LECTURE VII. FLAMES AS ILLUMINANTS. 56. Two main classes of illuminants exist: those producing radiation by the conversion of the chemical energy of com- bustion— the flames — and those deriving the energy of radia- tion from electric energy — the incandescent lamp and the arc lamp, and other less frequently used electric illuminants. Flames. To produce light from the chemical energy of combustion, almost exclusively hydrocarbon flames are used, as the gas flame, the candle, the oil lamp, the gasolene and kerosene lamp, etc.; that is, compounds of hydrogen and carbon or of hydrogen, carbon and some oxygen are burned. The hydrogen, H, com- bines with the oxygen, 0, of the air to water vapor, H20, and the carbon, C, with the oxygen of the air, to carbon dioxide, C02; or, if the air supply is insufficient, to carbon monoxide, CO, a very poisonous, combustible, odorless gas (coal gas), which thus appears in all incomplete combustions and is present, also, as intermediary stage, in complete combustion. The mechanism of the light production by the hydrocarbon flame I illustrate here on the luminous gas flame : where the gas issues from the burner into the air, it burns at the surface of the gas jet. By the heat of combustion the gas is raised to a high temperature. Most hydrocarbons, however, cannot stand high temperatures, but split up, dissociate into simpler hydro- carbons very rich in hydrogen : methane, CH4, and in free carbon. The carbon particles formed by this dissociation of hydrocar- bon gas float in the burning gases, that is, in the flame, and are raised to a high temperature by the heat of combustion of the gases, thereby made incandescent, and radiate light by tem- perature radiation; until ultimately, at the outer edge of the flame, they are burned by the oxygen of the air, and thus destroyed. We can see these carbon particles, which, floating 128 FLAMES AS ILLUMINANTS. 129 in the flame in anjncandescent state, give the light if, by passing a cold porcelain or glass plate through the luminous flame, we suddenly chill it and thereby preserve the carbon particles from combustion; they appear then on the plate as a carbon deposit, soot or lampblack. The light given by the luminous hydrocarbon flame thus is due to black-body radiation, and the flame makes its own radiator, and afterwards destroys it by combustion. To give a luminous flame, the hydrocarbon must be suffi- ciently rich in carbon to split off carbon at high temperatures. Thus methane, CH4, does not give a luminous flame, since it con- tains the smallest amount of carbon which can combine with hydrogen, and therefore does not deposit carbon at high tem- peratures. Ethylene, however, C2H4, which is the foremost light giving constituent of illuminating gas, dissociates in the flame into CH4 and C, and thus gives a luminous flame, as half of its carbon is set free and gives the incandescent radiator. If, however, the hydrocarbon is very rich in carbon, the amount of deposited carbon becomes so large that the energy of combustion of the remaining hydrocarbon is not sufficient to raise the carbon to very high temperatures, the luminosity therefore again decreases, the flame becomes reddish yellow, and a large amount of carbon escapes from the flame uncon- sumed, as smoke or soot, that is, the flame becomes smoky. To show you this, I pour some gasolene and some benzol in small glass dishes. The gasolene, having 2J hydrogen atoms per carbon atom, burns with a luminous flame and very little smoke. The benzol, having only one hydrogen atom per carbon atom, burns with a reddish-yellow flame, pouring out masses of black smoke. The proportion between the hydrogen and carbon required to give a luminous non-smoky flame, therefore can be varied only within narrow limits: too little carbon gives a less lumi- nous or non-luminous flame, too much carbon a smoky reddish flame. Hydrocarbons exist having almost any proportion between hydrogen and carbon, from a maximum of four hydrogen atoms to one carbon in methane, CH4, to practically pure carbon in anthracite coal. Some of them are shown in the following table, with the number of hydrogen atoms per carbon atom 130 RADIATION, LIGHT, AND ILLUMINATION. added in column a, and the percentage of carbon which is de- posited by dissociation, in column b ;* a thus may be called the luminosity index of the hydrocarbon. HYDROCARBONS. Name. State. Formula. Lumi- nosity Index (a). Car- bon Index (ft). Paraffines: Methane Gas. do.. CH4 C2H6 C3H8 r4H10 ^5n!2 C«H14 C10H22 ^14^30 C20H42 U24±150 C2H4 C2H2 C.H6 C10H8 C14H10 approx. 4.0 3.0 2.67 2.5 2.4 2.33 2.2 2.14 2.1 2.08 2 1 1 0.8 0.71 0 0.25 0.333 0.375 0.40 0.417 0.45 0.464 0.475 0.479 0.50 0.75 0.75 0.80 0.821 Ethane Propane . do... Butane . . do... Pentane . ... Liquid. ..do.. . . .do.... Gasolene Kerosene Mineral oil Vaseline .. .do.... Solid, do Paraffine Olefines: Ethylene Gas. Gas. Liquid. Solid, do.. Acetylenes: Acetylene Benzols: Benzol Naphthalene Anthracene 57. The proportion between carbon and hydrogen required to give a luminous non-smoky flame somewhat depends on the size of the flame, and, with a larger size, a higher proportion of hydrogen is required to avoid smoke than with a smaller flame, as in the latter, due to the larger surface compared with the volume, the combustion is more rapid. I show you this on the, gas flame : admitting a little gas, I get a small flame, which does not smoke, but if I open the stop-cock wide I get a large and smoky flame. With a moderate-sized flame without artificial ventilation, from 30 to 40 per cent of the carbon must be deposited to give good luminosity without smoke. This corresponds to a value a * Every four hydrogen atoms retain one carbon atom, while the rest of the carbon is set free. FLAMES AS ILLUMINANTS. 131 between 2.4 and somewhat less than three hydrogen atoms per carbon atom. Ethane, C2H6, with a = 3, still gives a luminous flame, but of somewhat lower luminosity, and, on the other side, the gasolene flame, a = 2.33, is slightly smoky. However, in very small flames in which the surface is larger compared with the volume, and the combustion thus very rapid, higher percentages of carbon can be used without smoke. Thus the flame of the parafnne candle a = 2.08 is still smokeless but begins to smoke if it gets large, and in extremely small flames, J in. or less diameter, even acetylene, a = 1, gives smokeless combustion. Increase of the rapidity of combustion by increasing the sur- face of the flame by using a flat or hollow cylindrical burner, and increasing the air supply by artificial draft, as by a chimney, gives smokeless flames even up to b = 50, or one carbon atom to two hydrocarbon atoms, a = 2. Thus kerosene, which, due to its high carbon content a = 2.14, smokes badly, except in very small flames, is burned smoke- lessly in lamps with chimneys and flat or hollow round burners, and then gives a high light intensity : with the rapid air supply and the large surface of the thin flame, the combustion is very rapid, a part of the free carbon is immediately consumed, the temperature is high, and thus the free carbon heated sufficiently to give considerable light, and to consume completely when leaving the flame. With a hydrocarbon still richer in carbon, as acetylene or benzol a = 1, artificial draft and large flame surface are no longer sufficient to give smokelessness, and the total range of hydrocarbons which can be burned with lumi- nous flames and without smoke thus is between from three to two hydrogen atoms per carbon atom. Hydrocarbons which are too rich in carbon to be burned smokelessly, as acetylene or benzol, obviously can be burned with a smokeless luminous flame by mixing them in the proper proportions with hydrocarbons deficient in carbon, which latter by themselves would give a non-luminous or nearly non-lumi- nous flame. Thus a mixture of one volume of acetylene, C2H2, with three volumes of methane, 3 CH4, (the number of mole- cules of gases are proportional to their volumes), gives a non- smoky luminous flame: 5 C to 14 H, or a = 2.8. Such hydrocarbons as acetylene, benzol, etc., which are rich 132 RADIATION, LIGHT, AND ILLUMINATION. in carbon, are used for enriching poor gas, that is, making it more luminous : gas which gives little free carbon, as water-gas (which is rich in H and CO — both giving non-luminous flames), and which therefore would give a non-luminous or only slightly luminous flame, thus is improved in its light-giving quality by admixture of acetylene, etc. 58. If the hydrocarbon contains oxygen, as alcohol, C2H60, etc., the presence of oxygen atoms reduces the luminosity or the tendency to smoke, by taking care of a corresponding num- ber of carbon atoms: the most stable compound is CO, and water vapor, H20, as well as carbon dioxide, C02, are reduced by carbon at high temperature with the formation of carbon monoxide, CO. During the dissociation of the hydrocarbon in the flame, each oxygen atom takes up one carbon atom, form- ing CO, which burns with a non-luminous flame. In approxi- mately estimating the luminosity or the tendency to smoke of a hydrocarbon containing oxygen, for each oxygen atom one car- bon atom is to be subtracted. To illustrate this I pour some aldehyde, C2H40, and some amyl acetate, C7H1402, in small glass dishes and ignite them. In both the ratio of hydrogen to car- bon atom is a = 2, corresponding to a luminous but smoky flame. You see, however, that the aldehyde burns with a per- fectly non-luminous flame : we have to put out the light to see it; while the amyl acetate burns with a luminous, non-smoky flame. Applying above reasoning, the oxygen accounts for one carbon atom in the aldehyde : C2H40 = CO + CH4, and in CH4 : a = 4, corresponding to a non-luminous flame, as observed. In amyl acetate, the two oxygen atoms take up two carbon atoms : C7H1402 = 2 CO + C5H14, and the ratio of hydrogen to carbon atoms is a = 2.8, or b = 30, corresponding to a luminous non- smoky flame, as observed. The same effect as given by oxygen contained in the hydro- carbon molecule obviously is obtained by mixing oxygen or air with the hydrocarbon. I illustrate this on the bunsen flame : closing the air supply, I have an ordinary luminous and somewhat smoky gas flame. I now gradually admit air, and you see first the smoke disappear, and then the luminosity decreases, and first the lower part, and then the entire flame, becomes non-luminous. When the luminosity has just disappeared, the amount of air mixed with the gas is just sufficient to take up FLAMES AS ILLUMINANTS. 133 all the carbon as CO, which would deposit otherwise and give the incandescent radiator, but it is far below the amount required for complete combustion, and, by still further increasing the air supply, you see the rapidity of combustion still further increase, as shown by the decreasing size of the flame. With increasing air supply, the size of the flame very greatly decreases, and, as the same total heat is produced by the combustion, this means that the heat is concentrated. in a smaller volume, that is, the temperature of the flame is increased, in other words, the non- luminous bunsen flame is of higher temperature than the lumi- nous gas flame. Hydrocarbons which are too rich in carbon to burn without smoke, as acetylene, can be burned with a smokeless flame by mixing them with oxygen or with air. Acetylene is always burned in this manner, and all acetylene-gas burners are con- structed so as to take in air with the acetylene gas before com- bustion, that is, are small bunsen burners or similar thereto. Since the temperature of the bunsen flame, due to the more rapid combustion resulting from the mixture with air, is higher than that of the ordinary gas flame, and in the acetylene flame in the acetylene air mixture a large part of the carbon is also immediately burned, the temperature of the acetylene flame is very high, and the deposited carbon therefore raised to a very high temperature, much higher than in the ordinary gas flame, and, as the result of the higher temperature, the black-body radiation of the free carbon in the acetylene flame is far more efficient, and of much whiter color than in the ordinary gas flame. Thus the hydrocarbons which are very rich in carbon, as acetylene, benzol, naphthalene, etc., if burned smokelessly by mixture with air, give whiter and more efficient flames, due to their higher temperature. Especially is this the case with acetylene, as the energy of combustion of acetylene is higher than that of other hydrocarbons of the same relative propor- tions of hydrogen and carbon: acetylene being endothermic, that is, requiring energy for its formation from the elements. 59. Since, as discussed in Lecture VI, chemical luminescence usually occurs where intense chemical reactions take place at high temperatures, — and this is the case in the flame, — chemical luminescence of the flame gases must be expected in the hydro- carbon flame. It does occur, but does not contribute anything 134 RADIATION, LIGHT, AND ILLUMINATION. to the light production, since the spectra of hydrogen and of carbon (or CO and CH4) are practically non-luminous. The luminescence of the hydrocarbon flame therefore can be observed only with those hydrocarbons which are sufficiently poor in car- bon as not to deposit free carbon, as methane, alcohol, etc., or in which, by the admixture of air, the deposition of free carbon and thereby the formation of an incandescent radiator, is avoided, as in the bunsen flame. In this case, the blue color of the chemical luminescence of carbon-flame gases is seen: all non-luminous hydrocarbon flames are blue. 60. While light, and radiation in general, can also be pro- duced by the combustion of other materials besides hydro- carbons, industrially other materials are very little used. Burning magnesium gives a luminous flame of extremely high brilliancy and whiteness. Its light is largely due to tem- perature radiation, and the flame makes its own incandescent radiator; but unlike the hydrocarbon flame, in which the radiator is again destroyed by combustion, the incandescent radiator of the magnesium flame is the product of combustion, magnesia, MgO, and escapes from the flame as white smoke. While, how- ever, in the hydrocarbon flame the incandescent radiator is a black body, — carbon, — giving the normal temperature radiation, the radiator of the magnesium flame, magnesia, is a colored radiator, and its radiation is deficient in intensity in the ultra- red, and very high in the visible range, and thereby of a much higher efficiency than given by black-body radiation. The magnesium flame therefore is far more efficient than the hydro- carbon flame, and its light whiter. So also burning aluminum, zinc, phosphorus, etc., give lu- minous flames containing incandescent radiators produced by the combustion: alumina, zinc oxide, etc. Superimposed upon the temperature radiation of the incan- descent radiator of those flames is the radiation of chemical luminescence. Since, however, magnesium, zinc, aluminum, give fairly luminous spectra, in these flames the chemical lumi- nescence contributes a considerable part of the light, and where the luminescent light, that is, the metal spectrum, is of a marked color — as green with zinc — the flame of the burning metal also is colored. Hence burning zinc gives a greenish-yellow flame, burning calcium an orange -yellow flame, etc. FLAMES AS ILLUMINANTS. 135 Obviously, where during the combustion no solid body is formed, the light given by the flame is entirely chemical lumi- nescence. Thus burning sulphur gives a blue flame, and, if the temperature of combustion is increased by burning the sulphur in oxygen, it gives a fairly intense light, of violet color, and a radiation which is very intense in the ultra-violet. Thus before development of the ultra-violet electric arcs, as the iron arc, for the production of ultra-violet radiation lamps were used, burning carbon bisulphide, CS2, in oxygen. Carbon bisulphide, has the advantage over sulphur that, as liquid, it can easier be handled in a lamp, and especially the combustion of carbon (without adding much to the light, due to the non-luminous character of the carbon spectrum) greatly increases the flame temperature, and thereby the intensity of the radiation. Flames with Separate Radiator. 61. The hydrocarbons are the only sources of chemical energy which by their cheapness are available for general use in light production. Carbon, however, is a black-body radiator, and its efficiency of light production therefore very low, es- pecially at the relatively low temperature of the luminous hydrocarbon flame, and such flames are, therefore, low in efficiency of light production, with the exception of the acety- lene flame and other similar flames. Separating the conversion into light from the heat production; that is, using the hydrocarbon flame merely for producing heat, and using a separate radiator for converting the heat into light, offers the great advantage (1) That a colored body can be used as radiator, and thereby a higher efficiency of light production, at the same temperature, secured, by selecting a body deficient in invisible and thereby useless radiation. (2) That the rapidity of combustion can be greatly increased by mixing the hydrocarbon with air in a bunsen burner, and thereby the temperature of the flame increased, which results in a further increase of the efficiency of light production. Thus, by the use of suitable external radiators, in a non- luminous hydrocarbon flame, far higher efficiencies of light production are reached than by the use of the luminous hydro- carbon flame. 136 RADIATION, LIGHT, AND ILLUMINATION. The first use of external radiators probably was the use of a lime cylinder in a hydro-oxygen flame, in the so-called "lime light/' for producing very large units of light. in the days before the electric arc was generally available. In the last quarter of a century the external radiator has come into extended use in the Welsbach mantle; the hydro- carbon is burned in a bunsen burner, that is, mixed with air, so as to get a non-luminous flame of the highest temperature, and in this flame is immersed a cone-shaped web of a highly effi- cient colored radiator: thoria with a small percentage of ceria, etc., the so-called "mantle." The higher temperature, com- bined with the deficiency of radiation in the invisible range, ex- hibited by this colored radiator, results in an efficiency of light production several times as high as that of the luminous gas flame. The distribution of intensity in the spectrum of the Welsbach mantle obviously is not that of black-body radiation, but differs therefrom slightly, and the radiation is somewhat more intense in the greenish yellow, that is, the light has a slightly greenish-yellow hue. The Welsbach mantle is very interesting as representing the only, very extensive industrial application of colored radiation.