volume of nitrogen, which has to be raised to the same temperature as the oxygen. Moreover, the nitrogen and the excess of oxygen occupy much s.p.a.ce in the flame, making it larger, and distributing that fixed quant.i.ty of heat which it is capable of generating over an unnecessarily large area. It is for this reason that any gas gives so much brighter a light when burnt in pure oxygen than in air, (1) because the flame is smaller and its heat more concentrated, and (2) because part of its heat is not being wasted in raising the temperature of a large ma.s.s of inert nitrogen. Thus, if the flame of a gas which naturally gives a luminous flame is supplied with an excess of air, its illuminating value diminishes; and this is true whether that excess is introduced at the base of the actual flame, or is added to the gas prior to ignition. In fact the method of adding some air to a naturally luminous gas before it arrives at its place of combustion is the principle of the Bunsen burner, used for incandescent lighting and for most forms of warming and cooking stoves. A well-made modern atmospheric burner, however, does not add an excess of air to the flame, as might appear from what has been said; such a burner only adds part of the air before and the remainder of the necessary quant.i.ty after the point of first ignition--the function of the primary supply being merely to insure thorough admixture and to avoid the production of elemental carbon within the flame.

ILLUMINATING POWER.--It is very necessary to observe that, as the combined losses of heat from a flame must be smaller in proportion to the total heat produced by the flame as the flame itself becomes larger, the more powerful and intense any single unit of artificial light is, the more economical does it become, because economy of heat spells economy of light. Conversely, the more powerful and intense any single unit of light is, the more is it liable to injure the eyesight, the deeper and, by contrast, the more impenetrable are the shadows it yields, and the less pleasant and artistic is its effect in an occupied room. For economical reasons, therefore, one large central source of light is best in an apartment, but for physiological and aesthetic reasons a considerable number of correspondingly smaller units are preferable. Even in the street the economical advantage of the single unit is outweighed by the inconvenience of its shadows, and by the superiority of a number of evenly distributed small sources to one central large source of light whenever the natural transmission of light rays through the atmosphere is interfered with by mist or fog. The illuminating power of acetylene is commonly stated to be "240 candles" (though on the same basis Wolff has found it to be about 280 candles). This statement means that when acetylene is consumed in the most advantageous self-luminous burner at the most advantageous rate, that rate (expressed in cubic feet per hour) is to 5 in the same ratio as the intensity of the light evolved (expressed in standard candles) is to the said "illuminating power."

Thus, Wolff found that when acetylene was burnt in the "0000 Bray" fish- tail burner at the rate of 1.377 cubic feet per hour, a light of 77 candle-power was obtained. Hence, putting x to represent the illuminating power of the acetylene in standard candles, we have:

1.377 / 5 = 77 / x hence x = 280.

Therefore acetylene is said to have, according to Wolff, an illuminating power of about 280 candles, or according to other observers, whose results have been commonly quoted, of 240 candles. The same method of calculating the nominal illuminating power of a gas is applied within the United Kingdom in the case of all gases which cannot be advantageously burnt at the rate of 5 cubic feet per hour in the standard burner (usually an Argand). The rate of 5 cubic feet per hour is specified in most Acts of Parliament relating to gas-supply as that at which coal-gas is to be burnt in testings of its illuminating power; and the illuminating power of the gas is defined as the intensity, expressed in standard candles, of the light afforded when the gas is burnt at that rate. In order to make the values found for the light evolved at more advantageous rates of consumption by other descriptions of gas--such as oil-gas or acetylene--comparable with the "illuminating power" of coal- gas as defined above, the values found are corrected in the ratio of the actual rate of consumption to 5 cubic feet per hour.

In this way the illuminating power of 240 candles has been commonly a.s.signed to acetylene, though it would be clearer to those unfamiliar with the definition of illuminating power in the Acts of Parliament which regulate the testing of coal-gas, if the same fact were conveyed by stating that acetylene affords a maximum illuminating power of 48 candles (_i.e._, 240 / 5) per cubic foot. Actually, by misunderstanding of the accepted though arbitrary nomenclature of gas photometry, it has not infrequently been a.s.sorted or implied that a cubic foot of acetylene yields a light of 240 candle-power instead of 48 candle-power. It should, moreover, be remembered that the ideal illuminating power of a gas is the highest realisable in any Argand or flat-flame burner, while the said burner may not be a practicable one for general use in house lighting.

Thus, the burners recommended for general use in lighting by acetylene do not develop a light of 48 candles per cubic foot of gas consumed, but considerably less, as will appear from the data given later in this chapter.

It has been stated that in order to avoid loss of heat from a flame through the burner, that burner should present only a small ma.s.s of material (_i.e._, be as light in weight as possible), and should be constructed of a bad heat-conductor. But if a small ma.s.s of a material very deficient in heat-conducting properties comes in contact with a flame, its temperature rises seriously and may approach that of the base of the flame itself. In the case of coal-gas this phenomenon is not objectionable, is even advantageous, and it explains why a burner made of steat.i.te, which conducts heat badly, in always more economical (of heat and therefore of light) than an iron one. In the case of acetylene the same rule should, and undoubtedly does, apply also; but it is complicated, and its effect sometimes neutralised, by a peculiarity of the gas itself. It has been shown in Chapters II. and VI. that acetylene polymerises under the influence of heat, being converted into other bodies of lower illuminating power, together with some elemental carbon.

If, now, acetylene is fed into a burner which, being composed of some material like steat.i.te possessed of low heat-conducting and radiating powers, is very hot, and if the burner comprises a tube of sensible length, the gas that actually arrives at the orifice may no longer be pure acetylene, but acetylene diluted with inferior illuminating agents, and accompanied by a certain proportion of carbon. Neglecting the effect of this carbon, which will be considered in the following paragraph, it is manifest that the acetylene issuing from a hot burner--a.s.suming its temperature to exceed the minimum capable of determining polymerisation-- may emit less light per unit of volume than the acetylene escaping from a cold burner. Proof of this statement is to be found in some experiments described by Bullier, who observed that when a small "Manchester" or fish-tail burner was allowed to become naturally hot, the quant.i.ty of gas needed to give the light of one candle (uncorrected) was 1.32 litres, but when the burner was kept cool by providing it with a jacket in which water was constantly circulating, only 1.13 litres of acetylene were necessary to obtain the same illuminating value, this being an economy of 16 per cent.

EARLY BURNERS.--One of the chief difficulties encountered in the early days of the acetylene industry was the design of a satisfactory burner which should possess a life of reasonable length. The first burners tried were ordinary oil-gas jets, which resemble the fish-tails used with coal- gas, but made smaller in every part to allow for the higher illuminating power of the oil-gas or acetylene per unit of volume. Although the flames they gave were very brilliant, and indeed have never been surpa.s.sed, the light quickly fell off in intensity owing to the distortion of their orifices caused by the deposition of solid matter at the edges. Various explanations have been offered to account for the precipitation of solid matter at the jets. If the acetylene pa.s.ses directly to the burner from a generator having carbide in excess without being washed or filtered in any way, the gas may carry with it particles of lime dust, which will collect in the pipes mainly at the points where they are constricted; and as the pipes will be of comparatively large bore until the actual burner is readied, it will be chiefly at the orifices where the deposition occurs. This cause, though trivial, is often overlooked. It will be obviated whenever the plant is intelligently designed. As the phosphoric anhydride, or pentoxide, which is produced when a gas containing phosphorus burns, is a solid body, it may be deposited at the burner jets. This cause may be removed, or at least minimised, by proper purification of the acetylene, which means the removal of phosphorus compounds. Should the gas contain hydrogen silicide siliciuretted hydrogen), solid silica will be produced similarly, and will play its part in causing obstruction. According to Lewes the main factor in the blocking of the burners is the presence of liquid polymerised products in the acetylene, benzene in particular; for he considers that these bodies will be absorbed by the porous steat.i.te, and will be decomposed under the influence of heat in that substance, saturating the steat.i.te with carbon which, by a "catalytic" action presumably, a.s.sists in the deposition of further quant.i.ties of carbon in the burner tube until distortion of the flame results. Some action of this character possibly occurs; but were it the sole cause of blockage, the trouble would disappear entirely if the gas were washed with some suitable heavy oil before entering the burners, or if the latter were constructed of a non-porous material. It is certainly true that the purer is the acetylene burnt, both as regards freedom from phosphorus and absence of products of polymerisation, the longer do the burners last; and it has been claimed that a burner constructed at its jets of some non-porous substance, e.g., "ruby," does not choke as quickly as do steat.i.te ones. Nevertheless, stoppages at the burners cannot be wholly avoided by these refinements. Gaud has shown that when pure acetylene is burnt at the normal rate in 1-foot Bray jets, growths of carbon soon appear, but do not obstruct the orifices during 100 hours' use; if, however, the gas-supply is checked till the flame becomes thick, the growths appear more quickly, and become obstructive after some 60 hours' burning. On the a.s.sumption that acetylene begins to polymerise at a temperature of 100 C., Gaud calculates that polymerisation cannot cause blocking of the burners unless the speed of the pa.s.sing gas is so far reduced that the burner is only delivering one- sixth of its proper volume. But during 1902 Javal demonstrated that on heating in a gas-flame one arm of a twin, non-injector burner which had been and still was behaving quite satisfactorily with highly purified acetylene, growths were formed at the jet of that arm almost instantaneously. There is thus little doubt that the princ.i.p.al cause of this phenomenon is the partial dissociation of the acetylene (i.e., decomposition into its elements) as it pa.s.ses through the burner itself; and the extent of such dissociation will depend, not at all upon the purity of the gas, but upon the temperature of the burner, upon the readiness with which the heat of the burner is communicated to the gas, and upon the speed at which the acetylene travels through the burner.

Some experiments reported by R. Granjon and P. Mauricheau-Beaupre in 1906 indicate, however, that phosphine in the gas is the primary cause of the growths upon non-injector burners. According to these investigators the combustion of the phosphine causes a deposit at the burner orifices of phosphoric acid, which is raised by the flame to a temperature higher than that of the burner. This hot deposit then decomposes some acetylene, and the carbon deposited therefrom is rendered incombustible by the phosphoric acid which continues to be produced from the combustion of the phosphine in the gas. The incombustible deposit of carbon and phosphoric acid thus produced ultimately chokes the burner.

It will appear in Chapter XI. that some of the first endeavours to avoid burner troubles were based on the dilution of the acetylene with carbon dioxide or air before the gas reached the place of combustion; while the subsequent paragraphs will show that the same result is arrived at more satisfactorily by diluting the acetylene with air during its actual pa.s.sage through the burner. It seems highly probable that the beneficial effect of the earliest methods was due simply or primarily to the dilution, the molecules of the acetylene being partially protected from the heat of the burner by the molecules of a gas which was not injured by the high temperature, and which attracted to itself part of the heat that would otherwise have been communicated to the hydrocarbon. The modern injector burner exhibits the same phenomenon of dilution, and is to the same extent efficacious in preventing polymerisation; but inasmuch as it permits a larger proportion of air to be introduced, and as the addition is made roughly half-way along the burner pa.s.sage, the cold air is more effectual in keeping the former part of the tip cool, and in jacketing the acetylene during its travel through the latter part, the bore of which is larger than it otherwise would be.

INJECTOR AND TWIN-FLAME BURNERS.--In practice it is neither possible to cool an acetylene burner systematically, nor is it desirable to construct it of such a large ma.s.s of some good heat conductor that its temperature always remains below the dissociation point of the gas. The earliest direct attempts to keep the burner cool were directed to an avoidance of contact between the flame of the burning acetylene and the body of the jet, this being effected by causing the current of acetylene to inject a small proportion of air through lateral apertures in the burner below the point of ignition. Such air naturally carries along with it some of the heat which, in spite of all precautions, still reaches the burner; but it also apparently forms a temporary annular jacket round the stream of gas, preventing it from catching fire until it has arrived at an appreciable distance from the jet. Other attempts were made by placing two non- injector jets in such mutual positions that the two streams of gas met at an angle, there to spread fan-fas.h.i.+on into a flat flame. This is really nothing but the old fish-tail coal-gas burner--which yields its flat flame by identical impingement of two gas streams--modified in detail so that the bulk of the flame should be at a considerable distance from the burner instead of resting directly upon it. In the fish-tail the two orifices are bored in the one piece of steat.i.te, and virtually join at their external ends; in the acetylene burner, two separate pieces of steat.i.te, three-quarters of an inch or more apart, carried by completely separate supports, are each drilled with one hole, and the flame stands vertically midway between them. The two streams of gas are in one vertical plane, to which the vertical plane of the flame is at right angles. Neither of these devices singly gave a solution of the difficulty; but by combining the two--the injector and the twin-flame principle--the modern flat-flame acetylene burner has been evolved, and is now met with in two slightly different forms known as the Billwiller and the Naphey respectively. The latter apparently ought to be called the Dolan.

[Ill.u.s.tration: FIG. 8.--TYPICAL ACETYLENE BURNERS.]