COMBUSTION EUGENE W. BLANK,' STATE COLLEGE, PENNSYLVANIA The Arabian chemist Geber as early as the eighth century stated that the calcination of mercury must he carried out in open vessels to secure the expected results. Early in the sixteenth century Leonardo da Vinci wrote, "there is smoke in the center of the flame of a burning candle because the air which enters into the composition of the flame cannot penetrate to the center. It stops a t the surface of the flame and condenses there." Later Robert Boyle noticed that a burning candle was quickly extinguished in an exhausted vessel. Subsequent experiments showed that air is necessary for comhustion. John Mayow, in a historic experiment, burned a candle in a glass receiver over water. The flame of the candle soon expired and the water level quickly rose in the jar. From this experiment Mayow rightly concluded that air is a mixture of at least two gases. Subsequently he repeated the experiment using a mouse instead of a candle. He found, as before, that the water level in the jar slowly rose and that it was impossible to ignite a combustible body in the residual gas. Mayow did not quite grasp the full significance of the phenomena with which he was dealing, but he came very neaR the present theory of combustion. The Arabian chemist Geher knew that the product formed when lead is roasted in the air weighs more than the original metal. The increase in weight was puzzling to him. It was not until 1630 that Jean Rey made some interesting experiments on this subject that were GAS extended by Antoine Laurent Lavoisier with AIR both decisive and far-reaching results. By FIGURE 1.-ARRANGEMENT REheating mercury for several days in a sealed retort he was able to show that air is composed of the two gases, oxygen and nitrogen, of which tbe'oxygen alone combines with the mercury. Following this line of work Lavoisier, in 1777, brought forth the oxidation theory of combustion stating that oxygen is necessary for combustion and that the increase in weight of the substance burnt is equal to the decrease in weight of the atmospheric air. By comhustion is usually meant some chemical combination accompanied by the evolution of both heat and light. Were we to adhere strictly to the * Winner of $10 award in undergraduate contest closing February 15, 1930. 1159
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theory of combustion of Lavoisier many chemical reactions would be excluded as not being cases of true oxidation. Hence it is customary to apply the term to any chemical reaction accompanied by the emission of thermal and actinic energy even though oxygen may not figure in the chemical change. Thus if a candle be lowered into a bottle of chlorine gas, comhustion continues, although the phenomenon undergoes a change. Volumes of black soot are evolved and the flame becomes orange in color, while a t the same time the greenish tinge of the chlorine gas is replaced by a white mist of hydrogen chloride gas. This is a case of combustion, in our sense of the word, even though no oxygen figures in the reaction. In most chemical reactions, however, oxidation plays an important r61e due to the fact that oxygen is the most chemically active constituent of the atmosphere in which we live. Combustion may take place with all conceivable velocities, occurring instantaneously or requiring many years. Practically every one is familiar with the extreme force and rapidity with which electrolytic gas explodes in contact with an open flame. Yet it has been observed that the same mixture of gases combines extremely slowly a t room temperature. As the temperaFIGURE2.-OXYGEN GAS BURNINGIN AN ture is the rate ATMOSPHERE COALGas of comhustion slowly increases and becomes a measurable phenomenon a t about 450°C. The slow decay of wood and the sudden detonation of an explosive are extreme opposite degrees of rapidity of comhustion. It is usual to speak of the huming substance as the combustible, and the atmosphere surrounding i t as the supporter of comhustion. This choice of terms is merely arbitrary for, from the chemist's point of view, it does not matter whether coal gas hums in air or air in coal gas. There are several ways of showing this reversed or reciprocal combustion. For lecture purposes an apparatus of the type shown in Figure 1 may be
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used. The coal gas is ignited a t the top of the apparatus and air is drawn in a t (B). By carefully inserting a lighted splint through (3) a flame appears a t the top of ( A ) whence we have the phenomenon of air burning in coal gas. This phenomenon may be shown in a more spectacular manner by inserting a deflagrating spoon of burning potassium chlorate into an atrnos~hereof coal gas. (See Fimre 2.) Combustion is greatly facilitated by the fineness of division of the reacting members. Pyrophoric powders consist of reduced metals in such an extremely finely divided condition that they inflame when tossed into the air. Lead, iron, and nickel furnish such pyrophoric metals. Finely divided organic matter suspended in the air forms a highly explosive mixture. Serious explosions, the so-called "dust explosions," have occurred in numerous factories due to this cause. Most substances reauire an initial temL perature rise before combustion can start &,RE 3,-ARRANGEMENT OF OXYGEN and successful~ycontinue. Many reactions APPARATUSroR BURNING are known in which the substances inflame IN COALGAS spontaneously with the evolution of light. Various organic compounds, as zinc ethyl and silicoLuminous Zone ethane, ignite spontaneously in contact with the air. Such reNon Luminous actions are examples of spontaInner Zone neous combustion. The temperature a t which rapid combustion becomes independent of external supplies of heat is known as the ignition temperature. In other words, the ignition, or kindling temperature as i t is variously called, is the temperature to which the substance must be heated in OP THB CANDLE FLAME order to start combustion or exFIGURE 4.-THE PARTS
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plosion. - Substances that are already in combustion can be extinguished by cooling below the ignition point. Thus a candle flame may be extinguished by placing a helix of copper wire about the flame. Sparks below 0.22 mm. in length will not ignite a mixture of hydrogen and oxygen gases. The ignition point of oils is an important physical property and is known as the flash point. It is hard to distinguish successfully between a rapid combustion and an explosive combustion. As renards aaseous mixtures an explosioh mav be defined as a reaction which proceeds with a rise of temperature and an ever-increasing velocity until the explosion travels with a uniform maximum rate. This maximum speed has a definite value for each explosive mixture under certain defined conditions. This constant is known as the velocity of the detonation wave and is usually several times as high as the speed of sound in the same mixture of gases. The candle ranks high as an object illustrative of the principal points of combustion. The great Faraday made it the subject of a series of lectures. I n the candle the fuel is usually solid paraffin wax that is gradually liquefied by the heat of &ombustion and rising by capillary attraction reahes a hotter zone where i t is vaporized and ignited. If a candle be suddenly blown out and the rising vapors brought into contact with a flame they will ignite and, flashing downward, will relight the candle. As the flame continues to burn, cold air is drawn inward and upward, cooling the outer edge of the candle and thus providing a shallow FIGURE5.-TAPPING THE INNERZ o m OF A CANDLE depression to hold the liquefied fuel. F L A M EP O R V A P O R I Z E D Four zones may be distinguished in a candle FUEL flame, namely: (1) An inner, non-luminous zone which consists of vaporized fuel that has not yet come intoeontact with the surrounding atmosphere. A short glass tube placed in this zone will allow some of this vapor to be drawn off and it may be ignited a t the upper end of the glass tube. (Figure 5.) (2) A luminous mantle where chemical reactions have begun to take place. (3) A non-luminous outer mantle where the carbon and hydrogen are completely oxidized by an excess of air to carbon dioxide and water. This is the hottest portion of the flame.
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(4) Lastly a small blue zone below the wick. Wicks for candles are plaited of several cords, one of which is under greater tension than the others. Thus, as the candle burns, the wick curls until the top is in the non-luminous outer mantle where it gradually burns away. If the wick did not curl and burn off it would project into the inner cone of the flame, interfering with proper combustion, and the candle would then require snuffing. A flame is a mass of gas raised to incandescence by heat. Its formation is not essential to combustion however, since we can have combustion take place with very little flame. It is of interest to inquire the causes of the luminosity of a candle flame. Formerly the theory of Davy, that the luminosity of a candle flame is due to the separation and raising to incandescence of free carbon particles, was considered correct. Frankland, in some experiments on the relation between luminosity and pressure, came to the conclusion that the luminosity of ordinary flames is due . to the glow of dense hydrocarbons r rather than to the presence of solid carbon particles. This is the generally accepted explanation. When a stream of gas issues from a circular orifice the gas can only burn a t its surface of contact with the surround1 ing air. The interior of the flame which does not come into contact with the air , is unburnt gas and is relatively cool. FICURE~.-THE PRINCIPLE OP THE Such is the case in the common Bunsen burner. A brief examination of the flame is all that is needed to show that i t consists of two parts, an inner zone of unburnt gas and an outer non-luminous mantle of vigorous combustion. Various experiments have been devised to show that the inner portion of a Bunseu flame is cool. A square of asbestos'paper is painted with red mercuric iodide and quickly pressed down on the flame. A yellow ring indicates the hottest portion of the flame. Gunpowder and matches may be placed in the center of the flame without being ignited.
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In 1859 Frankland made some experiments with burning candles to determine the influence of pressure on the rate of combustion and luminosity. He found that differences in pressure do not materially affect the rate of combustion. However, he did discover that the diminution of illuminating power is directly proportional to the diminution of atmospheric pressure. For every fall of 1 inch of the mercury barometer the luminosity falls by 5.1 per cent. The law is applicable to pressures up to three atmospheres but a t still higher pressures the luminosity rapidly increases. If a square of wire gauze be fixed some distance above a Bunsen burner the gas may be ignited above the Gauze gauze without striking back to the orifice of the burner. If the gas be ignited below the gauze i t will not catch fire above until the gauze becomes hot. Such gauze experiments ledHumphty D a y , in 1815, lass to the invention of the mining lamp. Numerous modifications have been made A from time to time hut the underlying principleremains the same. A n oil-fed wick is enclosed by a glass chimney, the upper portion of 7.-CONSTRUCTION OF A MODERN FIGURE which is made of wire gauze. M I N I N G LAMP Should any marsh gas be present in the atmosphere the flame will commence to flicker and with greater quantities of the gas present will assume a curiously elongated shape. The peculiar appearance of the flame is due to the formation of a flame-cap of burning firedamp which surmounts the flame proper. From the work of E. B. Whalley and W. M. Tweedie i t is now possible to form a fair estimate of the amount of marsh gas present in a mine from the elongation of the flame cap of a lamp that has been standardized in an atmosphere containing a known quantity of the gas. If a piece of platinum wire be heated to redness and then placed in a stream of coal gas and air issuing from a Bunsen burner, the wire will
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continue to glow brightly. The coal gas is uniting with the oxygen of the air directly on the surface of the platinum with the liberation of sufficient heat to raise i t to incandescence. This is a case of combustion without the formation of a free flame and is known as surface combustion. This phenomenon gives rise to a very interesting experiment. A short platinum wire is heated and lowered into a beaker containing several cubic centimeters of methyl alcohol. The whole is covered with a sheet of filter paper to prevent the alcohol frbm catching fire. Under ,these conditions the platinum glows brigbt1y.a~the alcohol burns on its surface. In the Welsbach mantle a like phenomenon takes place. The mantle is so arranged as to become heated to incandescence by the burning gas. The mantles are made by saturating frail cotton fabrics with a mixture of thoria and ceria. For protection in transit the mantles are dipped in collodion and dried. ~
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Acknowledgment The writer acknowledges his indebtedness to the following references: "Modern Inorganic Chemistry," by J. W. Mellor, Longmans, Green and Co., New York, 1922. "The Chemistry of Combustion." by J. N. Friend, Gurney and Jackson. London. ~
1922.
Articles on Combustion and Flame in the Encyclopaedia Britannica. 14th edition, New York, 1929. "The Chemical History of a Candle," by MicHacl Faraday, Volume 30, The Harvard E Classics, New York.
Liquid Crystals and Chemical Constitution. The property possessed by certain chemical compounds of forming liquid crystals has been studied by D. Vorlinder, who attempts in two papers in the November issue of the Berichle der Deutschen Chemischen Gesellschuft to correlate this property with certain deductions as to the relative orientations of two or more long chains of atoms to one another. It is shown that the nonappearance of liquid crystals in such derivatives of urea and thio-urea as are obtained by introducing two fi-amino-p'-ethoxyazobenzene groupings into the molecule may be due t o the angular divergence of these two long groups, which are linked together by the carbonyl or thiocarbonyl group a t an angle of log0. When, however, these compounds are converted t o the corresponding diimide by removing water (or hydrogen sulfide), this divergence disappears, since the double linking of each nitrogen atom t o the central carbon atom results in a linear confiauration -N=C=Nand the pound can exist in the liquid crystalline condition. This evidence is c a n h e d from a study of the esters which trimesic acid and the three phthalic acids ~ r o d u c ewith such lengthy chain compounds as 9-phenetoleazophenol. The star-shaped orientation of the trimesic esters and the angular divergence (609 of the chains in the ortho-esters seem t o preclude the possibility of liquid nystal formation, The meta esters with a divergence of 120' melt to liquid crystals which persist only over a short temperature range, while the para esters, in which the two chains are said to lie in a straight line, are described as supracrystalline, that is to say, the liquid crystalline condition is so stable that the amorphous liquid state is not even attained.-Nature
