358 FLAMES PUT OUT BY GOOD CONDUCTORS. duce it; consequently the temperature of the charcoal is soon reduced to below the kindling-point. A result similar to the foregoing is obtained when any fire is broken up and scattered about to such an extent that its several portions cannot assist in one another's combustion-though if, instead of being placed upon iron, the separate glowing coals be laid upon ashes or dry earth, they will be extinguished only after a much longer time, for ashes and dry earth are very poor conductors of heat in comparison with iron. In all ordinary fires the heat evolved by the combustion of the fuel is more than sufficient to maintain a temperature higher than the kindling-point of the fuel-though, generally speaking, the fuel becomes at last so clogged with ashes, that the oxygen of the air cannot get at the remaining combustible matter in sufficient quantity to maintain lively chemical action. 435. Precisely as coals can be extinguished by placing them upon cold metal, so flames may be put out. Fig. 71. Exp. 213.-Upon a ring of the iron stand place a sheet of clean wire gauze about 10 c.m. square; lower the ring so that the gauze shall be pressed down upon the flame of a lamp or candle almost to the wick, as shown in Fig. 71. No flame will be seen above the gauze, but instead of flame a cloud of smoke. The gauze is a mere open sieve; there is nothing about it which can prevent the gas, which was just now burning with flame above the wick of the candle, from passing through. Indeed it may be seen from the smoke that the particles of carbon which, in the original undisturbed flame, were becoming incandescent, and so affording light, do now actually come through the gauze. The explanation of the phenomenon is simply that the metallic sieve conducts away so much heat that the temperature of the candleflame is reduced to below the kindling-point. That this is really so is proved by the fact that, after the gauze has become sufficiently heated by long-continued contact with the flame below-after it has attained the kindling-point of the candle-gas, it will no longer extinguish the flame. In like manner, a candle-flame may be cooled to such an extent that it will go out, by placing over it a small coil of cold copper wire, while if the wire be previously heated the flame will continue to burn. If the smoke and unburned gas which has passed through the cold wire gauze be touched with a lighted match, and so brought to the kindling temperature, it will burst into flame. The power of wire gauze to prevent the passage of flame has been usefully applied in several ways, notably for the prevention of explosions in those coal-mines which are liable to accumulations of marshgas. For this purpose safety-lamps are constructed by enclosing an ordinary oil-lamp completely in wire gauze, so that the flame within the gauze cannot kindle any combustible or explosive gas into which it may be carried. In case such a lamp be carried into a place filled with explosive gas, the latter will, of course, pass into the lamp through the meshes of the gauze and burn within the cage. This combustion gives warning of the presence of the dangerous gas, and indicates to the workman that he should withdraw from the locality; the gas can then be expelled by appropriate methods of ventilation. The wire-gauze lamps employed in chemical laboratories (for one form of which see Appendix, Fig. viii.) are simply applications of the same general idea. Erp. 214.-Beneath the sheet of wire gauze of Exp. 213, place an unlighted ordinary gas-lamp (Bunsen's burner), at such distance that the gauze shall be 3 or 4 c.m. above the top of the lamp; turn on the gas and light it above the wire gauze; it will continue to burn on the top of the gauze for an indefinite period, for the gauze will, in this case, always be kept cool by the cold gas which is continually passing through it. Carefully and gradually lift the ring which carries the gauze, and determine how far it is possible to lift the gauze above the gas-jet without extinguishing the flame. The student will remember that other experiments illustrating the influence of cooling agencies in extinguishing combustion (Exps. 136 and 154), have already been performed. Compare also Exp. 196 and § 200, as regards kindling. 436. An effect somewhat similar to that produced by wire gauze is often seen in ordinary fires. When a mass of red-hot anthracite, charcoal, or coke is burning freely upon a grate in the open air, there is always a blue flame of carbonic oxide burning above the coal. This gas results from the reduction of carbonic acid by means of hot carbon, precisely as in Exp. 181. Air enters at the bottom of the grate and combines with the hot coal which it finds there to form carbonic acid (CO). This carbonic acid, as it rises through the hot coal in the middle of the fire, is deprived by the heated carbon of half its oxygen, 200, so that two molecules of carbonic oxide gas finally emerge at the top of the coal, instead of the single molecule of carbonic acid which was formed at first. The carbonic oxide being combustible, will at once take fire on coming in contact with the air, provided the temperature at the summit of the fire be equal to the kindling-temperature of carbonic oxide. But if the temperature of the fire is in any way reduced below this point, as, for example, by throwing on too large a quantity of cold fuel, which is, of course, equivalent to covering the fire with a sheet of wire gauze, then the carbonic oxide will be extinguished, and, escaping into the chimney, will produce no useful effect. Were it not for the formation of carbonic oxide, as above mentioned, neither anthracite coal, nor coke, nor charcoal would burn with flame after having once got well on fire; they would simply glow, as a single live coal, or a bar of metal, glows when taken from the fire. 437. In heating steam-boilers and other large vessels, it is often a point of great importance to obtain from the fuel a large flame, in order that the heat from the fuel may be quickly distributed and brought into contact with the matter to be heated. With anthracite and coke this result is effected by placing beneath the grate, upon which the fuel is burned, a quantity of water. From this water steam gradually rises, as hot ashes and cinders fall into it and as heat radiates down upon it from the fire above. The steam, as it enters the fire, is decomposed by the hot coal (see Exp. 156), in accordance with the following reaction, C + H2O = CO + 2H, and the combustible gases thus obtained are superadded to the carbonic oxide which is formed in due course from the action of air upon the coal. All these gases burn again to carbonic acid and water above the fire, where air is thrown in to meet them through appropriate orifices. In this use of water as an adjunct to the combustion of coal, the absolute amount of heat given off by the fuel is in nowise increased; but in many instances much heat may undoubtedly be saved by thus equally distributing and applying it by means of flame. 438. On the other hand, furnaces are sometimes seen con CARBONIC OXIDE SHOULD BE BURNED. 361 suming fuel under such conditions that all the carbonic oxide produced within them escapes unburned into the chimney. In such cases, more than two-thirds of the amount of heat which the fuel is capable of yielding must necessarily be lost; for while 1 gramme of charcoal gives off in burning to carbonic acid 8080 units of heat (§ 55), 1 gramme of carbon in burning to carbonic oxide gives off only 2473 units of heat. The number last given is determined as follows. It has been found, by direct experiment, that 1 gramme of carbonic oxide, on being burned to carbonic acid, yields 2403 units of heat; carbonic oxide is composed (§ 422) of one atom of carbon, weighing 12, and one atom of oxygen weighing 16,-the weight of the molecule of carbonic oxide being consequently 28. In one gramme of carbonic oxide, therefore, there can be only 120-4286 of a gramme of carbon; but 0.4286: 1=2403 : x=5607, whence it appears that there is evolved by one gramme of carbon in carbonic oxide 5607 units of heat when this carbon unites with the additional oxygen to form carbonic acid; and the difference between this number (5607) and the number (8080) denoting the amount of heat given off by one gramme of charcoal in burning to carbonic acid will show how much heat is evolved by one gramme of carbon burned to carbonic oxide: 8080-5607=2473, as above stated. In order to thoroughly burn the carbonic oxide in any case, the stove or furnace should be so arranged that a volume of air, as large as that which has already passed through the fire, can be constantly supplied to the carbonic oxide and nitrogen as they emerge from the coal, and be intimately mixed with these gases while they are still hot. 439. The amount of air needed for the complete combustion of coal or other fuel can always be readily calculated. We have only to determine how much oxygen will be needed by the combustible, and then how much air must be taken in order to supply this oxygen. Let it be supposed, for example, that we wish to learn how much air is needed in order to burn one kilogramme of charcoal. Having learned the full significance of the formula CO,, a moment's consideration of this formula informs us that, for every 12 parts by weight of carbon, 32 parts by weight of oxygen are needed in order to its complete combustion, or, for one part of carbon, 2.67 parts of oxygen. Air contains 23.1 per cent. of oxygen by weight; hence the proportion 23·1: 100= 2.67: x 11.558, from which it appears that to burn 1 kilo. of charcoal, 11.558 kilos. of air are needed. Since the weight of a litre of air, at the ordinary temperature, is only 1.2258 grm., these 11.558 kilos. of air will occupy about 9429 litres, or, in other words, nearly 9 cubic metres. In round numbers, it may be said that about 12 kilos., or 9 to burn one kilo. of charcoal. tions must, of course, be made for the ashes which they contain, as well as for a certain portion of hydrogen which may also be present. If a gramme of pure carbon will disengage 8080 units of heat, a gramme of well-burned coke, containing 15 per cent. of ashes, will disengage only 6868 units. cubic metres, of air are required For coke and anthracite, correc 440. Chloride of Carbon (CC1).—Chlorine does not unite directly with carbon; but several compounds of the two elements can be obtained by subjecting compounds of carbon and hydrogen to the action of chlorine. Of these compounds, only the socalled bichloride (CC1) need here be mentioned, the others being usually treated of in works upon organic chemistry. Bichloride of carbon may be obtained by the action of chlorine on marshgas, by subjecting chloroform or wood-spirit to the action of an excess of chlorine in sunlight, by passing a mixture of bisulphideof-carbon vapor and chlorine through a red-hot porcelain tube, or by the action of quinquichloride of antimony upon bisulphide of carbon : = CS2 + 2SbC1, CCl + 2SbC1, + 28. At the ordinary temperature of the air it is a transparent, colorless liquid, of pungent aromatic odor, boiling at 77°, and having a specific gravity of 1.56. At -23° it solidifies in the form of crystals of pearly lustre. The specific gravity of its vapor has been determined to be 76.96, which would indicate that a molecule of the vapor is composed of 1 atom of carbon and 4 volumes of chlorine, condensed to 2 unit-volumes : For, since the weight of one atom of carbon is The weight of the two volumes of gas produced would be 12 142 |