A review of gas-burners would scarcely be complete without some reference to the incandescent burners of M. Clamond and Mr. Lewis. Although their dependence upon an artificially produced blast or current of air removes them from the list of appliances applicable to ordinary conditions, the remarkable results which they afford, not less than their originality, demand for them at least a passing notice. The production of light by the agency of these burners is brought about in a manner altogether different, and is due to quite other causes than those which are concerned in the production of an ordinary illuminating gas flame. In the latter case, the illuminating power developed is solely due to the hydrocarbons contained in the gas, which are decomposed by the heat of the flame, the separated carbon being raised to a white heat. In the former, the illuminating power is not obtained directly from the gas; but advantage is taken of the heat of the flame, enhanced by the application of a blast of air, to raise to incandescence some refractory foreign material, which latter is thus made to give out light. In the Clamond burner this refractory substance is a basket composed of magnesia, spun into threads; in the Lewis burner it is a cage of platinum wire.
To the unthinking reader it may perhaps appear somewhat surprising that results so remarkable as are yielded by these burners should be obtained, while disregarding, as a source of light, the hydrocarbons contained in gas, and employing them, in common with the other constituents, solely as a source of heat. An explanation, however, is readily forthcoming. As was shown in a former part of this treatise,[ [16] the great bulk of ordinary coal gas consists of constituents which, in the act of combustion, produce considerable heat, but scarcely any light; the illuminating power developed in an ordinary gas flame being almost wholly dependent upon the very small proportion of heavy hydrocarbons which the gas contains. Thus, the quantity of heat-producing elements contained in the gas being quite disproportionate to the light-yielding hydrocarbons, there is always produced, in an ordinary gas flame, more heat than is necessary for effectively consuming the free carbon, which is liberated in the flame by the decomposition of the heavy hydrocarbons. This is shown by the fact that coal gas can usually be naphthalized—that is, impregnated with the vapour of naphtha—to a considerable extent before the limit of effective combustion is reached. The object aimed at in the incandescent burners about to be described is to utilize, in the development of illuminating power, the combined heat produced by the combustion of all the constituents of the gas. To this end the heat of combustion is brought to bear upon, and caused to raise to incandescence, some refractory material, extraneous to, but brought within the operation of the flame.
Effect of injecting a blast of air into a gas flame. A further explanation of the superior results yielded by these burners may be found in the employment of an artificial blast or current of air. Indeed, without some such arrangement the desired end could not be attained. The heat developed by the unaided flame is diffused over too wide an area to raise the temperature of the heated substance to the necessary degree of incandescence to enable it to give out sufficient light. By injecting a current of air into its midst, the flame is condensed into a smaller compass; and is brought to bear more directly upon the precise locality where its heat may be most effectively employed. Thus, although the total quantity of heat developed remains exactly the same as before, it is concentrated upon a smaller surface of the refractory substance; and the latter is consequently more intensely heated, or, in other words, raised to a more exalted temperature. The very superior illuminating power which is thereby obtained is due to the circumstance that the quantity of light yielded by an incandescent body increases in a higher ratio than the temperature to which it is raised.
Lewis's incandescent gas-burner. Proceeding now to describe the burners. The one invented by Mr. Lewis (various forms of which are illustrated on the next page) consists of an upright tube, connected at its base to the gas supply, and surmounted by a cap or cage of platinum wire gauze; which latter constitutes a combustion chamber, as it is there that the mixture of gas and air is consumed. Into the lower part of the upright tube the nozzle of an air-pipe is inserted, through which a supply of air can be injected, under pressure, into the burner, after the manner of a blowpipe. There are also small branch tubes leading into the upright gas-tube, and open to the atmosphere. Through these an additional quantity of air enters the burner; being drawn or sucked in by the agency of the main current, which flows through the upright tube. The resemblance to an ordinary Bunsen burner is, therefore, very close. The mixture of gas and air thus produced, when ignited, burns at the platinum cap; the heat which is developed causing the latter to become highly incandescent, and so to give out a brilliant light. To prevent the conduction of heat from the incandescent platinum, through the upright tube, a non-conducting material—such, for instance, as steatite or porcelain—is interposed between the gauze cap and the metal tube.
Fig. 26.—Lewis's Incandescent Gas-Burner.
The light produced by this burner is said to approximate more closely to daylight than that yielded by an ordinary gas flame (the colours of textile fabrics, for instance, being shown as well by its aid as by daylight); while, on account of its resulting from the incandescence of a fixed body, instead of being emitted from a flame, it is unaffected by a gust of wind, and maintains perfect steadiness under every condition of weather. The illuminating power developed is stated to be equal to 5 standard candles per cubic foot of gas consumed.
Clamond's incandescent gas-burner. M. Clamond's burner, which is shown in fig. 27, is a much more complicated apparatus than the preceding one, and not so easily described; but its main features may be briefly enumerated as follows:—The air (which, as in Mr. Lewis's burner, is supplied under pressure) is divided, as it enters the apparatus, into two portions. One portion is at once mixed with the gas; the remainder being conveyed, through a peculiarly constructed tube composed of small pieces of refractory material, to the combustion chamber, or "wick," as it is termed, of the burner. This "wick" is a small conical basket, made of a kind of lacework of spun magnesia, which, when raised to incandescence by the heat produced by the combustion of the gas, furnishes the desired illumination. The mixture of gas and air is subdivided, by a "distributor," into two portions, one of which goes direct to the magnesia "wick," there to be burnt, while the other is distributed among a number of tubes, forming so-called "auxiliary burners," the flames of which are utilized to heat the chief air supply; being directed upon the sides of the before-mentioned tube of refractory material, through which it is conveyed. By this means the air is raised to a very high temperature (1000° C., or 1800° Fahr., it is said) before it impinges upon the flame. The result is the production of a most intense heat within the magnesia basket; the latter being raised to brilliant incandescence, and so developing a high illuminating power.
