CHAPTER II.

FLAT-FLAME BURNERS.

THE FIRST GAS-BURNER.

The first gas-burner was a very simple and unpretentious contrivance. In one of the earliest works on gas lighting[1] we read: "The extremities of the pipes have small apertures, out of which the gas issues; and the streams of gas, being lighted at those apertures, burn with a clear and steady flame as long as the supply of gas continues." Familiar as it is to us, and from its familiarity unnoticed, the phenomenon presented by the flame thus produced continuing to burn "as long as the supply of gas continued," was doubtless, to the first experimenters, a wonderful sight. Though we may smile at the question, it is not difficult to understand the incredulity of the honourable member who, when Murdock was examined before a Committee of the House of Commons, in 1809, asked the witness: "Do you mean to tell us that it will be possible to have a light _without a wick_?" "Yes; I do indeed," replied Murdock. "Ah, my friend," replied the member, "you are trying to prove too much."

[1] Accum's "Treatise on Gas-Lights." Third edition, 1816.

[Sidenote: The dawn of gas lighting.]

It was but natural, seeing that oil-lamps and candles were the only forms of artificial illumination in use prior to the introduction of gas lighting, that the earliest attempts at illumination by gas should be in imitation of the effects produced by those means. Accordingly we find that one of the first gas-burners employed was the Argand, modelled upon the oil-lamp of that name, which had been found to give superior results; while in more general use, and for some time almost the sole apparatus available, were single jets, giving a flame similar in appearance to that of a common candle, together with various combinations of these jets. A fair idea of the mode of illumination practised during the earliest period of gas lighting may be gleaned from the following extract from a paper describing the lighting of Messrs. Phillips and Lee's cotton-mill at Manchester, read before the Royal Society, in 1808, by Mr. William Murdock:--

The gas-burners are of two kinds. The one is upon the principle of the Argand lamp, and resembles it in appearance; the other is a small curved tube with a conical end, having three circular apertures or perforations, of about 1-30th of an inch in diameter, one at the point of the cone, and two lateral ones, through which the gas issues, forming three divergent jets of flame, somewhat like a fleur-de-lis. The shape and general appearance of this tube has procured for it, among the workmen, the name of the "cockspur" burner.

Nor was much advance made upon these arrangements down to the year 1816, judging from Accum's "Treatise" (before cited), as the subjoined extract from that work, together with the above illustrations, will show:--

The burners are formed in various ways--either a tube ending with a simple orifice, at which the gas issues in a stream, and if once lighted will continue to burn with the most steady and regular light imaginable, as long as the gas is supplied; or two concentric tubes of brass or sheet iron are placed at a distance of a small fraction of an inch from each other, and closed at the bottom. The gas which enters between these cylinders, when lighted, forms an Argand lamp, which is supplied by an internal and external current of air in the usual manner. Or the two concentric tubes are closed at the top with a ring, having small perforations, out of which the gas can issue; thus forming small distinct streams of light.

It is interesting, in view of the present demand for increased illumination, and for burners of high illuminating power, to note the amount of light produced by the burners then in use. In Mr. Murdock's paper we find it stated that each of the Argands in use at Messrs. Phillips and Lee's establishment gave "a light equal to that of 4 candles (mould candles of 6 to the pound);" and each of the cockspurs "a light equal to 2-1/4 of the same candles." From which meagre results we conclude that, besides being burnt in an ignorant and wasteful manner, the gas consumed was wofully deficient in illuminating power.

THE BATSWING BURNER.

[Sidenote: Who invented the batswing burner?]

A notable advance was made when the batswing burner was invented. To whom we are indebted for this invention seems involved in some doubt. Although Clegg, in the historical introduction to his valuable work,[2] says, very distinctly, that "the batswing burner was introduced by a Mr. Stone, an intelligent workman employed by Mr. Winsor," it is not so much as mentioned by Accum, even in the third edition of his "Treatise;" and Accum, it may be remarked, was for some time closely associated with Winsor in the promotion of the latter's ambitious and visionary schemes. Yet, if Clegg's statement be correct, it would almost appear to fix the date of the introduction of this burner as prior to 1816. But to whomsoever is due the credit of its invention, certain is it that the batswing burner was a considerable improvement upon the old cockspur. Producing a better light for the gas consumed, it assisted to demonstrate still further the superiority of gas lighting over other methods of illumination; and as it could be supplied at a trifling cost, and contained no delicately adjusted nor easily injured parts, it enabled the benefits of the new method of lighting to be extended to wherever artificial light was required.

[2] Clegg's "Treatise on Coal Gas," 1841, p. 21.

[Sidenote: Superiority of the batswing over the cockspur burner.]

From the cockspur and single jet burners the gas ascended in streams, rising into the air until it came in contact with sufficient oxygen to completely consume it. In order that this might take place without producing a flame of an inordinate length, and without much smoke, the orifices were restricted to a very small size; and the gas issuing from these at considerable pressure tended to draw in, and mix with the air in its course. Besides the loss of illuminating power caused by this mixture of air with the gas flame (similar to what takes place in a Bunsen burner), the cooling influence upon the small body of flame of the mass of metal composing the burner, operated still further to reduce the quantity of light which the gas was calculated to yield. With the batswing the gas was spread out producing, when ignited, a thin sheet of flame, by which means the gas was enabled to combine more readily with the air necessary to effect complete combustion. The size of the flame being, in comparison with that of the cockspur, so much larger proportionately to the metal burner, the cooling effect of the latter was not so apparent. The increased size of flame, also, of itself, tended to improve the illuminating power; each portion of flame contributing to elevate and sustain the temperature of the whole, and so to heighten the intensity of incandescence to which the light-giving particles were raised.

[Sidenote: Batswing and Argand burners compared.]

Even with the Argands of that day, the batswing compared not unfavourably. The former burner, having the regulation of its air supply under complete control, gives the best results when the gas is supplied to it at a low pressure; as then the requisite quantity of air to ensure complete combustion of the gas can be delicately adjusted by means of a chimney of suitable length. When the gas and air have been nicely adjusted to each other, the flame becomes extremely sensitive to any change of pressure in the gas supply; a diminution of the supply, by reducing the quantity of gas issuing from the burner without at the same time proportionately diminishing the supply of air, tends to destroy the illuminating power by the cooling action of the surplus air; while an increased pressure, by allowing more gas to issue than the air can consume, causes the flame to smoke. But at the time to which I now refer the principles of combustion were little understood, still less applied in the construction of burners. Besides this, the pressure of the gas in the mains was excessive; and there being no method adopted of controlling it at the burner, the construction of a good Argand was, under the circumstances, almost impossible. The batswing was not so prejudicially affected by an excess of pressure. Pressure to some extent was, indeed, required to enable the flame to attain its normal shape; while any excess forced the gas through the flame without permitting it to be raised to incandescence before being consumed, and although necessitating loss of light, caused no inconvenience like a smoking flame. Another important advantage which the batswing possessed over the Argand burner was its simplicity of construction; and the absence of accessories, such as the glass chimney--dispensing with the cleaning and attention which the latter required. Had the benefits of gas lighting been dependent upon the use of apparatus so fragile, and requiring so much care and attention as the Argand, the range of its applicability must have been considerably limited, and its prospects of commercial success much less assured. The introduction of a series of cheap but effective burners, however, altered the conditions of gas lighting, and marked the commencement of a new era in artificial illumination. The possibility of obtaining, by means of a burner so simple and apparently insignificant as the batswing, results little, if at all, inferior to what could be obtained by the use of the most complicated and expensive, was of advantage alike to the consumer and the producer of gas. To the former it gave the benefits of an increased illumination, without requiring any corresponding outlay; to the latter it promised a growing extension of the use of coal gas, and thus furnished the surest guarantee of future progress and prosperity.

THE UNION-JET, OR FISHTAIL BURNER.

[Sidenote: Who invented the union-jet burner?]

The batswing had been for some years in extensive use before a burner was produced worthy in any degree to compare with it in respect to simplicity and efficiency. The invention of the union-jet, or fishtail burner, furnished a competitor equally simple; little, if at all, inferior as regards efficiency; and, to some extent, superior to the former burner in general adaptability. Although so much behind in point of time, the new burner speedily rivalled the older batswing in popular favour; and in its various modifications and improvements may be said, without fear of contradiction, to have received a wider application than any other gas-burner. As in the case of the batswing, so with regard to this burner: few details are recorded of its invention. But, slight as is the information available, such as we have is more satisfactory and more authentic than the meagre notice of Clegg, which is all that is known of the invention of the former burner. It appears to be established beyond doubt that the union-jet is the joint invention of Mr. James B. Neilson, the inventor of the hot-blast, and Mr. James Milne, of Glasgow, founder of the engineering firm of Milne and Son. About the year 1820, or soon after (as in that year Mr. Neilson was appointed Manager of the Glasgow Gas-Works), these gentlemen were experimenting with gas-burners, when they discovered that by allowing two jets of gas, of equal size, to impinge upon each other at a certain angle, a flat-flame was produced, with increased light. This was the origin of the union-jet; so called from the manner in which the flame is produced. At first separate nipples were employed for the two jets; but, very soon, Mr. Milne hit upon the expedient of drilling two holes, at the required angle, in the same nipple. In this manner, with slight modifications, the burner has continued to be constructed down to the present day.

The explanation of the preference accorded to this burner over its predecessor, the batswing, is to be found chiefly, I think, in the very different shapes of the respective flames produced by the two burners. The batswing, in its original form, produced a flame of great width, but of no corresponding height. The extremities of the flame, stretching out from the burner so far on either hand, were easily affected by an agitation of, or commotion in the surrounding atmosphere; a slight draught or current of air causing the flame to smoke at these points. The extreme width of flame also precluded the use of this burner in globes. The flame produced by the union-jet burner, as first constructed, was very different to the one just described. Longer than that of the batswing, and considerably narrower (but widening gradually from its base, at the burner, to its apex), it presented somewhat nearly the appearance of an isosceles triangle; or more closely, perhaps (with its slightly-forked apex), the tail of a fish, from which resemblance it is commonly designated the fishtail burner. This form of flame was better adapted for use in globes, and also better withstood the effects of draughts. And it is perhaps not unreasonable to suppose that as in shape it approached more closely to the kind of flame with which the people had been familiar in oil lamps, the flame produced by the union-jet burner was more agreeable to the eye than that of the batswing, and that this seemingly trivial consideration will account, to some extent at least, for the undue favour shown towards it. For it must not be assumed, because of the widespread popularity to which the union-jet so early attained, and which it has continued to enjoy, that it was of necessity a better burner (in the sense of developing more light for the gas consumed) than the one which preceded it. On the contrary, in this regard it was not quite so effective as the batswing. Nor is this result surprising, looking at the different methods adopted in the two burners for producing the same effects of light and flame.

[Sidenote: Union-jet and batswing burners compared.]

From the batswing burner the gas issued in a thin but widely-extending stream, presenting, when ignited, a continuous sheet of flame; its height and width depending upon the pressure at which the gas was supplied, but always offering an unbroken surface of flame to the air. Although, from the excessive pressures which, in the early days of gas lighting, were generally employed, the flame drew upon its surface too much air for the attainment of the fullest lighting efficiency obtainable from the gas; yet the form given to the issuing stream of gas precluded the air from entering the interior of the flame, and still further reducing its illuminating power. With the union-jet burner the conditions were greatly changed; and this latter evil, of the introduction of cold air into the interior of the flame, was one of the consequences entailed by the means it employed for producing its flame. From this burner the gas issued in two narrow streams, like single jets, which, directly after emerging from the burner, impinged upon each other at a given angle; the mutual shock given to the streams of gas when thus arrested causing them to spread out in a lateral direction, and (the high velocity at which the gas issued being expended) to unite, and ascend in a sluggish stream until consumed. That injury to the illuminating power of the flame should result from causes connected with the manner of producing it will be understood on considering some of the phenomena associated with the production of a gas flame.

[Sidenote: How air is drawn upon a gas flame.]

When a jet or stream of gas issues into a still atmosphere, it produces in its immediate neighbourhood, on all sides, an area of low pressure, to occupy which the contiguous air rushes in. Induced air currents are thus set up in close proximity to, and having the same direction as the issuing stream of gas, and varying in force with the pressure, or velocity, at which the gas issues. The non-luminous flame of the Bunsen burner, and of the so-called "atmospheric" burner employed in gas cooking and heating stoves (which is produced by burning a mixture of gas and air), is obtained by taking advantage of this tendency of a stream of gas, issuing under pressure, to draw air upon itself; and it is to the same circumstance that ordinary illuminating flames owe the continuous supply of air necessary to keep up combustion. For the effect is heightened when the gas is inflamed; because, the gaseous products of combustion being expanded by the intense heat to which they are subjected, their velocity of ascension is vastly increased. Having regard to these considerations, it will be clearly perceived how that, in producing the flame of the union-jet burner, the two streams of gas, in the act of combining together, drew into the very midst of the flame a portion of the air with which they were surrounded; and this air, reducing the temperature of the flame, and diluting the illuminating gas by the inert nitrogen introduced, as well as by its oxygen causing a too early oxidation of the carbon particles in the flame, operated to reduce the illuminating power otherwise obtainable from the gas.

The foregoing remarks, it must be borne in mind, refer to the union-jet burner in its original form. Numerous improvements have been effected, from time to time, in its construction, as well as in that of the batswing, which, by reducing its liability thus to convey air into the flame, have increased its efficiency; while, at the same time, the shape of the flame has been improved. Indeed, the result of successive improvements in the construction of both burners has been so to modify the shape of their respective flames that, in their latest and most improved form, the flames produced by the two burners are practically identical in appearance, although the manner of their production remains as widely diverse as at the first. The improvements that led up to, and the causes that produced this result, will be more fully explained in the sequel.

HOW LIGHT IS PRODUCED FROM COAL GAS.

I have before remarked that, in the early period of its use, one of the chief obstacles to the development of the lighting power of coal gas was the excessive pressure at which it was generally supplied. To understand the action of pressure in influencing the amount of light which a given quantity of gas will afford, it is necessary to know something of the nature and properties of flame. Moreover, the conditions upon which is dependent the illuminating power of a gas flame are so intimately related to each other, that the precise functions due to each cannot well be separated from the complete effect produced by the combined operation of all. I shall not, therefore, be needlessly digressing from my subject if, at this juncture, I explain the manner in which combustion takes place in the flame of an ordinary gas-burner. In doing this, I shall endeavour to clothe my remarks in very plain language; using no more technicalities than are absolutely required by the exigencies of the subject. In this way I hope to make my meaning clear to the simplest. At the same time, without pretending to be scientifically complete, the explanation of the phenomena of combustion which I shall furnish will, I trust, be sufficiently explicit to enable the reader to form a right estimate of the principles which regulate the production of light when coal gas is consumed. The end chiefly kept in view is to show clearly the extent to which the degree of light evolved is dependent upon the burner employed, and the manner in which the gas is consumed. If my remarks are the means of causing the reader to look with intelligent interest upon the familiar phenomena of gaslight, they will not have been written altogether in vain.

[Sidenote: What is coal gas?]

Seeing that this treatise is compiled especially for those whose knowledge as to what coal gas consists of is extremely limited, it may be of advantage to preface my observations on its combustion, and the production of light therefrom, by a few remarks as to its composition. Coal gas, as generally supplied, is made up of a variety of distinct gases; of which, however, only some three or four exist in any considerable proportion. About 50 per cent., by volume (or half of the whole), is hydrogen; from 30 to 40 per cent. consists of marsh gas; while carbonic oxide is usually present to the extent of from 5 to 15 per cent. These three gases, which constitute the great bulk of what is known as common gas--that is, gas made from ordinary bituminous coal, as distinguished from that produced from the more costly cannel--are of little or no value as regards the amount of light they are capable of affording. The flames produced by the burning of the two former gases evolve much heat, but are of very feeble illuminating power. The latter gives a flame of a deep blue colour, producing scarcely any light, but, like the other two, an intense heat. The power of coal gas to yield a luminous flame is dependent upon the small quantity of heavy hydrocarbons which it contains--a constituent, or series of constituents, of which common gas only contains a proportion varying between 3 and 7 per cent., although in cannel gas it reaches as high as 15 or 20 per cent. These heavy hydrocarbons are gases composed, like marsh gas, of carbon and hydrogen; but containing in their composition, for each unit of volume, a greater aggregate of the two elements, as well as a relatively higher proportion of carbon, than exists in marsh gas. One of the simplest members of the series, and that which is usually present in by far the largest amount, is called olefiant gas. It contains twice as much carbon, combined with only the same quantity of hydrogen, as is contained in marsh gas. But besides olefiant gas there are minute quantities of other gases of the same series, having an analogous composition, but differing in the amount and relative proportions they contain of the two elements of which they are composed. All the gases of this series, when properly burnt, are capable of affording a brightly luminous flame; but when consumed alone it is somewhat difficult, on account of the high proportion of carbon which they contain, to effect their combustion without the production of smoke. It is, then, to the heavy hydrocarbons which are part of it--insignificant as their amount may appear--that the luminosity of a gas flame is solely due. The other constituents which I have mentioned as forming so much larger a proportion of the whole, besides contributing to the heat of the flame, serve only to dilute these richer gases, and so promote their more complete combustion.

[Sidenote: How gas burns.]

The various simple gases which constitute ordinary coal gas do not all burn together in the flame; the temperature required to effect their ignition being lower for some of them than for others. Thus, hydrogen is the first to burn, taking fire readily as soon as it issues from the burner; while the combustion of the heavy hydrocarbons does not commence until they enter the hotter portions of the flame, and is not completed until they reach its farthest extremity. Neither is the process of combustion in both cases the same. The former gas is at once completely consumed; the latter first undergo decomposition by the heat of the flame, being resolved into their elements--hydrogen and carbon--before being fully consumed. This decomposition of the hydrocarbons is a factor of supreme importance in the development of the lighting power of the flame. The hydrogen they contain, being more easily ignited than the carbon, burns first; and the latter is set free, in the solid form, as minute particles of soot. These particles of solid carbon, being liberated in the midst of the flame, are immediately subjected to its most intense heat; they thus become white-hot before they reach the outer verge of the flame, and come in contact with sufficient oxygen to effect their complete combustion. The amount of light developed by any coal-gas flame is directly proportional to the degree of intensity to which the temperature of these carbon particles is raised, and the length of time they remain in the flame before being finally consumed. It becomes, therefore, a matter of considerable importance to know the conditions which are most conducive to the early liberation in the flame of free carbon, and the attainment by it of an exalted temperature.

[Sidenote: What is a gas flame?]

Looking at the flame (say) of a common slit burner, it is seen to be divided into two sharply defined and wholly distinct portions. First, there is--immediately surrounding the burner head, and extending to some distance from it--a dark, transparent area, which, on closer examination, is found to consist of unignited gas enclosed in a thin envelope of bright blue flame. Second, there is (beyond this central area) a zone, or belt, of brightly luminous flame, white and opaque; the latter property indicating the presence of solid matter at this part of the flame. That the dark central portion of the flame consists chiefly of unignited gas may be shown in various ways, in addition to the evidence afforded by its complete transparency. Thus, if a small glass tube be taken, and its lower end inserted in the flame at this point, the unburnt gas will pass up the tube, and may be lighted at its upper extremity. A splinter of wood thrust through this portion of the flame is charred first at the two edges of the flame; while, in like manner, a piece of platinum foil remains dull in the centre of the flame, and glows only at the points of contact with the outer air. The presence of solid carbon in the luminous portion of the flame may be shown by inserting therein any cold substance (such as a piece of metal or porcelain), which, reducing the temperature of the heated particles of carbon below the point at which they are consumed, becomes instantly coated on its under surface with a deposit of soot. Or, if the flame be suddenly cooled by gently blowing upon its surface, the same result is brought about; clouds of soot are given off, and the flame "smokes."[3]

[3] The behaviour of gas flames when exposed to the action of the wind (as exemplified in the naked lights of open markets and similar situations) affords an instructive illustration of the theory of luminous combustion. A sudden gust causes the flame to smoke, by reducing the temperature of the liberated carbon below the point at which it can combine with the oxygen of the air. A continuous wind blowing upon the flame destroys its luminosity altogether, because the heat-intensity of the flame is lowered below the temperature necessary to decompose the hydrocarbons; consequently, these latter burn without the preliminary separation of carbon, and a non-luminous flame is produced--exactly as in the Bunsen or "atmospheric" burner.

[Sidenote: How the flame is cooled.]

The existence, in the midst of the flame, of an area of unconsumed gas is due to the cold gas, as it issues from the burner, cooling the interior of the flame below the temperature required for its ignition, as well as to its not at once meeting with sufficient air for complete combustion. The causes which affect the luminous zone of the flame are not so readily explained. It has been stated that the luminosity of the flame is due to the particles of carbon, which are separated out of the hydrocarbons in the gas, being raised to a white heat. To decompose the hydrocarbons, a very high temperature is required; and, on account of the cooling effect of the stream of cold gas, this is not attained except at some distance from the burner. The abstraction of heat by the burner itself is also a cause of the reduction of the temperature of the flame; and, on this account, burners of porcelain, steatite, or similar composition, being bad conductors of heat, have an advantage over those made of metal. So considerable is the cooling influence of the gas stream, that, within certain limits, the distance, from the burner head, at which the luminosity of a flame commences, is proportionate to the velocity with which the gas issues; or, in other words, the pressure at which it is delivered from the burner. The effect is heightened by the tendency (which has been before remarked) of a stream of gas, issuing under pressure, to draw upon itself and mix with the surrounding air. Thus, with each increment of pressure the luminous zone of the flame is farther removed, until a point is reached at which the gas is so mixed with air before being consumed that the luminosity of the flame is completely destroyed.

[Sidenote: Effects of pressure in the gas supply.]

But it must not be assumed, because of the foregoing remarks, that the pressure at which the gas issues from the burner is altogether an unmixed evil. In flat-flame burners it fulfils the important function of promoting intensity of combustion, by bringing the white-hot particles of carbon into intimate and rapid contact with the air that is necessary for complete combustion. In Argand burners this duty is discharged by the glass chimney; but with flat-flame burners it devolves entirely upon the pressure at which the gas issues from the burner. It will be seen, therefore, that the pressure of the gas is a factor of considerable importance in determining the amount of light afforded by a gas flame, as it is a matter requiring careful adjustment with each and every burner. On the one hand, with an excessive pressure the intensity of combustion is increased; but the separated carbon does not remain so long in the flame. The area of luminosity is thereby decreased, and the total light yielded is reduced. On the other hand, with insufficient pressure the combustion is not energetic enough to raise the particles of carbon to a white heat; consequently, the illuminating power of the flame is feeble, or else the carbon escapes unconsumed as smoke.

The thickness of the flame produced by any burner has also an important bearing upon the degree of light afforded; and this property of thickness, again, is dependent upon the width of slit, in the case of batswings (or, in the case of union-jets, upon the size of orifices), and the pressure at which the gas is supplied. The thickness of the flame yielded by any burner will obviously vary inversely with the pressure at which the gas is supplied to it. With a thin flame, all parts of the flame are so completely exposed to the air, that the particles of carbon are no sooner raised to the temperature required to enable them to give out light than they are entirely consumed. With a thicker flame the carbon separated in the midst of the flame exists for a sensibly longer period of time in the white-hot state before it reaches the outside of the flame, and meets with sufficient oxygen for its complete combustion. Thus we find that the best flat-flame burners have comparatively wide orifices; while the pressure at which the gas is delivered from the burner is carefully reduced to the lowest point at which a firm flame is obtained, without smoke. Similarly, in the best Argands the pressure is considerably diminished within the burner, and the gas allowed to issue gently through relatively large holes; while the chimney is carefully adapted to draw upon the surface of the flame just sufficient air to completely consume the quantity of gas which the burner is calculated to deliver.

IMPROVEMENTS IN FLAT-FLAME BURNERS.

Although, there is no doubt, they were made empirically, and in ignorance of the real effects of pressure upon the flame, the first steps towards increasing the efficiency of flat-flame burners were in the right direction of reducing the excessive pressure at which the gas was formerly allowed to burn. They consisted in the adoption of simple arrangements for obstructing the passage of the gas through the burner, and so retarding its flow. The crudeness of the means which were employed is sufficient evidence that the end aimed at was, at best, but dimly discerned. The body of the burner was stuffed with wool, or pieces of wire gauze; which impeded the progress of the gas; reduced the quantity that would otherwise have been consumed; and, consequently, diminished the velocity with which it issued from the burner. Unfortunately, owing to the imperfect methods in use at that day for condensing and purifying the gas, the burners so constructed became choked with the tarry matters held in suspension, and carried forward by the gas; and so, after a comparatively short period of service, were rendered entirely inoperative. But, altogether apart from the inconvenience and loss thus entailed (which, when improved modes of manufacture had removed the cause, ceased to be experienced), the arrangement was ill adapted for the purpose which it was designed to serve. The rough and uneven nature of the material employed to stuff the burner caused the gas to eddy and swirl as it issued into the atmosphere, and prevented it being supplied equally to all parts of the flame. The consequence was that the advantages which ought to have been derived from the diminished pressure were neutralized by the unsteady flow acquired by the stream of gas; and the illuminating power developed by the flame was little improvement upon what could previously be obtained by the manipulation of the tap controlling the supply of gas to the burner. Besides which, from its unevenness, the appearance of the flame was not so satisfactory. It was not until the principles which regulate the production of light from coal gas came to be known and observed in the construction of burners, that a modification of the old idea was arrived at, which enabled the benefits of a reduced pressure to be obtained without any of the attendant evils hitherto experienced.

[Sidenote: The first real improvement of the union-jet burner.]

A modification in the construction of the union-jet which, though slight, was nevertheless a real improvement, appears to have been made at an early period in the history of this burner. Instead of having the top of the burner perfectly flat, it was made slightly concave; more especially at its centre, where the two jets of gas emerge. The effect of this alteration was to enable the stream of gas to spread out better; and thus to cause the flame to become broader at its base. The shape of the flame was thereby improved; and (what is of more consequence) its illuminating power increased, because air was not drawn so readily into the midst of the flame. The value of the arrangement is shown by the fact that it has been retained ever since, and is made use of in the latest and most improved burners of this class.

Prior to 1860, numerous novel contrivances were introduced as "improved" burners; but all were not equally valuable with the simple arrangement just described. The construction of many of them, indeed, betrayed a lamentable ignorance of the first principles of gas combustion. For instance, one is described as "a fishtail with four converging holes; and there is an aperture in the centre of the burner for the admission of atmospheric air into the flame!" Another was a batswing with two or more slits, producing a series of flames amalgamated into one; by which means it was supposed that an improved duty was obtained from the gas--unmindful, or, more probably, in ignorance of the fact that the same quantity of gas, properly consumed through one slit, would yield a better light.

[Sidenote: The double-flame burner.]

A burner which, at different times, and under various names, has been brought repeatedly into notice is the double-flame; consisting of two batswing or union-jet burners set at an angle to each other, so that their flames converge, and merge into one. When two gas flames are made to coalesce in this manner, a greater amount of light is developed than the sum of that yielded by the separate flames; provided that, in the combined flame, the gas is properly consumed, without smoke. The reason for this increase is twofold. First, the increased quantity of gas burnt in one flame enables a higher average temperature to be maintained; and, in addition, a smaller surface of flame is exposed to the cooling action of the atmosphere than when the same quantity of gas is consumed in two flames. Second, the pressure at which the gas burns is diminished, because the initial velocity with which the streams of gas issue from the two burners is expended in impinging against each other, and a thicker flame results; the apparatus being, as far as its effect is concerned, a union-jet burner on a large scale. The increase of light so obtained appears to have been noticed at an early period; as a burner embodying the same principle is described and figured in "Clegg's Treatise," published in 1848. In Clegg's burner the gas issued from two perforated parallel plates inclined to each other; but at a more recent period two fishtail burners were employed, being mounted on separate tubes which branched out to a short distance from each other. Occasionally, for experimental and show purposes, it has been constructed with the two branches hinged together, so as to show the different effects produced when the two burners are used separately and in combination. At the present day it is made, by various makers, as one burner with two nipples, as shown in the annexed illustration; which doubtless is its most perfect form.

The advantages of the double flame are not so obvious under the conditions which obtain at the present day as at the period when it was first introduced. The increase of light it affords is most apparent when the gas is being consumed at an excessive pressure. Although, in general, it may be taken that any two flames, when combined, will develop a higher duty, per cubic foot of gas consumed, than separately; yet it would appear that this is not so in every case. When the gas is being consumed at the critical pressure which gives the best results, the flames are so near the smoking point that the slight diminution of pressure experienced when the streams of gas impinge upon each other is sufficient to cause the combined flame to smoke. Moreover, to such a stage of perfection have the ordinary flat-flame burners now been brought, that, for all ordinary consumptions, it may be safely affirmed that equal, if not superior results can be obtained with a single as with a double flame. Where, however, larger quantities of gas are required to be dealt with than can be effectively consumed in a single burner, the principle of combining two or more burners together, so that their flames shall mutually assist each other, may be advantageously employed; as is seen in the combination of flat-flame burners in the large lamps now employed in improved street lighting.

[Sidenote: Scholl's "Platinum Light Perfecter."]

An ingenious device for improving the efficiency of union-jet burners was brought out some twenty years ago by a Mr. Scholl, of London, and known as Scholl's "Platinum Light Perfecter," which is shown in the accompanying illustration. It consisted of a little brass ring, carrying a plate of platinum about 0·4 inch long by 0·15 inch wide. The ring fitted on to the top of the burner in such a manner that the platinum plate was held, in a vertical position, between the two orifices from which the gas emerged. The jets of gas, instead of impinging upon each other, impinged against the plate, and united above to form the flame. By the interposition of the metal plate, the velocity of the gas was much reduced; and a thicker and more sluggish flame was produced, with the result of increasing its illuminating power. When the apparatus was used upon a burner having very small orifices, and delivering its gas at a high pressure, the increase of light obtained was very striking; but with lower pressures the advantage derived from its use was correspondingly diminished. This is very clearly shown by the following table, which is extracted from a report, by Captain Webber and Mr. Rowden, on experiments upon gas-burners, carried out at the Paris Universal Exhibition, 1867.[4]

[4] See _Journal of Gas Lighting_, Vol. XVIII., p. 88.

--------------------------+-------+--------+---------------------+-------- Kind of Burner. |Cubic |Pressure| Illuminating Power. | |Feet of| in +----------+----------+Increase |Gas |Inches. |Without |With | per |per | |Perfecter.|Perfecter.| Cent. |Hour. | | | | --------------------------+-------+--------+----------+----------+-------- Leoni's fishtail, No. 2 . | 3 | 0·84 | 1·3 | 4·1 | 215 Leoni's fishtail, No. 3 . | {3 | 0·46 | 2·4 | 4·6 | 91 | {4 | 0·70 | 2·8 | 6·5 | 132 | {3 | 0·31 | 3·4 | 5·0 | 47 Leoni's fishtail, No. 4 . | {4 | 0·47 | 4·5 | 7·6 | 68 | {5 | 0·71 | 5·0 | 9·2 | 84 | {4 | 0·42 | 5·3 | 6·9 | 30 Leoni's fishtail, No. 5 . | {5 | 0·60 | 6·1 | 8·3 | 36 | {6 | 0·81 | 7·1 | 10·0 | 40[5] Leoni's fishtail, No. 6 . | {4 | 0·31 | 6·2 | 8·0 | 29[6] | {5 | 0·46 | 8·0 | 10·4 | 30[7] --------------------------+-------+--------+----------+----------+--------

[5] Flame flickers.

[6] Do.

[7] Flame flickers a great deal.

Burners were also made with the metal plate forming part of the burner head; and, instead of being of platinum, it was sometimes formed of thin steel, or other commoner metal. Where platinum was used, some advantage probably accrued from its becoming incandescent; but, of course, any benefit arising from this source was not obtained when steel was employed. The remarks which have been made respecting the limited applicability of the double-flame burner will apply, with equal force, to the apparatus under notice. Although it effected an undoubted improvement when applied to burners ill adapted to the pressure at which the gas was supplied, equally good results could be obtained without its aid, when a burner was employed suited to the quality and pressure of the gas supplied.

[Sidenote: Leoni's flat-flame burners.]

Perhaps the most efficient flat-flame burners available prior to 1867 were those made by Mr. S. Leoni, of London. One of these is shown in fig. 7. This maker produced both batswing and union-jets; various sizes being made of each burner. Besides affording fairly good results from the gas consumed, the burners were supplied at a very moderate price. Their distinguishing feature was the peculiar substance of which the burner-tips were formed. This was a material invented by Mr. Leoni, and named by him "adamas." (The precise composition of "adamas" is a trade secret; but it appears to consist of a mixture of various minerals or earths, moulded in a clayey or plastic condition, and then burnt.) Previous to his invention, the tip of the burner, or the burner head, had been made, almost exclusively, of iron or brass. There were, however, some grave defects inherent in the use of metal for this purpose. The orifices of union-jets and the slits of batswings in course of time became much obstructed by the corrosion of the metal; and the efforts made to remove the obstruction only served to destroy the burner more quickly, by increasing the size and injuring the precise shape of the apertures. The "adamas" tips, on the other hand, perfectly withstood the high temperature to which they were exposed, were quite incorrodible, and were sufficiently hard to endure a considerable degree of even rough usage. By constructing the tip of this material, the efficiency of the burner was improved in many ways. The liability of the burner to corrosion being removed, and the inconvenience due to this cause done away with, the life of the burner was prolonged, and the expense of renewal consequently reduced. But, in addition to these advantages, there was yet another direction in which the "adamas" tip contributed to enhance the utility of the burner. This was in maintaining a higher temperature of the flame; and arose from its inferior capacity, compared with metal, for conducting heat from the flame. That the advantage derived from this source, although unimportant, was not altogether imaginary, will be apparent when it is mentioned that metal burners, when in operation, usually attain to a temperature of from 400° to 500° Fahr.--an indication of the amount of heat being continuously abstracted from the flame. The adoption of a non-conducting material for the burner-tip, while it did not entirely prevent, considerably reduced the loss of heat.

Two varieties of each class of burner were made by Mr. Leoni. In the one burner, the "adamas" tip was inserted into an iron stem; in the other, the tip was inserted in a brass body, which fitted on to the iron stem. Between the brass body and the iron stem of the latter burner there was affixed a layer of wool, designed to check the pressure at which the gas was supplied. Owing, very probably, to the unsuitability of the material (wool) used for this purpose, the result was not satisfactory; as, according to the statements of Messrs. Webber and Rowden, in the report previously cited, no difference could be detected, in many experiments, between the results yielded by the burner with or without the layer of wool. Some light is shed upon this apparent anomaly by certain experiments made by the writer to determine the pressure at which gas issues from various burners. With one of Leoni's No. 4 union-jets, under an initial pressure of 1 inch (the pressure at the inlet when the burner is in operation), the pressure at the outlet of the burner, when the layer of wool was employed, was 0·11 inch; but from the same burner, when the layer of wool was removed, the gas issued at a pressure of only 0·07 inch. Thus the effect of inserting the layer of wool in the burner was exactly the opposite of that which it was intended to produce; the pressure of the issuing gas stream being increased instead of diminished.

BRÖNNER'S BURNERS.

The credit of having produced the first flat-flame burners designed upon scientifically correct principles belongs undoubtedly to Herr Julius Brönner, of Frankfort-on-the-Maine. Long before the date of his invention, efforts had been made to reduce the pressure of the gas within the burner. But these endeavours were carried out in so hap-hazard a fashion as to lead to the belief that no definite conception was entertained as to what was really required. As we have seen, layers of wool had been employed; but the area of the interstices, or the gas-way through the material, was a matter of the merest accident. And there was not the slightest guarantee that the same conditions should prevail in any two burners. Herr Brönner shrewdly detected the cause of former failures, as he clearly perceived the end which it was requisite to attain, and towards which previous inventors had been but blindly groping. Having formed a right estimate of the requirements to be fulfilled, and the difficulties to be surmounted, he set about accomplishing the desired result by other means. There were two causes which had chiefly contributed to the unsuccessful issues of previous attempts. One was the uncertain and indefinite operation of the means employed for diminishing the pressure; the other was the inadequate provision for enabling the gas to lose the current, or swirl, acquired in passing the diminishing arrangement, and come to a state of comparative rest before issuing into the atmosphere. Both these errors were successfully avoided in Brönner's invention--the former by making the inlet to the burner of restricted and definite dimensions, and of less area than the outlet, or slit; the latter by enlarging the chamber, or place of expansion within the burner, as well as by the different arrangement adopted for diminishing the pressure.

[Sidenote: Construction of Brönner's burners.]

The general appearance of Brönner's burner is pear-shaped; and in size it is considerably larger than an ordinary burner designed to pass an equal quantity of gas. It consists of a cylindrical brass body surmounted by a steatite top, and tapering to a very small diameter at its lower end, or inlet; the latter being closed by a plug of steatite, in which is a rectangular slot, or aperture, of accurately defined dimensions. The size of this aperture determines the quantity of gas which, at any particular pressure, is admitted to the burner; and the slit, or outlet of the burner, being of greater area than the inlet, ensures the gas being delivered from the burner at a lower pressure than that at which it enters it. By varying the respective dimensions of these two openings, and their relation to each other, the burner may be regulated to deliver its gas at any required pressure short of the initial pressure at the entrance to the burner. The enlargement of the cylindrical body provides an expansion chamber, wherein the velocity of the stream of gas which rushes through the narrow opening at the inlet of the burner is checked, and any agitation or unsteadiness which may have been imparted to it is subdued before the gas issues into the atmosphere and is consumed. There are two kinds of tops for the burners, which are distinguished by the letters A and B. The B top is of the ordinary semi-spherical type, giving a true batswing-shaped flame; the A top is flatter, almost square in form, and yields a flame taller than, but not so broad as the former. In consequence of this difference in the shape of its flame, the latter burner is better adapted for use in globes. The general appearance of the burners, and their distinguishing peculiarities, will be clearly understood from the illustrations.

[Sidenote: Properties of steatite.]

The material of which the more important parts of the burner are constructed is eminently adapted for the purpose. Steatite is a mineral which, as found in nature, is so soft as to be readily turned in a lathe, and shaped to any design; but when heated up to about 2000° Fahr. it becomes almost as hard and durable as flint, while perfectly retaining its form and colour. These properties peculiarly qualify it for receiving a slit or orifice, which, though of minute proportions, must be accurately formed to precise dimensions. Besides which, like "adamas," its capacity for conducting heat away from the flame is so limited that, in this respect, it has a considerable advantage over metal for the purpose of being formed into gas-burners.

[Sidenote: Varied adaptability of the Brönner burner.]

The following tables, which are extracted from the report of the Committee of the British Association appointed to investigate the means for the development of light from coal gas of different qualities,[8] exhibit the very satisfactory results obtained by the use of these burners. In Table I., the gas operated upon was cannel gas (such as is generally supplied in Scotland), and possessed an illuminating power, when employed in the standard burner, of 26 candles per 5 cubic feet. Table II. contains the results of determinations with common gas (such as is used in London, and generally throughout the greater part of England); 5 cubic feet of which, in the standard burner, gave an illuminating power of 16 candles. The first and second columns of the tables refer to the different sizes of the tops and bottoms of the particular burners employed; there being in all some 16 sizes of the one, and 11 sizes of the other. These, being interchangeable, permit of a great variety of combinations; and enable a burner to be selected suited to any particular quality or pressure of gas. For as with pressure, so with illuminating power: In order to obtain the utmost lighting efficiency, different burners are required for gases differing in quality or their degree of richness. A burner which, with gas of one quality, will yield excellent results, may, under the same conditions of pressure and supply, be totally unsuited to gas of another quality. That this should be so will be evident from a consideration of what has been said as to the theory of burning gas to the best advantage; and, in brief, results from the richer gas containing in its composition a greater proportion of carbon, and so requiring an increased supply of air for its thorough combustion. This increased supply of air can only be obtained (with flat-flame burners) by causing the gas to issue into the atmosphere at a higher pressure; and so it comes about that, compared with the quantity of gas to be delivered through them, the slits of batswing and the orifices of union-jet burners must be considerably narrower when intended for cannel gas than when common gas is to be consumed. In other words, in order to develop its full illuminating power, it is essential that the pressure at which the gas issues from the burner should be proportioned to its quality. The gist of the matter is set forth in the general statement that "the poorer the quality of the gas, the lower must be the pressure at which it is consumed; and _vice versâ_."

[8] See _Journal of Gas Lighting_, Vol. XXXII., p. 423, and Vol. XXXVI., p. 376.

TABLE I.

-----------------------------------+----------------------------------- | AT 0·5-INCH | AT 1·0-INCH | AT 1·5-INCH | PRESSURE. | PRESSURE. | PRESSURE. -------+----+-----+-------+--------+-------+----+-----+-------+-------- No. |No. |Cubic|Illumi-|Illumi- |No. |No. |Cubic|Illumi-|Illumi- of |of |Feet |nating |nating |of |of |Feet |nating |nating Burner.|Top.|per |Power. |Power |Burner.|Top.|per |Power. |Power | |Hour.| |per Five| | |Hour.| |per Five | | | |Cub. Ft.| | | | |Cub. Ft. -------+----+-----+-------+--------+-------+----+-----+-------+-------- 2 | 2 | 1·20| 5·07 | 24·13 | 2 | 2 | 1·40| 5·25 | 18·75 2 | 3 | 1·40| 6·64 | 23·71 | 2 | 3 | 1·95| 7·37 | 18·90 2 | 4 | -- | Smokes| -- | 2 | 4 | 2·30| 10·33 | 22·46 2 | 5 | -- | " | -- | 2 | 5 | 2·40| 11·24 | 23·42 2 | 6 | -- | " | -- | 2 | 6 | -- | Smokes| -- -------+----+-----+-------+--------+-------+----+-----+-------+-------- 2-1/2 | 2 | 1·40| 5·53| 19·75 | 2-1/2 | 2 | 1·90| 8·30 | 21·84 2-1/2 | 3 | 1·70| 8·48| 24·94 | 2-1/2 | 3 | 2·30| 10·14 | 22·04 2-1/2 | 4 | 2·03| 10·33| 25·49 | 2-1/2 | 4 | 2·70| 12·08 | 22·37 2-1/2 | 5 | -- | Smokes| -- | 2-1/2 | 5 | 2·85| 14·29 | 25·07 2-1/2 | 6 | -- | " | -- | 2-1/2 | 6 | 3·00| 15·21 | 25·35 -------+----+-----+-------+--------+-------+----+-----+-------+-------- 3 | 2 | 1·45| 6·27| 21·62 | 3 | 2 | 2·00| 8·48 | 21·20 3 | 3 | 1·90| 8·66| 22·79 | 3 | 3 | 2·40| 11·34 | 23·63 3 | 4 | 2·13| 11·24| 26·39 | 3 | 4 | 2·80| 14·84 | 26·50 3 | 5 | -- | Smokes| -- | 3 | 5 | 3·15| 17·04 | 27·20 3 | 6 | -- | " | -- | 3 | 6 | 3·25| 18·07 | 27·80 -------+----+-----+-------+--------+-------+----+-----+-------+-------- 3-1/2 | 2 | 1·50| 5·81| 19·36 | 3-1/2 | 2 | 2·12| 8·85 | 20·87 3-1/2 | 3 | 1·95| 8·30| 21·28 | 3-1/2 | 3 | 2·55| 12·63 | 24·76 3-1/2 | 4 | 2·55| 12·08| 23·68 | 3-1/2 | 4 | 3·00| 14·47 | 26·12 3-1/2 | 5 | 2·80| 14·38| 25·68 | 3-1/2 | 5 | 3·50| 18·07 | 25·81 3-1/2 | 6 | 3·00| 15·58| 25·97 | 3-1/2 | 6 | 3·60| 19·45 | 27·01 -------+----+-----+-------+--------+-------+----+-----+-------+-------- 4 | 2 | 1·60| 6·36| 19·87 | 4 | 2 | 2·30| 9·77 | 21·24 4 | 3 | 2·10| 10·69| 25·45 | 4 | 3 | 2·90| 13·83 | 23·84 4 | 4 | 2·65| 13·37| 25·23 | 4 | 4 | 3·30| 17·06 | 25·85 4 | 5 | 3·45| 17·61| 25·52 | 4 | 5 | 4·10| 21·57 | 26·30 4 | 6 | 3·55| 18·07| 25·45 | 4 | 6 | 4·20| 22·40 | 26·66 -------+----+-----+-------+--------+-------+----+-----+-------+-------- 5 | 2 | 1·77| 7·38| 20·85 | 5 | 2 | 2·60| 9·68 | 18·81 5 | 3 | 2·30| 11·90| 25·87 | 5 | 3 | 3·30| 13·64 | 20·67 5 | 4 | 3·30| 15·40| 23·33 | 5 | 4 | 4·00| 19·91 | 24·14 5 | 5 | 4·10| 20·74| 25·29 | 5 | 5 | 5·00| 25·36 | 25·36 5 | 6 | 4·30| 22·68| 26·37 | 5 | 6 | 5·30| 27·66 | 26·10 -------+----+-----+-------+--------+-------+----+-----+-------+--------

TABLE II.

----+----+----------------------+---------------------+------------------ | | AT 0·5-INCH | AT 1·0-INCH | AT 1·5-INCH | PRESSURE. | PRESSURE. | PRESSURE. | +-----+-------+------+-----+-------+-------+-----+-------+------ No. |No. |Cubic|Illumi-|Illum.|Cubic|Illumi-|Illum. |Cubic|Illumi-|Illum. of |of |Feet |nating |Power |Feet |nating |Power |Feet |nating |Power Top.|Bot-|per |Power. |per |per |Power. |per |per |Power. |per |tom.|Hour.| |Five |Hour.| |Five |Hour.| |Five | | |Cub. | | |Cub. | | |Cub. | | |Ft. | | |Ft. | | |Ft. ----+----+-----+-------+------+-----+-------+-------+-----+-------+------ A2 | 1 | -- | -- | -- | 1·5 | 2·7 | 9·0 | 2·0 | 4·0 | 10·0 " | 2 | 1·6 | 2·9 | 9·1 | 2·4 | 5·2 | 10·8 | 3·1 | 6·8 | 11·0 " | 2½ | 2·0 | 3·9 | 9·8 | 2·9 | 6·8 | 11·7 | 3·8 | 9·4 | 12·4 A3 | 3 | 2·1 | 4·4 | 10·5 | 3·2 | 7·8 | 12·2 | 4·4 | 10·6 | 12·0 " | 3½ | 2·5 | 4·8 | 9·6 | 3·8 | 9·2 | 12·1 | 4·9 | 12·2 | 12·4 " | 4 | 2·5 | 5·4 | 10·8 | 3·8 | 9·6 | 12·7 | 5·2 | 13·6 | 13·1 " | 4½ | 3·0 | 6·4 | 10·7 | 4·5 | 10·8 | 12·0 | 5·9 | 14·8 | 12·5 " | 5 | 3·2 | 7·7 | 2·0 | 5·1 | 13·2 | 13·0 | 6·8 | 18·0 | 13·2 " | 6 | 3·7 | 8·7 | 11·8 | 5·8 | 15·5 | 13·3 | 7·7 | 21·0 | 13·6 " | 7 | 3·5 | 8·6 | 12·3 | 5·9 | 16·0 | 13·6 | 8·4 | 23·0 | 13·7 " | 8 | 3·7 | 9·0 | 12·2 | 6·2 | 16·8 | 13·5 | 8·6 | 23·4 | 13·6 B1 | 1 | -- | -- | -- | 1·3 | 2·3 | 8·8 | 1·8 | 3·5 | 9·7 B2 | 2 | 1·3 | 2·3 | 8·8 | 2·1 | 4·4 | 10·5 | 2·8 | 6·4 | 11·4 " | 2½ | 1·6 | 3·0 | 9·4 | 2·5 | 6·0 | 12·0 | 3·4 | 8·4 | 12·4 B3 | 3 | 2·0 | 3·8 | 9·0 | 3·0 | 7·2 | 12·0 | 4·1 | 10·1 | 12·3 " | 3½ | 2·3 | 4·3 | 9·3 | 3·4 | 7·7 | 11·3 | 4·5 | 11·0 | 12·2 B4 | 4 | 2·3 | 4·7 | 0·2 | 3·6 | 8·8 | 12·2 | 5·0 | 13·0 | 13·0 " | 4½ | 2·7 | 5·9 | 10·9 | 4·3 | 10·4 | 12·1 | 5·6 | 15·0 | 13·4 B5 | 5 | 3·1 | 7·0 | 11·3 | 4·9 | 12·9 | 13·2 | 6·5 | 18·0 | 13·8 B6 | 6 | 3·8 | 9·6 | 12·6 | 5·9 | 16·4 | 13·8 | 8·0 | 23·0 | 14·4 B7 | 7 | 4·0 | 10·2 | 12·8 | 6·6 | 19·0 | 14·4 | 9·0 | 26·0 | 14·4 B8 | 8 | 4·7 | 11·8 | 12·6 | 7·3 | 22·0 | 15·1 | 9·6 | 30·0 | 15·7 ----+----+-----+-------+------+-----+-------+-------+-----+-------+------

[Sidenote: Pressure of gas with the Brönner burner.]

Doubtless the chief cause of the remarkable efficiency of the Brönner over previous burners is to be found in the pressure at which the gas flows from the burner and is consumed. In the course of some experiments made to determine the pressure at which gas is delivered from various burners, the writer found that from a No. 4 Brönner, with an initial pressure--_i.e._, the pressure at the inlet when the burner is in operation--of 1 inch, the gas issued at a pressure of only 0·05 inch; and with an initial pressure of 0·5 inch, the outlet pressure was only 0·03 inch. On the other hand, a No. 4 steatite flat-flame burner, without any arrangement for retarding the flow of the gas, under the same initial pressure gave at the outlet 0·16 inch and 0·05 inch respectively. The absence of anything within the burner to cause the gas to swirl, or to issue with an unsteady flow, must also be credited with contributing, in no slight degree, to the favourable results yielded by these burners.

THE HOLLOW-TOP BURNER.

In the hollow-top burner we have one of the most notable improvements which have been effected in flat-flame burners. A simple modification of the batswing--the earliest of flat-flame burners--it is not more complicated in its details than is that burner. Yet, simple as it is, its construction exhibits an important advance upon the original batswing. Indeed, this burner may be said to embody the only considerable improvement that has been made in the distinctive features of the batswing since the introduction of the latter burner, which, as we have seen, took place as early as the year 1816.

[Sidenote: The hollow-top an improved batswing burner.]

In its outward form, the hollow-top burner differs little, if at all, from the batswing; but a slight modification which has been adopted in the arrangement of its interior has produced a very marked result in improving the shape of the flame yielded by the burner, and, to some extent, in the results, as regards illuminating power, which it is capable of affording. In this burner, as its name implies, the interior of the top or head of the burner is hollowed out, forming an enlargement of the cavity or chamber within the burner, and (what is chiefly important) making the shell of the dome-shaped burner head of equal thickness throughout. In the ordinary batswing, in consequence of the varying thickness of the burner at this part, the slit is much deeper in the middle than at any other part of its length, and gradually decreases in depth towards each end. As the resistance to the passage of the gas, or the friction which it encounters, increases with the depth of the slit, the gas passes out from the burner at the ends of the slit more readily than in the middle; producing a wide-stretching flame, of scanty height in proportion to its width. From the same cause the flame is not of equal thickness throughout; being thinner in the middle than at the ends. Moreover, the outer extremities of the flame, extending so far beyond the body of the burner, are unable to retain the form given to them by the lateral flow of the gas at the ends of the slit; the resistance, presented by the atmosphere, together with the natural tendency of the gas to ascend, causing the under portion of the flame to fold back upon itself. As one result of this combination of untoward circumstances, the flame is liable to smoke with a slight agitation of the surrounding air.

In the hollow-top burner, the slit is of equal depth throughout its length; and the resistance offered to the passage of the gas being the same in all parts of the slit, the gas flows through the middle as readily as at the ends--nay, in reality rather more so, owing to the innate ascensive power of the gas, due to its being lighter than air. The peculiar hollowing-out of the head of the burner, also, withdraws the ends of the slit out of the direct course or current of the gas through the burner; so that the tendency of the stream of gas to issue at these points, in preference to going through the middle of the slit, is further checked. The consequence is that the shape of the flame is considerably improved; it being taller, more compact, and not so broad as that of the batswing. In addition, the flame being of equal thickness throughout, its illuminating power is somewhat improved; while, from its compactness, it is better enabled to resist atmospheric influences. With this alteration in the shape of the flame all original resemblance to a batswing is entirely destroyed; but the appearance of the flame of the new burner is much more agreeable to the eye than that of the older batswing.

[Sidenote: Who invented the hollow-top burner.]

As has been exemplified in so many instances in the history of invention, the hollow-top burner was not appreciated at its true value until long after it had been brought into existence. It appears to have been originally invented by Joseph and James Wadsworth, of Marple and Salford, and was patented by them in 1860. According to the specification of the inventors, the burners might be made either in solid or sheet metal, as will be seen from the accompanying illustrations, copied from the drawings in the specification. But there were difficulties in the way of casting the burners in solid metal which do not seem to have been surmounted; and those produced under the patent appear to have been made exclusively of sheet brass. For many years these burners were made and sold without their peculiarities attracting any marked attention; which would seem to imply that their faulty construction precluded the attainment of all the advantages afforded by the burner as we know it.

[Sidenote: Sugg's hollow-top burner.]

The superior results which the hollow-top burner was calculated to afford did not become fully apparent until the burner was made of non-conducting material, and greater care exercised in its construction. This appears to have been done in Germany earlier than in this country. But, in England, it was undoubtedly Mr. Sugg who first turned his attention to the improvement of the burner, and demonstrated its merits. Mr. Sugg commenced the manufacture of this burner in steatite in the year 1868; and since that time the burner has been extensively employed, and its advantages widely recognized. The superiority of hollow-top burners produced by Mr. Sugg to those previously manufactured, is chiefly the result of their being made in steatite instead of in metal. With this material, greater exactness and uniformity are obtained in the shape and dimensions of the burner than when metal is employed; besides which there is (what has been before referred to) the advantage arising from its inferior conductive capacity for heat, and its non-liability to corrosion. Another improvement, also due to Mr. Sugg, and which is productive of noticeable results, consists in cutting the slit of the burner with a circular saw, applied from above, so as to make the ends of the slit curved instead of horizontal; by which means the tendency of the gas to issue laterally at the ends of the slit, and form horns to the flame, is lessened. Mr. Sugg's table-top burner (which was introduced in 1880), in addition to the characteristic features of the hollow-top, has a rim-like projection from the burner, below the slit; its object being to protect the flame from the disturbing influence of the uprush of air in its immediate vicinity, and so preserve its shape unaltered, while diminishing its liability to smoke. Prior to Mr. Sugg--namely, in the early part of 1879--Mr. Bray had successfully obviated this injurious action upon the flame of the ascending current of air, by affixing to the burner two arms of brass, so placed as to be immediately under the projecting wings of the flame.

BRAY'S BURNERS.

The burners of Messrs. George Bray and Co. have deservedly acquired a world-wide reputation, and are in extensive use wherever gas lighting is known. Their distinguishing characteristic, and that which has won for them the high repute in which they are held, is the union of cheapness with remarkable efficiency. In all the various descriptions and classes of burners which are produced by this firm, the characteristic referred to is preserved; although it is needless to add that the different varieties are not equally efficient. Of the three forms of flat-flame burners we have been considering--batswing, union-jet, and hollow-top--the one which, more than any other, has been the speciality of the firm is the union-jet; and it is with the development of this class of burner that the name of Bray is most intimately and honourably associated.

[9] The name "slit-union," by which Mr. Bray prefers to designate this burner, he states to be derived from the resemblance of its flame to that of the union-jet burner; while it is produced by means of a slit.

[Sidenote: Bray's "regulator" burner.]

[Sidenote: Bray's "special" burner.]

The "regulator" union-jet, which was the first notable burner produced by Messrs. Bray, has received, perhaps, a wider application than any other single gas-burner. It consists of a cylindrical brass case, screwed at one end for insertion into the fittings, and at the other containing a tip of "enamel"--a material invented by Mr. Bray, and apparently of somewhat similar composition to the "adamas" of Mr. Leoni--the "enamel" tip being perforated, in the usual manner, with two holes, set at an angle to each other, for the outflow of the gas. The distinctive feature of this burner is the introduction into the lower part of the brass case of a layer, or layers, of muslin; designed to check in some degree, and to steady the current or flow of the gas through the burner. At the time of its introduction, this burner compared very favourably, as regards the results it yielded, with other burners in common use; and its fairly good performances, together with the very low price at which it can be sold, cause it still to be extensively employed wherever the attainment, from the gas consumed, of the highest obtainable results may be subordinated to cheapness, or in situations where, from delicacy of construction or from the care and attention demanded, a more efficient burner may not be so suitable. But in the matter of developing the illuminating power of the gas employed, the "regulator" is far surpassed by the more recently introduced "special" burner of the same makers.

Mr. Bray's series of "special" burners--embracing union-jet, hollow-top, and batswing--are constructed upon the principle of, and in form are somewhat similar to Brönner's burners, which have already been fully described. Apart from its being of greater bulk, the main divergence in the construction of the "special" burner from that of the earlier "regulator" is the introduction, into the lower part of the brass case, of a plug or washer of enamel, pierced by a small circular hole for the admission of gas into the burner; the diameter of this hole determining the quantity of gas which, at any particular pressure, is admitted into the burner. Just above the enamel washer, a layer of muslin is inserted, as in the "regulator" burner; which, in this case, is for the purpose of subduing the agitation, or swirl, acquired by the current of gas in passing through the narrow aperture in the washer. A tip of enamel, made of the particular description (union-jet, hollow-top, or batswing) required, fitting into the upper part of the brass case, completes the burner. The objects aimed at in the "special" burner are to cause the gas to be consumed at the lowest pressure compatible with the maintenance of a firm flame, and with the least agitation, or swirl, in the current of gas as it issues from the burner. The former is attained, as in Brönner's burners, by diminishing the area of the opening admitting into the burner, without a corresponding diminution of the orifices through which the gas issues into the atmosphere; the latter, by the interposition of the layer of muslin which is immediately above the diminishing arrangement, as well as by the enlargement of the gas chamber in the upper part of the burner. The improvement thus effected in the illuminating power developed from the gas is well shown in the following tables extracted from an exhaustive series of tests of gas-burners carried out by Mr. T. Fairley, F.R.S.E., Borough Analyst of Leeds, and embodied by him in a report presented to the Leeds Corporation. The full text of the report will be found in the _Journal of Gas Lighting_ for February 6, 1883.

_Medium Lighting Power Union-Jets._

-----------------------------------+----------------------------------- "Regulator" Burners. | "Special" Burners. ------+------+-----+-------+-------+------+------+-----+-------+------- No. |Pres- |Cubic|Illumi-|Illumi-|No. |Pres- |Cubic|Illumi-|Illumi- of |sure |Feet |nating |nating |of |sure |Feet |nating |nating Burner|in |per |Power |Power |Burner|in |per |Power |Power |Inches|Hour |in |per 5 | |Inches|Hour |in |per 5 | | |Stand. |Cubic | | | |Stand. |Cubic | | |Candls.|Feet. | | | |Candls.|Feet. ------+------+-----+-------+-------+------+------+-----+-------+------- 3 | 0·5 | 3·50| 6·8 | 9·7 | 3 | 0·5 | 3·43| 11·3 | 16·4 3 | 1·0 | 4·80| 6·9 | 7·2 | 3 | 1·0 | 4·90| 15·6 | 15·8 3 | 1·5 | 6·20| 7·5 | 6·05 | 3 | 1·5 | 6·03| 17·6 | 14·6 4 | 0·5 | 4·65| 12·2 | 13·1 | 4 | 0·5 | 3·73| 13·3 | 17·8 4 | 1·0 | 6·67| 14·2 | 10·6 | 4 | 1·0 | 5·15| 17·4 | 16·9 4 | 1·5 | 8·16| 14·2 | 8·8 | 4 | 1·5 | 6·57| 22·4 | 17·1 5 | 0·5 | 5·72| 17·0 | 14·9 | 5 | 0·5 | 4·80| 17·6 | 18·3 5 | 1·0 | 7·97| 20·0 | 12·6 | 5 | 1·0 | 6·67| 24·4 | 18·3 5 | 1·5 | 9·73| 21·8 | 11·2 | 5 | 1·5 | 8·30| 30·0 | 18·2 6 | 0·5 | 5·90| 18·0 | 15·2 | 6 | 0·5 | 5·48| 20·1 | 18·3 6 | 1·0 | 8·35| 23·0 | 13·8 | 6 | 1·0 | 7·65| 28·4 | 18·6 6 | 1·5 |10·60| 28·0 | 13·2 | 6 | 1·5 | 9·20| 34·2 | 18·7 ------+------+-----+-------+-------+------+------+-----+-------+-------

_Medium Lighting Power Slit-Unions._

-----------------------------------+----------------------------------- "Regulator" Burners. | "Special" Burners. ------+------+-----+-------+-------+------+------+-----+-------+------- No. |Pres- |Cubic|Illumi-|Illumi-|No. |Pres- |Cubic|Illumi-|Illumi- of |sure |Feet |nating |nating |of |sure |Feet |nating |nating Burner|in |per |Power |Power |Burner|in |per |Power |Power |Inches|Hour |in |per 5 | |Inches|Hour |in |per 5 | | |Stand. |Cubic | | | |Stand. |Cubic | | |Candls.|Feet. | | | |Candls.|Feet. ------+------+-----+-------+-------+------+------+-----+-------+------- 3 | 0·5 | 4·22| 13·8 | 16·4 | 3 | 0·5 | 3·04| 10·8 | 17·8 3 | 1·0 | 6·37| 20·2 | 15·9 | 3 | 1·0 | 4·61| 16·4 | 17·6 3 | 1·5 | 8·14| 25·8 | 15·9 | 3 | 1·5 | 5·88| 19·9 | 16·9 4 | 0·5 | 4·25| 14·8 | 17·4 | 4 | 0·5 | 3·82| 14·2 | 18·6 4 | 1·0 | 5·88| 20·6 | 17·5 | 4 | 1·0 | 5·69| 20·8 | 18·3 4 | 1·5 | 7·95| 26·5 | 16·6 | 4 | 1·5 | 7·35| 25·6 | 17·5 5 | 0·5 | 5·25| 19·0 | 18·2 | 5 | 0·5 | 4·12| 15·4 | 18·7 5 | 1·0 | 8·14| 28·4 | 17·45| 5 | 1·0 | 6·37| 23·4 | 18·4 5 | 1·5 |10·20| 36·4 | 17·8 | 5 | 1·5 | 7·94| 28·5 | 18·0 6 | 0·5 | 5·67| 22·2 | 19·6 | 6 | 0·5 | 5·00| 19·6 | 19·6 6 | 1·0 | 8·60| 33·6 | 19·4 | 6 | 1·0 | 7·55| 29·0 | 19·2 6 | 1·5 |11·10| 39·5 | 17·8 | 6 | 1·5 | 9·70| 37·0 | 19·1 ------+------+-----+-------+-------+------+------+-----+-------+-------

_Medium Lighting Power Batswings._

-----------------------------------+----------------------------------- "Regulator" Burners. | "Special" Burners. ------+------+-----+-------+-------+------+------+-----+-------+------- No. |Pres- |Cubic|Illumi-|Illumi-|No. |Pres- |Cubic|Illumi-|Illumi- of |sure |Feet |nating |nating |of |sure |Feet |nating |nating Burner|in |per |Power |Power |Burner|in |per |Power |Power |Inches|Hour |in |per 5 | |Inches|Hour |in |per 5 | | |Stand. |Cubic | | | |Stand. |Cubic | | |Candls.|Feet. | | | |Candls.|Feet. ------+------+-----+-------+-------+------+------+-----+-------+------- 3 | 0·5 | 4·16| 12·6 | 15·1 | 3 | 0·5 | 3·37| 12·4 | 18·4 3 | 1·0 | 5·64| 16·6 | 14·8 | 3 | 1·0 | 5·25| 20·4 | 19·4 3 | 1·5 | 7·83| 21·0 | 13·4 | 3 | 1·5 | 7·13| 24·0 | 16·8 4 | 0·5 | 4·26| 14·0 | 16·4 | 4 | 0·5 | 3·67| 13·0 | 17·7 4 | 1·0 | 6·74| 21·2 | 15·6 | 4 | 1·0 | 5·55| 20·6 | 18·6 4 | 1·5 | 7·81| 24·0 | 15·3 | 4 | 1·5 | 7·13| 26·0 | 18·2 5 | 0·5 | 4·76| 15·4 | 16·2 | 5 | 0·5 | 3·86| 14·6 | 18·9 5 | 1·0 | 6·93| 20·4 | 14·7 | 5 | 1·0 | 5·85| 22·6 | 19·4 5 | 1·5 | 8·72| 25·8 | 14·7 | 5 | 1·5 | 7·53| 28·0 | 18·6 6 | 0·5 | 6·04| 20·0 | 16·5 | 6 | 0·5 | 4·86| 19·4 | 20·0 6 | 1·0 | 8·82| 29·4 | 16·6 | 6 | 1·0 | 7·53| 31·6 | 21·0 6 | 1·5 |11·10| 31·6 | 14·2 | 6 | 1·5 | 9·60| 39·0 | 20·4 ------+------+-----+-------+-------+------+------+-----+-------+-------

The quality of the gas operated upon averaged about 19 candles when tested with the Standard London Argand Burner.

In a former part of this treatise it was remarked that the flames produced by the modern representatives[10] of the batswing and fishtail burners have lost the original resemblance to the objects whence the names of those burners were derived; and that the two flames have gradually approached each other in shape, until, in their latest developments, they are practically identical. We have seen how that, by the invention of the hollow-top, a burner is obtained apparently, to all outward appearance, the same as the batswing, yet giving a greatly improved form of flame. We have now to learn how the fishtail, or union-jet burner has been modified so as to yield a flame closely agreeing with that produced by the improved slit burner.

[10] Although the true batswing is still in common use, I look upon the hollow-top as being its "modern representative;" seeing that, in a great many instances, it has superseded the former burner--of which, indeed, it is only an improved form.

[Sidenote: How the union-jet burner has been improved.]

As first constructed, the union-jet burner gave a tall, narrow flame; its extremity being forked and jagged like the tail of a fish. Besides being unsightly, this form of flame was ill-adapted to develop, to anything like its full extent, the illuminating power of the gas. In order to obtain the best results, as regards illuminating power, the heat-intensity of the flame must be very high, so as to bring up the temperature of the particles of carbon liberated in the flame to the necessary degree of incandescence. To this end there must be concentration of the flame, in order to utilize to the full the heat of combustion. With the tall flame produced by the original union-jet burner there was too much exposure to the atmosphere for the flame to attain to the requisite intensity of heat; as well as considerable liability of the gas being brought too early into intimate contact with air, and so oxidized, or fully consumed, before its carbon had been raised to the temperature necessary to enable it to give out light. With the burner in its improved form the height of the flame is much curtailed, while it is broadened, and made more even and compact. This alteration has been chiefly brought about by two modifications in the construction of the burner-tip--first, by hollowing out its flat upper surface; and, second, by altering the angle at which the two streams of gas emerge from the burner. By scooping out the central portion of the flat top of the burner, so as to form a hollow or depression where the gas emerges, the flat sheet of flame which is formed when the two streams of gas impinge upon each other obtains a broader base, and at the same time is preserved from drawing air into its midst. But the chief share of the improvement is due to the alteration in the angle formed by the two channels in the burner-tip. It will be readily apparent that the more obtuse this angle--that is, the nearer the two streams of gas are to impinging against each other in a horizontal line--the more will the flame tend to spread out, or the lower the pressure required to obtain any desired spread of flame. It is by taking advantage of this circumstance that Mr. Bray has been enabled to improve the union-jet burner. Twenty years ago this burner was usually made with the two channels in the burner-tip placed at an angle of about 60°. In Bray's "regulator" burner, introduced in 1869, they were placed at an angle of 90°; with the result of obtaining a more satisfactory flame, both as regards its appearance and illuminating power. In the "special" burner, which was not brought out till 1876, the angle is increased to 120°; thus enabling the necessary spread of flame to be obtained with the gas issuing at a low pressure. Another minor improvement in the latter burner consists in making the holes in the burner-tip elliptical instead of circular.